Boleslaus I of Masovia

Bolesław I of Masovia (Polish: Bolesław I Mazowiecki; 1208 – ca. 25 February 1248), was Polish prince member of the Polish House of Piast, Duke of Sandomierz (only a part) during 1229-1232, Duke of Dobrzyń during 1233-1247 and Duke over whole Masovia during 1247-1248.

He was the second son of Konrad I of Masovia and Agafia of Rus. He was probably named after his great-grandfather Bolesław III Wrymouth. The death of his older brother Przemysł in 1228 left him as the eldest surviving child of his family.

From a young age, Bolesław I became part of the ambitious political projects of his father. In 1227, after the assassination of the High Duke of Poland Leszek I the White, he accompanied his father to Lesser Poland to participate in negotiations on the succession with his widow Grzymisława. In view of the failure of the talks, Bolesław I received from his father the stronghold of Dobrzyń nad Wisłą until 1229, when Konrad I finally forced Grzymisława to surrender to Bolesław I a part of the Duchy of Sandomierz, the district of Radom. Immediately, Bolesław gave Dobrzyń nad Wisłą to his youngest brother, Siemowit I.

In 1231, Bolesław I became involved with his father and brothers in the installation of the Teutonic Knights in the region of Chełmno Land. At the same time, he supported his father in the war to conquer Kraków, after the death of Władysław III Spindleshanks in 1231. However, the quick intervention of Henry I the Bearded (High Duke of Poland from 1232 to 1238) and the invasion of Masovia by Prussians forced to Konrad I to abandon his plans; in addition, Henry I was able to take control of the entire Duchy of Sandomierz.

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