In the middle of the 13th century, the Silesian branch of the Piast dynasty (Henry I the Bearded and Henry II the Pious, ruled 1238–41) nearly succeeded in uniting the Polish lands, but the Mongols invaded the country from the east and defeated the combined Polish forces at the Battle of Legnica where Duke Henry II the Pious died.
These were most likely expeditionary missions sent out to protect the amber trade.
The exact time and routes of the original migration and settlement of Slavic peoples lacks written records and can only be defined as fragmented.
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.
Poland began to form into a recognizable unitary and territorial entity around the middle of the 10th century under the Piast dynasty.
Poland's first historically documented ruler, Mieszko I, accepted Christianity with the Baptism of Poland in 966, as the new official religion of his subjects.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.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.These groups are identified as Celtic, Sarmatian, Slavic, Baltic, and Germanic tribes.