Siemowit I of Masovia

Siemowit appears clearly in documents only in 1239, when at the behest of his father, he probably killed the canon of Płock Jan Czapla; however, and despite his personal involvement in this matter, current historiography completely blamed his father for his action.

Konrad I died on 31 August 1247, leaving the districts of Sieradz, Łęczyca and Czersk to his surviving sons. Using the confusion who followed this death, Siemowit I's older brother Casimir I managed to seized the greater part of their inheritance (Sieradz and Łęczyca); however, Siemowit I could kept Czersk thanks to the military support of his other older brother, Bolesław I.

Bolesław I died unexpectedly in early 1248, and under his will he gave all his domains to Siemowit I; after this, he avoided an open conflict with his brother Casimir I, accepting his rule over Sieradz-Łęczyca and focusing all his attention on the military conflicts in his frontiers with Kievan Rus', Yotvingia and the Teutonic Order.

Faced with the constant raids of Baltic tribes on his territory, Siemowit I made an alliance with Prince Daniel of Halych, who suffered the same problem; shortly after he married with Daniel's daughter Pereyaslava. In autumn 1248, Siemowit I, together with Daniel, his brother Vasilko and Bolesław V the Chaste (whose domains are also affected by the Baltic invasions), organized the first major military campaign against the Yotvingians. This was a complete victory, removed the threat of the Yotvingians for a few years. The allies held other military campaigns against the Yotvingians in 1253 and 1255, but then realiez that their forces are too weak to definitely solve the growing conflict; for this in 1254 Siemowit I and Daniel arranged a meeting in Raciąż with the Teutonic Order, represented by the komtur Burchard von Hornhausen, in with Siemowit I granted the Order 1/3 of the Yotvingians lands who could conquer. Siemowit I made two further agreements with the Teutonic Knights on 4 August 1257 at Włocławek and on 15 June 1260 at Troszyn.

The close collaboration between Siemowit I and the Teutonic Order (expressed, for example, in the defense that the Masovian ruler made for the Order before Pope Alexander IV, who accuses them of cruelty in 1258), didn't produce the expected results. Siemowit I was still targeting the Baltic tribes, while the Teutonic Knights received their territorial conquests. Particularly dangerous proved to be the growing of Lithuania, who after the destruction of the Yotvingians became in the direct neighbor of Siemowit I's domains.

Another problem that plagued Siemowit I's reign was the relationship with his brother Casimir I, who feared (perhaps not without reason) that the alliance with the Teutonic Knights may also be directed against him. So in December 1254, Casimir I and his wife where captured his brother and held them prisoners when they returned from Kraków after the celebration of the canonization of Stanislaus of Szczepanów. Siemowit I and Pereyaslava were released the following year, following the intervention of Bolesław V the Chaste and others who promised to support Casimir I in his war against Swietopelk II, Duke of Pomerelia-Gdańsk.

The extremely aggressive policy pursued by Casimir I turned sour in 1259, when Bolesław the Pious, ally of Pomerania, set up a coalition against him with Boleslaw V the Shy, Siemowit I and Daniel of Halych. The victory allowed Siemowit I to recover the district of Sieradz; however, after the signing of the peace treaty at Przedbórz on 2 December 1260, Siemowit I was forced to give Sieradz to Casimir I's eldest son Leszek II the Black, and the Kujavian princes promised that they never resolved their conflicts with wars.

In the meanwhile, Siemowit I face more internal problems. In the spring of 1262 Lithuania and Kievan Rus', under the command of Mindaugas, launched a major offensive campaign against the Duchy of Mazovia. The Masovians where caught by surprise, since virtually without any defense the capital, Płock, and after this crossed the Vistula river and captured the fortress of Jazdów. Siemowit I and his oldest son, Konrad II, who are Jazdów in a visit, prepared for a long siege of the fortress (especially when they expected that soon their allies could came to rescue them), however, due to the betrayal of the townspeople, the troops managed to enter Jazdów (23 June 1262); in the confusion of the balled, Siemowit I was killed and his son was captured; according to one version, the Masovian Duke was executed by his own brother-in-law Shvarn.

During the captivity of Konrad II (who lasted two years), the regency of Siemowit I's domains was held by his ally Bolesław the Pious.

By 1248, Siemowit I married with Pereyaslava (d. 12 April 1283), a daughter of Rurikid Prince Daniel of Halych. They had three children:

1. Konrad II (ca. 1250 – 23 June 1294).
2. Bolesław II (1253/58 – 20 April 1313).
3. Salomea (ca. 1262 – 1301), a nun.