Siemowit I of Masovia

Siemowit I of Masovia (Polish: Siemowit (Ziemowit) I mazowiecki) (c. 1224/28 – 23 June 1262), was a Polish prince member of the House of Piast, Duke of Czersk during 1247-1248, Duke of Masovia (except Dobrzyń) during 1248-1262, ruler over Sieradz during 1259-1260

He was the sixth son of Konrad I of Masovia and his wife Agafia of Rus. He was named after his older brother Siemowit, who died young in 1224.

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.

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