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Poland ( Polish : Polska [ˈpɔlska] ( listen)), officially the Republic of Poland ( Polish : Rzeczpospolita Polska [ʐɛt͡ʂpɔˈspɔlita ˈpɔlska] ( listen)), is a country located in Central Europe . It is divided into 16 administrative subdivisions, covering an area of 312,679 square kilometres (120,726 sq mi), and has a largely temperate seasonal climate . With a population of approximately 38.5 million people , Poland is the sixth most populous member state of the European Union . Poland's capital and largest metropolis is Warsaw . Other major cities include Kraków , Łódź , Wrocław , Poznań , Gdańsk , and Szczecin . The establishment of the Polish state can be traced back to A.D. 966, when Mieszko I, ruler of the realm coextensive with the territory of present- day Poland, converted to Christianity . 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 km2) and most populous countries of 16th and 17th century Europe , with a uniquely liberal political system which adopted Europe 's first written national constitution, the Constitution of 3 May 1791. 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.