Pomerania was first conquered by the Polish duke Mieszko I in the second half of the 10th century. Pagan uprisings in 1005 and 1038 resulted in independence for Western Pomerania and Pomerelia, respectively. Regained by Poland in 1116/1121, the Polish could not hold the Pomeranian duchy longer than 1135, whereas Pomerelia after the 1138 partition of Poland among the sons of Boleslaus Wrymouth became a part of the Polish seniorat, which was declared fief of the Holy Roman Empire in 1156. 