Period of fragmentation[edit]
Duke Bolesław III the Wrymouth, who reigned from 1102 to 1138, tried to end the repeated struggles between various claimants by setting the government of Poland on a more formal footing. In his testament, he divided his lands into five Duchies and distributed them among his sons.
To ensure unity, he established the senioral principle, which stated that the eldest member of the dynasty should be High Duke and have supreme power over the other Dukes. The High Duke ruled, in addition to the Duchy he inherited, over the indivisible senioral part, a vast strip of land running north-south down the middle of Poland, with Kraków as the chief city. The High Duke's prerogatives also included control over Pomerania, a fief of the Holy Roman Empire.
Church in Poland, 12th-13th century.
While the senorial part always fell to that member of the Dynasty that happened to be senior, the other four Duchies were inherited in the usual way among the descendants of Boleslaw's sons. These provisions were soon broken, however, with the various Dukes trying to gain the position of High Duke for themselves, regardless of actual seniority. The provisions, meant to ensure unity fragmented the country even further and resulted in a decline of monarchical power. Poland even came under the influence of the Přemyslid kings of Bohemia, whose dynasty died out before they could gain a stable foothold in Poland.
The accession of the Piast Duke Władysław I the Elbow-high ended the power struggle amongst the Polish nobility. He united the various principalities of the Kingdom of Poland, and in 1320 he was crowned King. His son Casimir III the Great greatly strengthened the Polish state in both foreign and domestic affairs.