Poland, officially the Republic of Poland, is a country in Central Europe. Poland is bordered by Germany to the west; the Czech Republic and Slovakia to the south; Ukraine, Belarus and Lithuania to the east; and the Baltic Sea and Kaliningrad Oblast, a Russian exclave, to the north. The total area of Poland is 312,679 square kilometres (120,726 sq mi), making it the 69th largest country in the world and the 9th largest in Europe. Poland has a population of over 38 million people, which makes it the 34th most populous country in the world and the most populous Eastern European Member State of the EU. The establishment of a Polish state is often identified with the adoption of Christianity by its ruler Mieszko I, in 966 (see Baptism of Poland), when the state covered territory similar to that of present-day Poland. In 1025, Poland became a kingdom and in 1569, it cemented a long association with the Grand Duchy of Lithuania, by signing the Union of Lublin, forming the Polish–Lithuanian Commonwealth. The Commonwealth collapsed in 1795 and Poland's territory was partitioned among the Kingdom of Prussia, the Russian Empire, and Austria. Poland regained its independence as the Second Polish Republic in 1918, after World War I, but was later occupied by Nazi Germany and the Soviet Union during World War II. Poland lost over six million citizens in World War II, emerging several years later as the socialist People's Republic of Poland within the Eastern Bloc, under strong Soviet influence. During the Revolutions of 1989, communist rule was overthrown and Poland became what is constitutionally known as the "Third Polish Republic". Poland is a unitary state, made up of sixteen voivodeships (Polish: województwo). Poland is also a member of the European Union, NATO, United Nations, World Trade Organization, and the Organisation for Economic Co-operation and Development.