Casimir I the Restorer
Mieszko's son and successor, Duke Bolesław I Chrobry, upon the 1018 Peace of Bautzen expanded the southern part of the realm, but lost control over the lands of Western Pomerania on the Baltic coast. After fragmentation, pagan revolts and a Bohemian invasion in the 1030s, Duke Casimir I the Restorer (reigned 1040-1058) again united most of the former Piast realm, including Silesia and Lubusz Land on both sides of the middle Oder River, but without Western Pomerania, which became part of the Polish state again under Bolesław III Wrymouth from 1116 until 1121, when the noble House of Griffins established the Duchy of Pomerania. On Bolesław's death in 1138, Poland for almost 200 years was subjected to fragmentation, being ruled by Bolesław's sons and by their successors, who were often in conflict with each other. Władysław I the Elbow-high, crowned King of Poland in 1320, achieved partial reunification, although the Silesian and Masovian duchies remained independent Piast holdings.