Rudolf of Rheinfelden (c. 1025 – 15 October 1080), a German aristocrat, was the Duke of Swabia (1057–1079), a brother-in-law of the Emperor Henry IV and the king-elect of a faction of the nobility in opposition to Henry from 1077. His election as King of Germany marked the outbreak of the Great Saxon Revolt, the first phase of open conflict in the contest between Empire and Papacy over the issue of episcopal investiture. Rudolf died as a result of battlefield injuries after his forces defeated Henry's at the Battle of Elster.
Rudolf was the son of Count (Graf) Kuno of Rheinfelden. He was first mentioned in a 1048 deed issued by the Salian emperor Henry III as a count in the Swabian Sisgau on the High Rhine (in present-day Northwestern Switzerland), an estate then held by the Prince-Bishopric of Basel. Rudolf's family had large possessions up to Sankt Blasien Abbey in the Black Forest and down to the Aargau beyond the border with the Kingdom of Burgundy. He probably was related to King Rudolph II of Burgundy (d. 937), the Dukes of Lorraine and the Ottonian dynasty.
Duke of Swabia
When Duke Otto III of Swabia died without male heirs in 1057, Empress Agnes, consort of late Henry III, appointed him Swabian duke and administrator of Burgundy. Rivalling with the Zähringen count Berthold, Rudolf according to Frutolf of Michelsberg had taken advantage of the minority of Agnes' son Henry IV, elected King of the Romans, by kidnapping his sister Matilda. Rudolf demanded, and received, Matilda's hand in marriage (1059). In 1061 Berthold received the Duchy of Carinthia instead. When Matilda died in 1060, Rudolf subsequently, in 1066, married Adelaide of Savoy (d. 1079), a daughter of Count Otto of Savoy and Adelaide of Susa. When Adelaide's sister Bertha of Savoy married Henry IV in 1066, Rudolf became brother-in-law to the king twice over.
During Agnes' regency, the Princes of the Holy Roman Empire could further strengthen their position against the Imperial authority. In the 1062 Coup of Kaiserswerth, several princes led by Archbishop Anno II of Cologne even abducted the minor king to enforce the surrender of the Imperial Regalia. When Henry came of age in 1065, he continued the policies of his father against the reluctant Saxon nobility, sparking the Saxon Rebellion in 1073. While other princes like the Carinthian duke Berthold of Zähringen or Duke Welf of Bavaria distanced themselves, Rudolf supported Henry's campaigns in Thuringia, when he was a primary force in the 1075 Battle of Langensalza against the insurgents. However, after the joint victory, Rudolf became estranged to the king and rumours occurred that he was involved in adversarial conspiracies. Empress Agnes repeatedly had to arbitrate between the parties.
Finally when the Investiture Controversy broke out and King Henry was excommunicated by Pope Gregory VII in February 1076, Duke Rudolf met with Berthold, Welf and several other princes in Trebur in order to decide on a course of action and to arrange a new election. Henry, observing the proceedings from his camp in Oppenheim on the other side of the Rhine, had to face a massive loss of support among the German nobles and realized that he had to achieve the lifting of his ban. Pope Gregory agreed to meet with the princes at Augsburg in February 1077.
Already in January, Henry hastened to see the pope on his way to the Empire from Rome. Duke Rudolf attempted to have the Alpine passes closed, nevertheless the king through wintry weather made his Walk to Canossa, where Gregory, fearing an armed attack by Henry's forces, had found refuge with Matilda of Tuscany. By doing penance Henry managed to achieve absolution, buying time at the price of his reputation and secular authority. The rebels continued with their plans. Rudolf was elected anti-king on 15 March 1077 at the Kaiserpfalz in Forchheim, where already Louis the Child and Conrad I of Germany had been crowned. The first anti-king in the history of the Empire, he promised to respect the investiture solely according to canon law as well as the concept of the elective monarchy. Further claims raised by the princes were rejected as simony by the attendant papal legates.
Rudolf was supported by the Archbishops of Mainz, Salzburg and Magdeburg as well as by the Dukes of Carinthia and Bavaria, the Saxon rebel Otto of Nordheim and possibly also by Duke Magnus of Saxony. He proceeded to Mainz, where on 25 May he was crowned by Archbishop Siegfried I, but soon after was forced to flee to Saxony, when the Mainz citizens revolted. This presented a problem, since the Saxon duchy was cut off from his Swabian homelands by the king's Salian territory. Moreover, the pope avoided taking sides and adopted a waiting attitude. Rudolf was accused of greed, treason and usurpation by Henry's liensmen, while his own support crumbled.
Rudolf gave Swabia to his son Berthold and attempted to rectify his situation by stalking Henry's forces near Würzburg, but to little effect. Meanwhile, he was deprived of Swabia by the Hoftag diet at Ulm in May, and the king gave the duchy to Frederick of Büren, the first Hohenstaufen ruler.
The next year Henry waged a successful campaign to Bavaria, while Pope Gregory rejected to excommunicate Rudolf. The Battle of Mellrichstadt on 7 August 1078 proved indecisive: though the opposition forces under Otto of Nordheim were victorious, the troops of Berthold and Welf were stuck in a peasants' revolt. Rudolf found it difficult to convince the Saxons to fight beyond their borders; they viewed him as a southerner and distrusted him. He was also frustrated by the apparent reluctance of the pope to recognize his cause. In order to gain and maintain supporters, he was forced to grant large parts of the crown lands, as well as those of the church, to his followers. Nevertheless, things seemed to be improving in 1080. The battle of Flarchheim (27 January 1080) went well in his favor. On 7 March, the pope excommunicated Henry again and recognized Rudolf as king.
Emboldened, Rudolf's forces met Henry's at the White Elster river in the Battle of Elster. The battle, which took place on 14 October 1080, would have been a huge victory for the anti-royalists. However, Rudolf lost his right hand in the battle and was mortally wounded in the abdomen. He withdrew to nearby Merseburg, where he died the next day and was buried. The majority of the support for the rebellion against Henry IV soon evaporated, but the struggle continued on in effect into 1085, with a final flare up in 1088 under Rudolph's successor, the second anti-king, Herman of Luxembourg.
Rudolph's daughter Adelaide married Ladislaus I of Hungary.