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Boleslaus I of Masovia

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Title: Boleslaus I of Masovia  
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Subject: Bolesław V the Chaste, Duke of Masovia, Konrad I of Masovia, Boleslaus of Masovia, Duchy of Masovia, Duchy of Sieradz
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Boleslaus I of Masovia

Boleslaw I of Masovia (Polish: Bolesław I Mazowiecki) (1208 – spring 1248), a member of the Polish House of Piast, was the eldest son of Konrad I of Masovia and of Konrad's wife Agafia of Rus.

He was the Duke of Sandomierz (he reigned over part of the duchy) from 1229 to 1232. In 1233 he became the Duke Northern Masovia and in 1247 he inherited all of the Duchy of Masovia.


From a young age, Boleslaw became part of ambitious political projects involving 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. Władysław III Spindleshanks had plans to seize the Duchy of Cracow from Masovia. Despite this setback, Konrad gave his son the stronghold of Dobrzyń nad Wisłą. In 1229 Konrad took a part of the Duchy of Sandomierz with the city of Radom; Konrad offered this in exchange for Boleslaw giving Dobrzyń nad Wisłą to Konrad's other son, Siemowit. In 1231, Boleslaw became involved with his father and brothers in the installation of the Teutonic Knights in the region of Chełmno Land. At the same time, Boleslaw supported his father in the war to conquer Cracow, after the death of Władysław III in 1231. In 1232 Henry I the Bearded (High Duke of Poland from 1232 to 1238) invaded Masovia by Prussians forced to abandon Konrad. In addition, Henry I the Bearded assumed control of the entire Duchy of Sandomierz.

To compensate for the loss of the Duchy of Sandomierz, Konrad offered Boleslaw part of Masovia, with Sieradz, the capital. Nevertheless, Konrad retained control of foreign policy and of the army. In 1234, Boleslaw won the north of Masovia in exchange for the region of Sieradz, near Lesser Poland and strategically important for his father who aimed to become Duke of Kraków. In subsequent years, Boleslaw attacked his Prussian and Russian neighbors. He installed the Order of Dobrin in Drohiczyn. In 1238 he took part in the war against Daniel of Galicia and Mindaugas of Lithuania.

In 1241, after the death of his father-in-law Henry II the Pious at the Battle of Legnica, Boleslaw maintained his father's military seizure of Kraków. He was expelled two years later.

On 31 August 1247, Konrad died; this opened new horizons for Boleslaw. In Konrad's will, he received most of Masovia. He faced an assault from his brother Casimir I of Kuyavia because Casimir was unhappy with the size of Boleslaw's inheritance. Casimir chased after his brothers, Boleslaw and Siemowit and Dobrzyń nad nad Wisla.

Boleslaw died shortly after the spring 1248. In his will he bequeathed all his possessions to his younger brother Siemowit. This however annoyed Casimir, and so wars continued between the brothers.[1]


Boleslaw I of Masovia married twice. Firstly he wed Gertrude, daughter of Henry II the Pious in 1234. Gertrude died ten years later; the couple had no children. Two years later in 1246, Boleslaw married Anastasia, daughter of Prince Alexander of Belz. This marriage also remained childless.


Preceded by
Konrad I
Duke of Masovia
Succeeded by
Siemowit I
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