With the Baptism of Poland the Polish rulers accepted Christianity and the religious authority of the Roman Church.
However, the transition from paganism was not a smooth and instantaneous process for the rest of the population as evident from the pagan reaction of the 1030s.
The significance of the event was documented by Gallus Anonymus in his 1118 chronicle.
In 1138, Poland fragmented into several smaller duchies when Bolesław divided his lands among his sons.
He built extensively during his reign, and reformed the Polish army along with the country's civil and criminal laws, 1333–70.
The Golden Liberty of the nobles began to develop under Casimir's rule, when in return for their military support, the king made a series of concessions to the nobility, and establishing their legal status as superior to that of the townsmen.Early Bronze Age in Poland begun around 2400 BC, while the Iron Age commenced in approximately 750 BC.During this time, the Lusatian culture, spanning both the Bronze and Iron Ages, became particularly prominent.More than a century after the Partitions of Poland at the end of the 18th century, Poland regained its independence in 1918 with the Treaty of Versailles.In September 1939, World War II started with the invasion of Poland by Germany, followed by the Soviet Union invading Poland in accordance with the Molotov–Ribbentrop Pact. In the aftermath of the Revolutions of 1989, most notably through the emergence of the Solidarity movement, Poland reestablished itself as a democratic republic.The bulk of the population converted in the course of the next few centuries.In 1000, Boleslaw the Brave, continuing the policy of his father Mieszko, held a Congress of Gniezno and created the metropolis of Gniezno and the dioceses of Kraków, Kołobrzeg, and Wrocław.The Kingdom of Poland was founded in 1025, and in 1569 it cemented its longstanding political association with the Grand Duchy of Lithuania by signing the Union of Lublin.This union formed the Polish–Lithuanian Commonwealth, one of the largest (about 1 million km which adopted Europe's first written national constitution, the Constitution of .His efforts to create an institution of higher learning in Poland were finally rewarded when Pope Urban V granted him permission to open the University of Kraków.Casimir III the Great is the only Polish king to receive the title of Great.