Poland (Polska ), officially the Republic of Poland (Rzeczpospolita Polska ), is a country located in Central Europe. It is divided into 16 administrative subdivisions, covering an area of and has a largely temperate seasonal climate. With the passing of prominence and prosperity, the country was partitioned by neighbouring states at the end of the 18th century, and regained independence in 1918 with the Treaty of Versailles. After a series of territorial conflicts, the new multi-ethnic Poland restored its position as a key player in European politics. In September 1939, World War II began with the invasion of Poland by Nazi Germany, followed by the Soviet Union invading Poland in accordance with the Molotov–Ribbentrop Pact. Approximately six million Polish citizens, including three million of the country's Jews, perished during the course of the war. As member of the Eastern Bloc, the Polish People's Republic proclaimed forthwith was a chief signatory of the Warsaw Treaty amidst global Cold War tensions. In the wake of the 1989 events, notably through the emergence and contributions of the Solidarity movement, the communist government was dissolved and Poland reestablished itself as a presidential democratic republic. Poland has a developed market and is a regional power in Central Europe, with the largest stock exchange in the East-Central European zone. It has the sixth largest economy by GDP (nominal) in the European Union and the tenth largest in all of Europe. It's one of the most dynamic economies in the world, simultaneously achieving a very high rank on the Human Development Index. which maintains a high-income economy safety, education, and economic freedom. Alongside a developed educational system, the state also provides free university education, social security, and a universal health care system. The country has 16 UNESCO World Heritage Sites, 15 of which are cultural. Poland is a member state of the Schengen Area, the United Nations, NATO, the OECD, the Three Seas Initiative, the Visegrád Group, and guested at the G20.