Poland, 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 and Belarus to the east; and the Baltic Sea, Kaliningrad Oblast (a Russian exclave) and Lithuania to the north. The total area of Poland is 312679km2, making it the 71st 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.The establishment of a Polish state can be traced back 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. This union formed the Polish–Lithuanian Commonwealth, one of the largest and most populous countries of 16th and 17th-century Europe. The Commonwealth ceased to exist in the years 1772–1795, when its territory was partitioned among Prussia, the Russian Empire, and Austria. Poland regained its independence (as the Second Polish Republic) at the end of World War I, in 1918.