Diocletian

Diocletian (/ˌdaɪ.əˈkliːʃən/; Latin: Gaius Aurelius Valerius Diocletianus Augustus; born Diocles; 22 December 244 – 3 December 311), was a Roman emperor from 284 to 305. Born to a family of low status in Dalmatia, Diocletian rose through the ranks of the military to become a cavalry commander of the Emperor Carus's army. After the deaths of Carus and his son Numerian on campaign in Persia, Diocletian was proclaimed emperor. The title was also claimed by Carus's surviving son, Carinus, but Diocletian defeated him in the Battle of the Margus.

Diocletian's reign stabilized the empire and marks the end of the Crisis of the Third Century. He appointed fellow officer Maximian as Augustus, co-emperor, in 286. Diocletian reigned in the Eastern Empire, and Maximian reigned in the Western Empire. Diocletian delegated further on 1 March 293, appointing Galerius and Constantius as Caesars, junior co-emperors, under himself and Maximian respectively. Under this 'tetrarchy', or "rule of four", each emperor would rule over a quarter-division of the empire. Diocletian secured the empire's borders and purged it of all threats to his power. He defeated the Sarmatians and Carpi during several campaigns between 285 and 299, the Alamanni in 288, and usurpers in Egypt between 297 and 298. Galerius, aided by Diocletian, campaigned successfully against Sassanid Persia, the empire's traditional enemy. In 299 he sacked their capital, Ctesiphon. Diocletian led the subsequent negotiations and achieved a lasting and favourable peace.

Diocletian separated and enlarged the empire's civil and military services and reorganized the empire's provincial divisions, establishing the largest and most bureaucratic government in the history of the empire. He established new administrative centres in Nicomedia, Mediolanum, Sirmium, and Trevorum, closer to the empire's frontiers than the traditional capital at Rome. Building on third-century trends towards absolutism, he styled himself an autocrat, elevating himself above the empire's masses with imposing forms of court ceremonies and architecture. Bureaucratic and military growth, constant campaigning, and construction projects increased the state's expenditures and necessitated a comprehensive tax reform. From at least 297 on, imperial taxation was standardized, made more equitable, and levied at generally higher rates.

Not all of Diocletian's plans were successful: the Edict on Maximum Prices (301), his attempt to curb inflation via price controls, was counterproductive and quickly ignored. Although effective while he ruled, Diocletian's tetrarchic system collapsed after his abdication under the competing dynastic claims of Maxentius and Constantine, sons of Maximian and Constantius respectively. The Diocletianic Persecution (303–312), the empire's last, largest, and bloodiest official persecution of Christianity, failed to eliminate Christianity in the empire; indeed, after 324, Christianity became the empire's preferred religion under Constantine. Despite these failures and challenges, Diocletian's reforms fundamentally changed the structure of Roman imperial government and helped stabilize the empire economically and militarily, enabling the empire to remain essentially intact for another 150 years despite being near the brink of collapse in Diocletian's youth. Weakened by illness, Diocletian left the imperial office on 1 May 305, and became the first Roman emperor to abdicate the position voluntarily. He lived out his retirement in his palace on the Dalmatian coast, tending to his vegetable gardens. His palace eventually became the core of the modern-day city of Split in Croatia.

Diocletian was born near Salona in Dalmatia (Solin in modern Croatia), some time around 244. His parents gave him the Greek name Diocles, or possibly Diocles Valerius. The modern historian Timothy Barnes takes his official birthday, 22 December, as his actual birthdate. Other historians are not so certain. His parents were of low status; Eutropius records "that he is said by most writers to have been the son of a scribe, but by some to have been a freedman of a senator called Anulinus." The first forty years of his life are mostly obscure. The Byzantine chronicler Joannes Zonaras states that he was Dux Moesiae, a commander of forces on the lower Danube. The often-unreliable Historia Augusta states that he served in Gaul, but this account is not corroborated by other sources and is ignored by modern historians of the period. The first time Diocletian's whereabouts are accurately established, in 282, the Emperor Carus made him commander of the Protectores domestici, the elite cavalry force directly attached to the Imperial household – a post that earned him the honour of a consulship in 283. As such, he took part in Carus's subsequent Persian campaign.

Death of Numerian

Carus's death, amid a successful war with Persia and in mysterious circumstances – he was believed to have been struck by lightning or killed by Persian soldiers – left his sons Numerian and Carinus as the new Augusti. Carinus quickly made his way to Rome from his post in Gaul as imperial commissioner and arrived there by January 284, becoming legitimate Emperor in the West. Numerian lingered in the East. The Roman withdrawal from Persia was orderly and unopposed. The Sassanid king Bahram II could not field an army against them as he was still struggling to establish his authority. By March 284, Numerian had only reached Emesa (Homs) in Syria; by November, only Asia Minor. In Emesa he was apparently still alive and in good health: he issued the only extant rescript in his name there, but after he left the city, his staff, including the prefect (Numerian's father-in-law, and as such the dominant influence in the Emperor's entourage) Aper, reported that he suffered from an inflammation of the eyes. He travelled in a closed coach from then on. When the army reached Bithynia, some of the soldiers smelled an odor emanating from the coach. They opened its curtains and inside they found Numerian dead. Both Eutropius and Aurelius Victor describe Numerian's death as an assassination.

Aper officially broke the news in Nicomedia (İzmit) in November. Numerianus's generals and tribunes called a council for the succession, and chose Diocles as Emperor, in spite of Aper's attempts to garner support. On 20 November 284, the army of the east gathered on a hill 5 kilometres (3.1 mi) outside Nicomedia. The army unanimously saluted Diocles as their new Augustus, and he accepted the purple imperial vestments. He raised his sword to the light of the sun and swore an oath disclaiming responsibility for Numerian's death. He asserted that Aper had killed Numerian and concealed it. In full view of the army, Diocles drew his sword and killed Aper. According to the Historia Augusta, he quoted from Virgil while doing so. Soon after Aper's death, Diocles changed his name to the more Latinate "Diocletianus"—in full, Gaius Aurelius Valerius Diocletianus.

Conflict with Carinus

After his accession, Diocletian and Lucius Caesonius Bassus were named as consuls and assumed the fasces in place of Carinus and Numerianus. Bassus was a member of a senatorial family from Campania, a former consul and proconsul of Africa, chosen by Probus for signal distinction. He was skilled in areas of government where Diocletian presumably had no experience. Diocletian's elevation of Bassus as consul symbolized his rejection of Carinus's government in Rome, his refusal to accept second-tier status to any other emperor, and his willingness to continue the long-standing collaboration between the empire's senatorial and military aristocracies. It also tied his success to that of the Senate, whose support he would need in his advance on Rome.

Diocletian was not the only challenger to Carinus's rule; the usurper M. Aurelius Julianus, Carinus's corrector Venetiae, took control of northern Italy and Pannonia after Diocletian's accession. Julianus minted coins from the mint at Siscia (Sisak, Croatia) declaring himself emperor and promising freedom. It was all good publicity for Diocletian, and it aided in his portrayal of Carinus as a cruel and oppressive tyrant. Julianus's forces were weak, however, and were handily dispersed when Carinus's armies moved from Britain to northern Italy. As leader of the united East, Diocletian was clearly the greater threat. Over the winter of 284–85, Diocletian advanced west across the Balkans. In the spring, some time before the end of May, his armies met Carinus's across the river Margus (Great Morava) in Moesia. In modern accounts, the site has been located between the Mons Aureus (Seone, west of Smederevo) and Viminacium, near modern Belgrade, Serbia.

Despite having the stronger, more powerful army, Carinus held the weaker position. His rule was unpopular, and it was later alleged that he had mistreated the Senate and seduced his officers' wives. It is possible that Flavius Constantius, the governor of Dalmatia and Diocletian's associate in the household guard, had already defected to Diocletian in the early spring. When the Battle of the Margus began, Carinus's prefect Aristobulus also defected. In the course of the battle, Carinus was killed by his own men. Following Diocletian's victory, both the western and the eastern armies acclaimed him as Emperor. Diocletian exacted an oath of allegiance from the defeated army and departed for Italy.

Diocletian may have become involved in battles against the Quadi and Marcomanni immediately after the Battle of the Margus. He eventually made his way to northern Italy and made an imperial government, but it is not known whether he visited the city of Rome at this time. There is a contemporary issue of coins suggestive of an imperial adventus (arrival) for the city, but some modern historians state that Diocletian avoided the city, and that he did so on principle, as the city and its Senate were no longer politically relevant to the affairs of the empire and needed to be taught as much. Diocletian dated his reign from his elevation by the army, not the date of his ratification by the Senate, following the practice established by Carus, who had declared the Senate's ratification a useless formality. However, Diocletian was to offer proof of his deference towards the Senate by retaining Aristobulus as ordinary consul and colleague for 285 (one of the few instances during the Late Empire in which an emperor admitted a privatus as his colleague) and by creating senior senators Vettius Aquilinus and Junius Maximus ordinary consuls for the following year – for Maximus, it was his second consulship.

Nevertheless, if Diocletian ever did enter Rome shortly after his accession, he did not stay long; he is attested back in the Balkans by 2 November 285, on campaign against the Sarmatians.

Diocletian replaced the prefect of Rome with his consular colleague Bassus. Most officials who had served under Carinus, however, retained their offices under Diocletian. In an act of clementia denoted by the epitomator Aurelius Victor as unusual, Diocletian did not kill or depose Carinus's traitorous praetorian prefect and consul Ti. Claudius Aurelius Aristobulus, but confirmed him in both roles. He later gave him the proconsulate of Africa and the post of urban prefect for 295. The other figures who retained their offices might have also betrayed Carinus.

Maximian made co-emperor

The assassinations of Aurelian and Probus demonstrated that sole rulership was dangerous to the stability of the empire. Conflict boiled in every province, from Gaul to Syria, Egypt to the lower Danube. It was too much for one person to control, and Diocletian needed a lieutenant. At some time in 285 at Mediolanum (Milan), Diocletian raised his fellow-officer Maximian to the office of caesar, making him co-emperor.

The concept of dual rulership was nothing new to the Roman Empire. Augustus, the first Emperor, had nominally shared power with his colleagues, and more formal offices of Co-Emperor had existed from Marcus Aurelius onward. Most recently, Emperor Carus and his sons had ruled together, albeit unsuccessfully. Diocletian was in a less comfortable position than most of his predecessors, as he had a daughter, Valeria, but no sons. His co-ruler had to be from outside his family, raising the question of trust. Some historians state that Diocletian adopted Maximian as his filius Augusti, his "Augustan son", upon his appointment to the throne, following the precedent of some previous Emperors. This argument has not been universally accepted.

The relationship between Diocletian and Maximian was quickly couched in religious terms. Around 287 Diocletian assumed the title Iovius, and Maximian assumed the title Herculius. The titles were probably meant to convey certain characteristics of their associated leaders. Diocletian, in Jovian style, would take on the dominating roles of planning and commanding; Maximian, in Herculian mode, would act as Jupiter's heroic subordinate. For all their religious connotations, the emperors were not "gods" in the tradition of the Imperial cult—although they may have been hailed as such in Imperial panegyrics. Instead, they were seen as the gods' representatives, effecting their will on earth. The shift from military acclamation to divine sanctification took the power to appoint emperors away from the army. Religious legitimization elevated Diocletian and Maximian above potential rivals in a way military power and dynastic claims could not.

Conflict with Sarmatia and Persia

After his acclamation, Maximian was dispatched to fight the rebel Bagaudae, insurgent peasants of Gaul. Diocletian returned to the East, progressing slowly. By 2 November, he had only reached Civitas Iovia (Botivo, near Ptuj, Slovenia). In the Balkans during the autumn of 285, he encountered a tribe of Sarmatians who demanded assistance. The Sarmatians requested that Diocletian either help them recover their lost lands or grant them pasturage rights within the empire. Diocletian refused and fought a battle with them, but was unable to secure a complete victory. The nomadic pressures of the European Plain remained and could not be solved by a single war; soon the Sarmatians would have to be fought again.

Diocletian wintered in Nicomedia. There may have been a revolt in the eastern provinces at this time, as he brought settlers from Asia to populate emptied farmlands in Thrace. He visited Syria Palaestina the following spring,His stay in the East saw diplomatic success in the conflict with Persia: in 287, Bahram II granted him precious gifts, declared open friendship with the Empire, and invited Diocletian to visit him. Roman sources insist that the act was entirely voluntary.

Around the same time, perhaps in 287, Persia relinquished claims on Armenia and recognized Roman authority over territory to the west and south of the Tigris. The western portion of Armenia was incorporated into the empire and made a province. Tiridates III, Arsacid claimant to the Armenian throne and Roman client, had been disinherited and forced to take refuge in the empire after the Persian conquest of 252–53. In 287, he returned to lay claim to the eastern half of his ancestral domain and encountered no opposition. Bahram II's gifts were widely recognized as symbolic of a victory in the ongoing conflict with Persia, and Diocletian was hailed as the "founder of eternal peace". The events might have represented a formal end to Carus's eastern campaign, which probably ended without an acknowledged peace. At the conclusion of discussions with the Persians, Diocletian re-organized the Mesopotamian frontier and fortified the city of Circesium (Buseire, Syria) on the Euphrates.

Maximian made Augustus

Maximian's campaigns were not proceeding as smoothly. The Bagaudae had been easily suppressed, but Carausius, the man he had put in charge of operations against Saxon and Frankish pirates on the Saxon Shore, had, according to literary sources, begun keeping the goods seized from the pirates for himself. Maximian issued a death warrant for his larcenous subordinate. Carausius fled the Continent, proclaimed himself Augustus, and agitated Britain and northwestern Gaul into open revolt against Maximian and Diocletian.

Far more probable, according to the archaeological evidence available, is that Carausius probably had held some important military post in Britain and had already a firm basis of power in both Britain and Northern Gaul (a coin hoard found in Rouen proves that he was in control of that mainland area at the beginning of his rebellion) and that he profited from the lack of legitimacy of the central government. Carausius strove to have his legitimacy as a junior emperor acknowledged by Diocletian: in his coinage (of far better quality than the official one, especially his silver pieces) he extolled the "concord" between him and the central power (PAX AVGGG, "the Peace of the three Augusti", read one bronze piece from 290, displaying, on the other side, Carausius together with Diocletian and Maximian, with the caption CARAVSIVS ET FRATRES SVI, "Carausius & his brothers" [1] ). However, Diocletian could not allow elbow room to a breakaway regional usurper following in Postumus's footprints to enter, solely of his own accord, the imperial college. So Carausius had to go.

Spurred by the crisis, on 1 April 286, Maximian took up the title of Augustus. His appointment is unusual in that it was impossible for Diocletian to have been present to witness the event. It has even been suggested that Maximian usurped the title and was only later recognized by Diocletian in hopes of avoiding civil war. This suggestion is unpopular, as it is clear that Diocletian meant for Maximian to act with a certain amount of independence. It may be posited, however, that Diocletian felt the need to bind Maximian closer to him, by making him his empowered associate, in order to avoid the possibility of having him striking some sort of deal with Carausius.

Maximian realized that he could not immediately suppress the rogue commander, so in 287 he campaigned solely against tribes beyond the Rhine instead. As Carausius was allied to the Franks, Maximian's campaigns could be seen as an effort to deny the separatist emperor in Britain a basis of support on the mainland. The following spring, as Maximian prepared a fleet for an expedition against Carausius, Diocletian returned from the East to meet Maximian. The two emperors agreed on a joint campaign against the Alamanni. Diocletian invaded Germania through Raetia while Maximian progressed from Mainz. Each emperor burned crops and food supplies as he went, destroying the Germans' means of sustenance. The two men added territory to the empire and allowed Maximian to continue preparations against Carausius without further disturbance. On his return to the East, Diocletian managed what was probably another rapid campaign against the resurgent Sarmatians. No details survive, but surviving inscriptions indicate that Diocletian took the title Sarmaticus Maximus after 289.

In the East, Diocletian engaged in diplomacy with desert tribes in the regions between Rome and Persia. He might have been attempting to persuade them to ally themselves with Rome, thus reviving the old, Rome-friendly, Palmyrene sphere of influence, or simply attempting to reduce the frequency of their incursions. No details survive for these events. Some of the princes of these states were Persian client kings, a disturbing fact in light of increasing tensions with the Sassanids. In the West, Maximian lost the fleet built in 288 and 289, probably in the early spring of 290. The panegyrist who refers to the loss suggests that its cause was a storm, but this might simply have been an attempt to conceal an embarrassing military defeat. Diocletian broke off his tour of the Eastern provinces soon thereafter. He returned with haste to the West, reaching Emesa by 10 May 290, and Sirmium on the Danube by 1 July 290.

Diocletian met Maximian in Milan in the winter of 290–91, either in late December 290 or January 291. The meeting was undertaken with a sense of solemn pageantry. The emperors spent most of their time in public appearances. It has been surmised that the ceremonies were arranged to demonstrate Diocletian's continuing support for his faltering colleague. A deputation from the Roman Senate met with the emperors, renewing its infrequent contact with the Imperial office. The choice of Milan over Rome further snubbed the capital's pride. But then it was already a long established practice that Rome itself was only a ceremonial capital, as the actual seat of the Imperial administration was determined by the needs of defense. Long before Diocletian, Gallienus (r. 253–68) had chosen Milan as the seat of his headquarters. If the panegyric detailing the ceremony implied that the true center of the empire was not Rome, but where the emperor sat ("...the capital of the empire appeared to be there, where the two emperors met"), it simply echoed what had already been stated by the historian Herodian in the early third century: "Rome is where the emperor is". During the meeting, decisions on matters of politics and war were probably made in secret. The Augusti would not meet again until 303.

Foundation of the Tetrarchy

Some time after his return, and before 293, Diocletian transferred command of the war against Carausius from Maximian to Flavius Constantius, a former Governor of Dalmatia and a man of military experience stretching back to Aurelian's campaigns against Zenobia (272–73). He was Maximian's praetorian prefect in Gaul, and the husband to Maximian's daughter, Theodora. On 1 March 293 at Milan, Maximian gave Constantius the office of caesar. In the spring of 293, in either Philippopolis (Plovdiv, Bulgaria) or Sirmium, Diocletian would do the same for Galerius, husband to Diocletian's daughter Valeria, and perhaps Diocletian's Praetorian Prefect. Constantius was assigned Gaul and Britain. Galerius was initially assigned Syria, Palestine, Egypt, and responsibility for the eastern borderlands.

This arrangement is called the tetrarchy, from a Greek term meaning "rulership by four". The Tetrarchic Emperors were more or less sovereign in their own lands, and they travelled with their own imperial courts, administrators, secretaries, and armies. They were joined by blood and marriage; Diocletian and Maximian now styled themselves as brothers. The senior Co-Emperors formally adopted Galerius and Constantius as sons in 293. These relationships implied a line of succession. Galerius and Constantius would become Augusti after the departure of Diocletian and Maximian. Maximian's son Maxentius and Constantius's son Constantine would then become Caesars. In preparation for their future roles, Constantine and Maxentius were taken to Diocletian's court in Nicomedia.

Demise of Carausius's breakaway Roman Empire

Just before his creation as Caesar, Constantius proceeded to cut Carausius from his base of support in Gaul, recovering Boulogne after a hotly fought siege, a success that would result in Carausius being murdered and replaced by his aide Allectus, who would hold out in his Britain stronghold for a further three years until a two-pronged naval invasion resulted in Allectus's defeat and death at the hands of Constantius's praetorian prefect Julius Asclepiodotus, during a land battle somewhere near Farnham. Constantius himself, after disembarking in the south east, delivered London from a looting party of Frankish deserters in Allectus's pay, something that allowed him to assume the role of liberator of Britain. A famous commemorative medallion depicts a personification of London supplying the victorious Constantius on horseback in which he describes himself as redditor lucis aeternae, 'restorer of the eternal light (viz., of Rome).' The suppression of this threat to the Tetrarchs' legitimacy allowed both Constantius and Maximian to concentrate on outside threats: by 297 Constantius was back on the Rhine and Maximian engaged in a full-scale African campaign against Frankish pirates and nomads, eventually making a triumphal entry into Carthage on 10 March 298. However, Maximian's failure to deal with Carausius and Allectus on his own had jeopardized the position of Maxentius as putative heir to his father's post as Augustus of the West, with Constantius's son Constantine appearing as a rival claimant.

Conflict in the Balkans and Egypt

Diocletian spent the spring of 293 travelling with Galerius from Sirmium (Sremska Mitrovica, Serbia) to Byzantium (Istanbul, Turkey). Diocletian then returned to Sirmium, where he would remain for the following winter and spring. He campaigned against the Sarmatians again in 294, probably in the autumn, and won a victory against them. The Sarmatians' defeat kept them from the Danube provinces for a long time. Meanwhile, Diocletian built forts north of the Danube, at Aquincum (Budapest, Hungary), Bononia (Vidin, Bulgaria), Ulcisia Vetera, Castra Florentium, Intercisa (Dunaújváros, Hungary), and Onagrinum (Begeč, Serbia). The new forts became part of a new defensive line called the Ripa Sarmatica. In 295 and 296 Diocletian campaigned in the region again, and won a victory over the Carpi in the summer of 296. Later during both 299 and 302, as Diocletian was then residing in the East, it was Galerius's turn to campaign victoriously on the Danube. By the end of his reign, Diocletian had secured the entire length of the Danube, provided it with forts, bridgeheads, highways, and walled towns, and sent fifteen or more legions to patrol the region; an inscription at Sexaginta Prista on the Lower Danube extolled restored tranquilitascode: lat promoted to code: la to the region. The defense came at a heavy cost, but was a significant achievement in an area difficult to defend.

Galerius, meanwhile, was engaged during 291–293 in disputes in Upper Egypt, where he suppressed a regional uprising. He would return to Syria in 295 to fight the revanchist Persian empire. Diocletian's attempts to bring the Egyptian tax system in line with Imperial standards stirred discontent, and a revolt swept the region after Galerius's departure. The usurper L. Domitius Domitianus declared himself Augustus in July or August 297. Much of Egypt, including Alexandria, recognized his rule. Diocletian moved into Egypt to suppress him, first putting down rebels in the Thebaid in the autumn of 297, then moving on to besiege Alexandria. Domitianus died in December 297, by which time Diocletian had secured control of the Egyptian countryside. Alexandria, however, whose defense was organized under Domitianus's former corrector Aurelius Achilleus, was to hold out until a later date, probably March 298.

Bureaucratic affairs were completed during Diocletian's stay: a census took place, and Alexandria, in punishment for its rebellion, lost the ability to mint independently. Diocletian's reforms in the region, combined with those of Septimius Severus, brought Egyptian administrative practices much closer to Roman standards. Diocletian travelled south along the Nile the following summer, where he visited Oxyrhynchus and Elephantine. In Nubia, he made peace with the Nobatae and Blemmyes tribes. Under the terms of the peace treaty Rome's borders moved north to Philae and the two tribes received an annual gold stipend. Diocletian left Africa quickly after the treaty, moving from Upper Egypt in September 298 to Syria in February 299. He met with Galerius in Mesopotamia.

War with Persia

Invasion, counterinvasion

In 294, Narseh, a son of Shapur who had been passed over for the Sassanid succession, came to power in Persia. Narseh eliminated Bahram III, a young man installed in the wake of Bahram II's death in 293. In early 294, Narseh sent Diocletian the customary package of gifts between the empires, and Diocletian responded with an exchange of ambassadors. Within Persia, however, Narseh was destroying every trace of his immediate predecessors from public monuments. He sought to identify himself with the warlike kings Ardashir (r. 226–41) and Shapur I (r. 241–72), who had defeated and imprisoned Emperor Valerian (r. 253–260) following his failed invasion of the Sasanian Empire.

Narseh declared war on Rome in 295 or 296. He appears to have first invaded western Armenia, where he seized the lands delivered to Tiridates in the peace of 287. Narseh moved south into Roman Mesopotamia in 297, where he inflicted a severe defeat on Galerius in the region between Carrhae (Harran, Turkey) and Callinicum (Raqqa, Syria) (and thus, the historian Fergus Millar notes, probably somewhere on the Balikh River). Diocletian may or may not have been present at the battle, but he quickly divested himself of all responsibility. In a public ceremony at Antioch, the official version of events was clear: Galerius was responsible for the defeat; Diocletian was not. Diocletian publicly humiliated Galerius, forcing him to walk for a mile at the head of the Imperial caravan, still clad in the purple robes of the Emperor.

Galerius was reinforced, probably in the spring of 298, by a new contingent collected from the empire's Danubian holdings. Narseh did not advance from Armenia and Mesopotamia, leaving Galerius to lead the offensive in 298 with an attack on northern Mesopotamia via Armenia. It is unclear if Diocletian was present to assist the campaign; he might have returned to Egypt or Syria. Narseh retreated to Armenia to fight Galerius's force, to Narseh's disadvantage; the rugged Armenian terrain was favorable to Roman infantry, but not to Sassanid cavalry. In two battles, Galerius won major victories over Narseh. During the second encounter, Roman forces seized Narseh's camp, his treasury, his harem, and his wife. Galerius continued moving down the Tigris, and took the Persian capital Ctesiphon before returning to Roman territory along the Euphrates.

Peace negotiations

Narseh sent an ambassador to Galerius to plead for the return of his wives and children in the course of the war, but Galerius dismissed him. Serious peace negotiations began in the spring of 299. The magister memoriae (secretary) of Diocletian and Galerius, Sicorius Probus, was sent to Narseh to present terms. The conditions of the resulting Peace of Nisibis were heavy: Armenia returned to Roman domination, with the fort of Ziatha as its border; Caucasian Iberia would pay allegiance to Rome under a Roman appointee; Nisibis, now under Roman rule, would become the sole conduit for trade between Persia and Rome; and Rome would exercise control over the five satrapies between the Tigris and Armenia: Ingilene, Sophanene (Sophene), Arzanene (Aghdznik), Corduene (Carduene), and Zabdicene (near modern Hakkâri, Turkey). These regions included the passage of the Tigris through the Anti-Taurus range; the Bitlis pass, the quickest southerly route into Persian Armenia; and access to the Tur Abdin plateau.

A stretch of land containing the later strategic strongholds of Amida (Diyarbakır, Turkey) and Bezabde came under firm Roman military occupation. With these territories, Rome would have an advance station north of Ctesiphon, and would be able to slow any future advance of Persian forces through the region. Many cities east of the Tigris came under Roman control, including Tigranokert, Saird, Martyropolis, Balalesa, Moxos, Daudia, and Arzan – though under what status is unclear. At the conclusion of the peace, Tiridates regained both his throne and the entirety of his ancestral claim. Rome secured a wide zone of cultural influence, which led to a wide diffusion of Syriac Christianity from a center at Nisibis in later decades, and the eventual Christianization of Armenia.

To strengthen the defence of the east Diocletian had a fortified road constructed at the southern border, where the empire bordered the Arabs, in the year 300. This road would remain in use for centuries but proved ineffective in defending the border as conventional armies could not operate in the region.

Early persecutions

At the conclusion of the Peace of Nisibis, Diocletian and Galerius returned to Syrian Antioch. At some time in 299, the emperors took part in a ceremony of sacrifice and divination in an attempt to predict the future. The haruspices were unable to read the entrails of the sacrificed animals and blamed Christians in the Imperial household. The emperors ordered all members of the court to perform a sacrifice to purify the palace. The emperors sent letters to the military command, demanding the entire army perform the required sacrifices or face discharge. Diocletian was conservative in matters of religion, a man faithful to the traditional Roman pantheon and understanding of demands for religious purification, but Eusebius, Lactantius and Constantine state that it was Galerius, not Diocletian, who was the prime supporter of the purge, and its greatest beneficiary. Galerius, even more devoted and passionate than Diocletian, saw political advantage in the politics of persecution. He was willing to break with a government policy of inaction on the issue.

Antioch was Diocletian's primary residence from 299 to 302, while Galerius swapped places with his Augustus on the Middle and Lower Danube. Diocletian visited Egypt once, over the winter of 301–2, and issued a grain dole in Alexandria. Following some public disputes with Manicheans, Diocletian ordered that the leading followers of Mani be burnt alive along with their scriptures. In a 31 March 302 rescript from Alexandria, he declared that low-status Manicheans must be executed by the blade, and high-status Manicheans must be sent to work in the quarries of Proconnesus (Marmara Island, Turkey) or the mines of Phaeno in southern Palestine. All Manichean property was to be seized and deposited in the imperial treasury. Diocletian found much to be offended by in Manichean religion: its novelty, its alien origins, the way it corrupted the morals of the Roman race, and its inherent opposition to long-standing religious traditions. The reasons he disliked Manichaeanism was at least equally applicable to his next target, Christianity.

Great Persecution

Diocletian returned to Antioch in the autumn of 302. He ordered that the deacon Romanus of Caesarea have his tongue removed for defying the order of the courts and interrupting official sacrifices. Romanus was then sent to prison, where he was executed on 17 November 303. Diocletian believed that Romanus of Caesarea was arrogant, and he left the city for Nicomedia in the winter, accompanied by Galerius. According to Lactantius, Diocletian and Galerius entered into an argument over imperial policy towards Christians while wintering at Nicomedia in 302. Diocletian argued that forbidding Christians from the bureaucracy and military would be sufficient to appease the gods, but Galerius pushed for extermination. The two men sought the advice of the oracle of Apollo at Didyma. The oracle responded that the impious on Earth hindered Apollo's ability to provide advice. Rhetorically Eusebius records the Oracle as saying "The just on Earth..." These impious, Diocletian was informed by members of the court, could only refer to the Christians of the empire. At the behest of his court, Diocletian acceded to demands for universal persecution.

On 23 February 303, Diocletian ordered that the newly built church at Nicomedia be razed. He demanded that its scriptures be burned, and seized its precious stores for the treasury. The next day, Diocletian's first "Edict against the Christians" was published. The edict ordered the destruction of Christian scriptures and places of worship across the empire, and prohibited Christians from assembling for worship. Before the end of February, a fire destroyed part of the Imperial palace. Galerius convinced Diocletian that the culprits were Christians, conspirators who had plotted with the eunuchs of the palace. An investigation was commissioned, but no responsible party was found. Executions followed anyway, and the palace eunuchs Dorotheus and Gorgonius were executed. One individual, Peter Cubicularius, was stripped, raised high, and scourged. Salt and vinegar were poured in his wounds, and he was slowly boiled over an open flame. The executions continued until at least 24 April 303, when six individuals, including the bishop Anthimus, were decapitated. A second fire occurred sixteen days after the first. Galerius left the city for Rome, declaring Nicomedia unsafe. Diocletian would soon follow.

Although further persecutory edicts followed, compelling the arrest of the Christian clergy and universal acts of sacrifice, the persecutory edicts were ultimately unsuccessful; most Christians escaped punishment, and pagans too were generally unsympathetic to the persecution. The martyrs' sufferings strengthened the resolve of their fellow Christians. Constantius and Maximian did not apply the later persecutory edicts, and left the Christians of the West unharmed. Galerius rescinded the edict in 311, announcing that the persecution had failed to bring Christians back to traditional religion. The temporary apostasy of some Christians, and the surrendering of scriptures, during the persecution played a major role in the subsequent Donatist controversy. Within twenty-five years of the persecution's inauguration, the Christian Emperor Constantine would rule the empire alone. He would reverse the consequences of the edicts, and return all confiscated property to Christians. Under Constantine's rule, Christianity would become the empire's preferred religion. Diocletian was demonized by his Christian successors: Lactantius intimated that Diocletian's ascendancy heralded the apocalypse, and in Serbian mythology, Diocletian is remembered as Dukljan, the adversary of God.

Illness and abdication

Diocletian entered the city of Rome in the early winter of 303. On 20 November, he celebrated, with Maximian, the twentieth anniversary of his reign (vicennaliacode: lat promoted to code: la ), the tenth anniversary of the tetrarchy (decennalia), and a triumph for the war with Persia. Diocletian soon grew impatient with the city, as the Romans acted towards him with what Edward Gibbon, following Lactantius, calls "licentious familiarity". The Roman people did not give enough deference to his supreme authority; it expected him to act the part of an aristocratic ruler, not a monarchic one. On 20 December 303, Diocletian cut short his stay in Rome and left for the north. He did not even perform the ceremonies investing him with his ninth consulate; he did them in Ravenna on 1 January 304 instead. There are suggestions in the Panegyrici Latini and Lactantius's account that Diocletian arranged plans for his and Maximian's future retirement of power in Rome. Maximian, according to these accounts, swore to uphold Diocletian's plan in a ceremony in the Temple of Jupiter.

From Ravenna, Diocletian left for the Danube. There, possibly in Galerius's company, he took part in a campaign against the Carpi. He contracted a minor illness while on campaign, but his condition quickly worsened and he chose to travel in a litter. In the late summer he left for Nicomedia. On 20 November 304, he appeared in public to dedicate the opening of the circus beside his palace. He collapsed soon after the ceremonies. Over the winter of 304–5 he kept within his palace at all times. Rumours alleging that Diocletian's death was merely being kept secret until Galerius could come to assume power spread through the city. On 13 December, it appeared that he had finally died. The city was sent into a mourning from which it recovered after public declarations that Diocletian was still alive. When Diocletian reappeared in public on 1 March 305, he was emaciated and barely recognizable.

Galerius arrived in the city later in March. According to Lactantius, he came armed with plans to reconstitute the tetrarchy, force Diocletian to step down, and fill the Imperial office with men compliant to his will. Through coercion and threats, he eventually convinced Diocletian to comply with his plan. Lactantius also claims that he had done the same to Maximian at Sirmium. On 1 May 305, Diocletian called an assembly of his generals, traditional companion troops, and representatives from distant legions. They met at the same hill, 5 kilometres (3.1 mi) out of Nicomedia, where Diocletian had been proclaimed emperor. In front of a statue of Jupiter, his patron deity, Diocletian addressed the crowd. With tears in his eyes, he told them of his weakness, his need for rest, and his will to resign. He declared that he needed to pass the duty of empire on to someone stronger. He thus became the first Roman emperor to voluntarily abdicate his title.

Most in the crowd believed they knew what would follow; Constantine and Maxentius, the only adult sons of reigning emperors, men who had long been preparing to succeed their fathers, would be granted the title of caesar. Constantine had travelled through Palestine at the right hand of Diocletian, and was present at the palace in Nicomedia in 303 and 305. It is likely that Maxentius received the same treatment. In Lactantius's account, when Diocletian announced that he was to resign, the entire crowd turned to face Constantine. It was not to be: Severus and Maximinus were declared caesars. Maximinus appeared and took Diocletian's robes. On the same day, Severus received his robes from Maximian in Milan. Constantius succeeded Maximian as augustus of the West, but Constantine and Maxentius were entirely ignored in the transition of power. This did not bode well for the future security of the tetrarchic system.

Retirement and death

Diocletian retired to his homeland, Dalmatia. He moved into the expansive Diocletian's Palace, a heavily fortified compound located by the small town of Spalatum on the shores of the Adriatic Sea, and near the large provincial administrative center of Salona. The palace is preserved in great part to this day and forms the historic core of Split, the second-largest city of modern Croatia.

Maximian retired to villas in Campania or Lucania. Their homes were distant from political life, but Diocletian and Maximian were close enough to remain in regular contact with each other. Galerius assumed the consular fasces in 308 with Diocletian as his colleague. In the autumn of 308, Galerius again conferred with Diocletian at Carnuntum (Petronell-Carnuntum, Austria). Diocletian and Maximian were both present on 11 November 308, to see Galerius appoint Licinius to be Augustus in place of Severus, who had died at the hands of Maxentius. He ordered Maximian, who had attempted to return to power after his retirement, to step down permanently. At Carnuntum people begged Diocletian to return to the throne, to resolve the conflicts that had arisen through Constantine's rise to power and Maxentius's usurpation. Diocletian's reply: "If you could show the cabbage that I planted with my own hands to your emperor, he definitely wouldn't dare suggest that I replace the peace and happiness of this place with the storms of a never-satisfied greed."

He lived on for four more years, spending his days in his palace gardens. He saw his tetrarchic system fail, torn by the selfish ambitions of his successors. He heard of Maximian's third claim to the throne, his forced suicide, and his damnatio memoriae. In his own palace, statues and portraits of his former companion emperor were torn down and destroyed. Deep in despair and illness, Diocletian may have committed suicide. He died on 3 December 312.

Tetrarchic and ideological

Diocletian saw his work as that of a restorer, a figure of authority whose duty it was to return the empire to peace, to recreate stability and justice where barbarian hordes had destroyed it. He arrogated, regimented and centralized political authority on a massive scale. In his policies, he enforced an Imperial system of values on diverse and often unreceptive provincial audiences. In the Imperial propaganda from the period, recent history was perverted and minimized in the service of the theme of the tetrarchs as "restorers". Aurelian's achievements were ignored, the revolt of Carausius was backdated to the reign of Gallienus, and it was implied that the tetrarchs engineered Aurelian's defeat of the Palmyrenes; the period between Gallienus and Diocletian was effectively erased. The history of the empire before the tetrarchy was portrayed as a time of civil war, savage despotism, and imperial collapse. In those inscriptions that bear their names, Diocletian and his companions are referred to as "restorers of the whole world", men who succeeded in "defeating the nations of the barbarians, and confirming the tranquility of their world". Diocletian was written up as the "founder of eternal peace". The theme of restoration was conjoined to an emphasis on the uniqueness and accomplishments of the tetrarchs themselves.

The cities where emperors lived frequently in this period—Milan, Trier, Arles, Sirmium, Serdica, Thessaloniki, Nicomedia and Antioch—were treated as alternate imperial seats, to the exclusion of Rome and its senatorial elite. A new style of ceremony was developed, emphasizing the distinction of the emperor from all other persons. The quasi-republican ideals of Augustus's primus inter pares were abandoned for all but the tetrarchs themselves. Diocletian took to wearing a gold crown and jewels, and forbade the use of purple cloth to all but the emperors. His subjects were required to prostrate themselves in his presence (adoratio); the most fortunate were allowed the privilege of kissing the hem of his robe (proskynesis, προσκύνησις). Circuses and basilicas were designed to keep the face of the emperor perpetually in view, and always in a seat of authority. The emperor became a figure of transcendent authority, a man beyond the grip of the masses. His every appearance was stage-managed. This style of presentation was not new—many of its elements were first seen in the reigns of Aurelian and Severus—but it was only under the tetrarchs that it was refined into an explicit system.

Administrative

In keeping with his move from an ideology of republicanism to one of autocracy, Diocletian's council of advisers, his consilium, differed from those of earlier emperors. He destroyed the Augustan illusion of imperial government as a cooperative affair among emperor, army, and senate. In its place he established an effectively autocratic structure, a shift later epitomized in the institution's name: it would be called a consistorium, not a council. Diocletian regulated his court by distinguishing separate departments (scrinia) for different tasks. From this structure came the offices of different magistri, like the magister officiorum ("Master of Offices"), and associated secretariats. These were men suited to dealing with petitions, requests, correspondence, legal affairs, and foreign embassies. Within his court Diocletian maintained a permanent body of legal advisers, men with significant influence on his re-ordering of juridical affairs. There were also two finance ministers, dealing with the separate bodies of the public treasury and the private domains of the emperor, and the praetorian prefect, the most significant person of the whole. Diocletian's reduction of the Praetorian Guards to the level of a simple city garrison for Rome lessened the military powers of the prefect – although a prefect like Asclepiodotus was still a trained general – but the office retained much civil authority. The prefect kept a staff of hundreds and managed affairs in all segments of government: in taxation, administration, jurisprudence, and minor military commands, the praetorian prefect was often second only to the emperor himself.

Altogether, Diocletian effected a large increase in the number of bureaucrats at the government's command; Lactantius was to claim that there were now more men using tax money than there were paying it. The historian Warren Treadgold estimates that under Diocletian the number of men in the civil service doubled from 15,000 to 30,000. The classicist Roger S. Bagnall estimated that there was one bureaucrat for every 5–10,000 people in Egypt based on 400 or 800 bureaucrats for 4 million inhabitants (no one knows the population of the province in 300 AD; Strabo 300 years earlier put it at 7.5 million, excluding Alexandria). (By comparison, the ratio in 12th-century Song dynasty China was one bureaucrat for every 15,000 people.) Jones estimated 30,000 bureaucrats for an empire of 50–65 million inhabitants, which works out to approximately 1,667 or 2,167 inhabitants per imperial official as averages empire-wide. The actual numbers of officials and ratios per inhabitant varied, of course, per diocese depending on the number of provinces and population within a diocese. Provincial and diocesan paid officials (there were unpaid supernumeraries) numbered about 13–15,000 based on their staff establishments as set by law. The other 50% were with the emperor(s) in his or their comitatus, with the praetorian prefects, or with the grain supply officials in the capital (later, the capitals, Rome and Constantinople), Alexandria, and Carthage and officials from the central offices located in the provinces.

To avoid the possibility of local usurpations, to facilitate a more efficient collection of taxes and supplies, and to ease the enforcement of the law, Diocletian doubled the number of provinces from fifty to almost one hundred. The provinces were grouped into twelve dioceses, each governed by an appointed official called a vicarius, or "deputy of the praetorian prefects". Some of the provincial divisions required revision, and were modified either soon after 293 or early in the fourth century. Rome herself (including her environs, as defined by a 100 miles (160 km)-radius perimeter around the city itself) was not under the authority of the praetorian prefect, as she was to be administered by a city prefect of senatorial rank – the sole prestigious post with actual power reserved exclusively for senators, except for some governors in Italy with the titles of corrector and the proconsuls of Asia and Africa. The dissemination of imperial law to the provinces was facilitated under Diocletian's reign, because Diocletian's reform of the Empire's provincial structure meant that there were now a greater number of governors (praesides) ruling over smaller regions and smaller populations. Diocletian's reforms shifted the governors' main function to that of the presiding official in the lower courts: whereas in the early Empire military and judicial functions were the function of the governor, and procurators had supervised taxation, under the new system vicarii and governors were responsible for justice and taxation, and a new class of duces ("dukes"), acting independently of the civil service, had military command. These dukes sometimes administered two or three of the new provinces created by Diocletian, and had forces ranging from two thousand to more than twenty thousand men. In addition to their roles as judges and tax collectors, governors were expected to maintain the postal service (cursus publicus) and ensure that town councils fulfilled their duties.

This curtailment of governors' powers as the Emperors' representatives may have lessened the political dangers of an all-too-powerful class of Imperial delegates, but it also severely limited governors' ability to oppose local landed elites, especially those of senatorial status, which, although with reduced opportunities for office holding, retained wealth, social prestige, and personal connections, particularly in relatively peaceful regions without a great military presence. On one occasion, Diocletian had to exhort a proconsul of Africa not to fear the consequences of treading on the toes of the local magnates of senatorial rank. If a governor of senatorial rank himself felt these pressures, one can imagine the difficulties faced by a mere praeses. That accounts for the strained relationship between the central power and local elites: sometime during 303, an attempted military sedition in Seleucia Pieria and Antioch made Diocletian to extract a bloody retribution on both cities by putting to death a number of their council members for failing their duties of keeping order in their jurisdiction.

Legal

As with most emperors, much of Diocletian's daily routine rotated around legal affairs—responding to appeals and petitions, and delivering decisions on disputed matters. Rescripts, authoritative interpretations issued by the emperor in response to demands from disputants in both public and private cases, were a common duty of second- and third-century emperors. Diocletian was awash in paperwork, and was nearly incapable of delegating his duties. It would have been seen as a dereliction of duty to ignore them. In the "nomadic" imperial courts of the later Empire, one can track the progress of the imperial retinue through the locations from whence particular rescripts were issued – the presence of the Emperor was what allowed the system to function. Whenever the imperial court would settle in one of the capitals, there was a glut in petitions, as in late 294 in Nicomedia, where Diocletian kept winter quarters.

Admittedly, Diocletian's praetorian prefects—Afranius Hannibalianus, Julius Asclepiodotus, and Aurelius Hermogenianus—aided in regulating the flow and presentation of such paperwork, but the deep legalism of Roman culture kept the workload heavy. Emperors in the forty years preceding Diocletian's reign had not managed these duties so effectively, and their output in attested rescripts is low. Diocletian, by contrast, was prodigious in his affairs: there are around 1,200 rescripts in his name still surviving, and these probably represent only a small portion of the total issue. The sharp increase in the number of edicts and rescripts produced under Diocletian's rule has been read as evidence of an ongoing effort to realign the whole Empire on terms dictated by the imperial center.

Under the governance of the jurists Gregorius, Aurelius Arcadius Charisius, and Hermogenianus, the imperial government began issuing official books of precedent, collecting and listing all the rescripts that had been issued from the reign of Hadrian (r. 117–38) to the reign of Diocletian. The Codex Gregorianus includes rescripts up to 292, which the Codex Hermogenianus updated with a comprehensive collection of rescripts issued by Diocletian in 293 and 294. Although the very act of codification was a radical innovation, given the precedent-based design of the Roman legal system, the jurists were generally conservative, and constantly looked to past Roman practice and theory for guidance. They were probably given more free rein over their codes than the later compilers of the Codex Theodosianus (438) and Codex Justinianeus (529) would have. Gregorius and Hermogenianus's codices lack the rigid structuring of later codes, and were not published in the name of the emperor, but in the names of their compilers. Their official character, however, was clear in that both collections were subsequently acknowledged by courts as authoritative records of imperial legislation up to the date of their publication and regularly updated.

After Diocletian's reform of the provinces, governors were called iudex, or judge. The governor became responsible for his decisions first to his immediate superiors, as well as to the more distant office of the emperor. It was most likely at this time that judicial records became verbatim accounts of what was said in trial, making it easier to determine bias or improper conduct on the part of the governor. With these records and the Empire's universal right of appeal, Imperial authorities probably had a great deal of power to enforce behavior standards for their judges. In spite of Diocletian's attempts at reform, the provincial restructuring was far from clear, especially when citizens appealed the decisions of their governors. Proconsuls, for example, were often both judges of first instance and appeal, and the governors of some provinces took appellant cases from their neighbors. It soon became impossible to avoid taking some cases to the emperor for arbitration and judgment. Diocletian's reign marks the end of the classical period of Roman law. Where Diocletian's system of rescripts shows an adherence to classical tradition, Constantine's law is full of Greek and eastern influences.

Military

It is archaeologically difficult to distinguish Diocletian's fortifications from those of his successors and predecessors. The Devil's Dykes, for example, the Danubian earthworks traditionally attributed to Diocletian, cannot even be securely dated to a particular century. The most that can be said about built structures under Diocletian's reign is that he rebuilt and strengthened forts at the Upper Rhine frontier (where he followed the works built under Probus along the Lake Constance-Basel and the Rhine–Iller–Danube line), on the Danube- where a new line of forts on the far side of the river, the Ripa Sarmatica, was added to older, rehabilitated fortresses – in Egypt, and on the frontier with Persia. Beyond that, much discussion is speculative and reliant on the broad generalizations of written sources. Diocletian and the tetrarchs had no consistent plan for frontier advancement, and records of raids and forts built across the frontier are likely to indicate only temporary claims. The Strata Diocletiana, built after the Persian Wars, which ran from the Euphrates North of Palmyra and South towards northeast Arabia in the general vicinity of Bostra, is the classic Diocletianic frontier system, consisting of an outer road followed by tightly spaced forts – defensible hard-points manned by small garrisons – followed by further fortifications in the rear. In an attempt to resolve the difficulty and slowness of transmitting orders to the frontier, the new capitals of the tetrarchic era were all much closer to the empire's frontiers than Rome had been: Trier sat on the Moselle, a tributary of the Rhine, Sirmium and Serdica were close to the Danube, Thessaloniki was on the route leading eastward, and Nicomedia and Antioch were important points in dealings with Persia.

Lactantius criticized Diocletian for an excessive increase in troop sizes, declaring that "each of the four [tetrarchs] strove to have a far larger number of troops than previous emperors had when they were governing the state alone". The fifth-century pagan Zosimus, by contrast, praised Diocletian for keeping troops on the borders, rather than keeping them in the cities, as Constantine was held to have done. Both these views had some truth to them, despite the biases of their authors: Diocletian and the tetrarchs did greatly expand the army, and the growth was mostly in frontier regions, where the increased effectiveness of the new Diocletianic legions seem to have been mostly spread across a network of strongholds. Nevertheless, it is difficult to establish the precise details of these shifts given the weakness of the sources. The army expanded to about 580,000 men from a 285 strength of 390,000, of which 310,000 men were stationed in the East, most of whom manned the Persian frontier. The navy's forces increased from approximately 45,000 men to approximately 65,000 men.

Diocletian's expansion of the army and civil service meant that the empire's tax burden grew. Since military upkeep took the largest portion of the imperial budget, any reforms here would be especially costly. The proportion of the adult male population, excluding slaves, serving in the army increased from roughly 1 in 25 to 1 in 15, an increase judged excessive by some modern commentators. Official troop allowances were kept to low levels, and the mass of troops often resorted to extortion or the taking of civilian jobs. Arrears became the norm for most troops. Many were even given payment in kind in place of their salaries. Were he unable to pay for his enlarged army, there would likely be civil conflict, potentially open revolt. Diocletian was led to devise a new system of taxation.

Economic

Taxation

In the early empire (30 BC – AD 235) the Roman government paid for what it needed in gold and silver. The coinage was stable. Requisition, forced purchase, was used to supply armies on the march. During the third century crisis (235–285), the government resorted to requisition rather than payment in debased coinage, since it could never be sure of the value of money. Requisition was nothing more or less than seizure. Diocletian made requisition into tax. He introduced an extensive new tax system based on heads (capita) and land (iugera) – with one iugerum equal to approximately 0.65 acres – and tied to a new, regular census of the empire's population and wealth. Census officials traveled throughout the empire, assessed the value of labor and land for each landowner, and joined the landowners' totals together to make citywide totals of capita and iuga. The iugum was not a consistent measure of land, but varied according to the type of land and crop, and the amount of labor necessary for sustenance. The caput was not consistent either: women, for instance, were often valued at half a caput, and sometimes at other values. Cities provided animals, money, and manpower in proportion to its capita, and grain in proportion to its iuga.

Most taxes were due on each year on 1 September, and levied from individual landowners by decuriones (decurions). These decurions, analogous to city councilors, were responsible for paying from their own pocket what they failed to collect. Diocletian's reforms also increased the number of financial officials in the provinces: more rationales and magistri privatae are attested under Diocletian's reign than before. These officials represented the interests of the fisc, which collected taxes in gold, and the Imperial properties. Fluctuations in the value of the currency made collection of taxes in kind the norm, although these could be converted into coin. Rates shifted to take inflation into account. In 296, Diocletian issued an edict reforming census procedures. This edict introduced a general five-year census for the whole empire, replacing prior censuses that had operated at different speeds throughout the empire. The new censuses would keep up with changes in the values of capita and iuga.

Italy, which had long been exempt from taxes, was included in the tax system from 290/291 as a diocesis. The city of Rome itself, however, remained exempt; the "regions" (i.e., provinces) South of Rome (generally called "suburbicarian", as opposed to the Northern, "annonaria" region) seem to have been relatively less taxed, in what probably was a sop offered to the great senatorial families and their landed properties.

Diocletian's edicts emphasized the common liability of all taxpayers. Public records of all taxes were made public. The position of decurion, member of the city council, had been an honor sought by wealthy aristocrats and the middle classes who displayed their wealth by paying for city amenities and public works. Decurions were made liable for any shortfall in the amount of tax collected. Many tried to find ways to escape the obligation. By 300, civilians across the empire complained that there were more tax collectors than there were people to pay taxes.

Currency and inflation

Aurelian's attempt to reform the currency had failed; the denarius was dead. Diocletian restored the three-metal coinage and issued better quality pieces. The new system consisted of five coins: the aureus/solidus, a gold coin weighing, like its predecessors, one-sixtieth of a pound; the argenteus, a coin weighing one ninety-sixth of a pound and containing ninety-five percent pure silver; the follis, sometimes referred to as the laureatus A, which is a copper coin with added silver struck at the rate of thirty-two to the pound; the radiatus, a small copper coin struck at the rate of 108 to the pound, with no added silver; and a coin known today as the laureatus B, a smaller copper coin struck at the rate of 192 to the pound. Since the nominal values of these new issues were lower than their intrinsic worth as metals, the state was minting these coins at a loss. This practice could be sustained only by requisitioning precious metals from private citizens in exchange for state-minted coin (of a far lower value than the price of the precious metals requisitioned).

By 301, however, the system was in trouble, strained by a new bout of inflation. Diocletian therefore issued his Edict on Coinage, an act re-tariffing all debts so that the nummus, the most common coin in circulation, would be worth half as much. In the edict, preserved in an inscription from the city of Aphrodisias in Caria (near Geyre, Turkey), it was declared that all debts contracted before 1 September 301 must be repaid at the old standards, while all debts contracted after that date would be repaid at the new standards. It appears that the edict was made in an attempt to preserve the current price of gold and to keep the Empire's coinage on silver, Rome's traditional metal currency. This edict risked giving further momentum to inflationary trends, as had happened after Aurelian's currency reforms. The government's response was to issue a price freeze.

The Edict on Maximum Prices (Edictum De Pretiis Rerum Venalium) was issued two to three months after the coinage edict, somewhere between 20 November and 10 December 301. The best-preserved Latin inscription surviving from the Greek East, the edict survives in many versions, on materials as varied as wood, papyrus, and stone. In the edict, Diocletian declared that the current pricing crisis resulted from the unchecked greed of merchants, and had resulted in turmoil for the mass of common citizens. The language of the edict calls on the people's memory of their benevolent leaders, and exhorts them to enforce the provisions of the edict, and thereby restore perfection to the world. The edict goes on to list in detail over one thousand goods and accompanying retail prices not to be exceeded. Penalties are laid out for various pricing transgressions.

In the most basic terms, the edict was ignorant of the law of supply and demand: it ignored the fact that prices might vary from region to region according to product availability, and it ignored the impact of transportation costs in the retail price of goods. In the judgment of the historian David Potter, the edict was "an act of economic lunacy". The fact that the edict began with a long rhetorical preamble betrays at the same time a moralizing stance as well as a weak grasp of economics – perhaps simply the wishful thinking that criminalizing a practice was enough to stop it.

There is no consensus about how effectively the edict was enforced. Supposedly, inflation, speculation, and monetary instability continued, and a black market arose to trade in goods forced out of official markets. The edict's penalties were applied unevenly across the empire (some scholars believe they were applied only in Diocletian's domains), widely resisted, and eventually dropped, perhaps within a year of the edict's issue. Lactantius has written of the perverse accompaniments to the edict; of goods withdrawn from the market, of brawls over minute variations in price, of the deaths that came when its provisions were enforced. His account may be true, but it seems to modern historians exaggerated and hyperbolic, and the impact of the law is recorded in no other ancient source.

Social and professional mobility

Partly in response to economic pressures and in order to protect the vital functions of the state, Diocletian restricted social and professional mobility. Peasants became tied to the land in a way that presaged later systems of land tenure and workers such as bakers, armourers, public entertainers and workers in the mint had their occupations made hereditary. Soldiers' children were also forcibly enrolled, something that followed spontaneous tendencies among the rank-and-file, but also expressed increasing difficulties in recruitment.

The historian A.H.M. Jones observed that "It is perhaps Diocletian's greatest achievement that he reigned twenty-one years and then abdicated voluntarily, and spent the remaining years of his life in peaceful retirement." Diocletian was one of the few emperors of the third and fourth centuries to die naturally, and the first in the history of the empire to retire voluntarily. Once he retired, however, his tetrarchic system collapsed. Without the guiding hand of Diocletian, the empire fell into civil wars. Stability emerged after the defeat of Licinius by Constantine in 324. Under the Christian Constantine, Diocletian was maligned. Constantine's rule, however, validated Diocletian's achievements and the autocratic principle he represented: the borders remained secure, in spite of Constantine's large expenditure of forces during his civil wars; the bureaucratic transformation of Roman government was completed; and Constantine took Diocletian's court ceremonies and made them even more extravagant.

Constantine ignored those parts of Diocletian's rule that did not suit him. Diocletian's policy of preserving a stable silver coinage was abandoned, and the gold solidus became the empire's primary currency instead. Diocletian's persecution of Christians was repudiated and changed to a policy of toleration and then favoritism. Christianity eventually became the official religion in 380. Most importantly, Diocletian's tax system and administrative reforms lasted, with some modifications, until the advent of the Muslims in the 630s. The combination of state autocracy and state religion was instilled in much of Europe, particularly in the lands which adopted Orthodox Christianity.

In addition to his administrative and legal impact on history, the Emperor Diocletian is considered to be the founder of the city of Split in modern-day Croatia. The city itself grew around the heavily fortified Diocletian's Palace the emperor had built in anticipation of his retirement.

The Era of Martyrs (Latin: anno martyrum or AM), also known as the Diocletian era (Latin: anno Diocletiani), is a method of numbering years used by the Church of Alexandria beginning in the 4th century anno Domini and by the Coptic Orthodox Church of Alexandria from the 5th century to the present. In this system of counting, the beginning of Diocletian's reign in 284 was used as the epoch, making Diocletian's first year in power into the Year 1 of that calendar. Western Christians were aware of this count but did not use it; Dionysius Exiguus replaced the anno Diocletiani era with his anno Domini era because he did not wish to continue the memory of a tyrant who persecuted Christians. The anno Domini era became dominant in the Latin West but was not used in the Greek East until modern times.

Dukljan, a major villain in Serbian mythology who is presented as the adversary of God is considered to be a mythological reflection of the historical Diocletian.