Many historians trace the establishment of a Polish state to 966, when Mieszko I, ruler of a territory roughly coextensive with that of present-day Poland, converted to Christianity. TheKingdom of Poland was founded in 1025, and in 1569 it cemented a longstanding political association with the Grand Duchy of Lithuania by signing the Union of Lublin, forming the Polish–Lithuanian Commonwealth. The Commonwealth ceased to exist in 1795, when thePolish territory was partitioned among the Kingdom of Prussia, the Russian Empire, and Old Austria. Poland regained independence (as the Second Polish Republic) at the end of World War I, in 1918.
Two decades later, in September 1939, World War II started with the invasions of Poland byNazi Germany and the Soviet Union (as part of the Molotov–Ribbentrop Pact). More than six million Polish citizens died in the war. Poland became a client state of theSoviet Union in 1944 and was accordingly renamed the People's Republic of Poland in 1952. During the Revolutions of 1989, Poland's communist government was overthrown and Poland adopted a new constitution establishing itself as a democracy and renaming itself the "Third Polish Republic".
Despite the vast destruction the country experienced during World War II, Poland managed to preserve much of its cultural wealth. There are currently 14 heritage sites inscribed on theUNESCO World Heritage list in Poland and 54 Historical Monuments. Since the end of the communist period, Poland has achieved a "very high" ranking in terms of human development.