While Siemowit's son Duke Konrad II (1264–1294) moved his residence to Czersk he and his brother Boleslaus II entered into a long-term conflict over the Polish seniorate with their Kuyavian relatives and the Silesian Piasts, which estranged them from the Piast monarchy.
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At first it was part of another Polish fiefdom, the Duchy of Masovia, as in 1388 the king of Poland, Władysław Jagiełło, granted Belz to Siemowit IV, Duke of Masovia, for his recognition of Masovia as a fiefdom of Poland and as a dowry for Siemowit's marriage with Jagiełło's sister, Alexandra.
In 1326 the Grand Master formed an anti-Polish coalition at Brodnica, consisting of the Masovian dukes Siemowit II, Trojden I and Wenceslaus of Płock as well as of several Silesian Piasts and King John of Bohemia.