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.Other metropolises include Kraków, Wrocław, Poznań, Gdańsk and Szczecin.
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 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.
In September 1939, World War II started with the invasion of Poland by Nazi Germany, followed shortly thereafter by invasion by the Soviet Union, both in accordance with the Molotov–Ribbentrop Pact.