Poland /ˈpoʊlənd/, officially the Republic of Poland, is a country in Central Europe, bordered by Germany to the west; the Czech Republic and Slovakia to the south; Ukraine, Belarus to the east; and the Baltic Sea and Kaliningrad Oblast and Lithuania to the north. The total area of Poland is 312,679 square kilometres, making it the 69th largest country in the world and the 9th largest in Europe. With a population of over 38.5 million people, Poland is the 34th most populous country in the world, the sixth most populous member of the European Union, and the most populous post-communist member of the European Union. Poland is a unitary state divided into 16 administrative subdivisions.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. The Kingdom 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 gradually ceased to exist in the years 1772–1795, when the Polish territory was partitioned among Prussia, the Russian Empire, and Austria. Poland regained its independence at the end of World War I, in 1918.Two decades later, in September 1939, World War II started with the invasions of Poland by Nazi Germany and the Soviet Union. More than six million Polish citizens died in the war. In 1944, a Soviet-backed Polish provisional government was formed, which, after a period of conflict, falsified referendum and elections, gave rise to a satellite state of the Soviet Union, Polish Republic, renamed to the People's Republic of Poland in 1952. During the Revolutions of 1989, Poland's Marxist-Leninist government was overthrown and Poland adopted a new constitution establishing itself as a democracy under the name Rzeczpospolita Polska, often referred to as 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 14 heritage sites inscribed on the UNESCO World Heritage and 54 Historical Monuments and many objects of cultural heritage. Since the end of the communist period, Poland has achieved a "very high" ranking in terms of human development.