Romania is a country located at the intersection of Central and Southeastern Europe, bordering on the Black Sea. Romania shares a border with Hungary and Serbia to the west, Ukraine and Moldova to the northeast and east, and Bulgaria to the south. At 238,400 square kilometers (92,000 sq mi), Romania is the ninth largest country of the European Union by area, and has the seventh largest population of the European Union with over 19 million people. Its capital and largest city is Bucharest, the tenth largest city in the EU, with a population of around two million. The United Principalities emerged when the principalities of Moldavia and Wallachia were united under Prince Alexander Ioan Cuza in 1859. In 1881, Carol I of Romania was crowned, forming the Kingdom of Romania. Independence from the Ottoman Empire was declared on 9 May 1877, and was internationally recognized the following year. At the end of World War I, Transylvania, Bukovina and Bessarabia united with the Kingdom of Romania. Greater Romania emerged into an era of progression and prosperity that would continue until the eve of World War II. That war caused the rise of a military dictatorship in Romania, leading it to fight on the side of the Axis powers from 1941 to 1944. It then switched sides in 1944 and joined the Allies. By the end of the war, many north-eastern areas of Romania's territories were occupied by the Soviet Union, and Romania forcibly became a socialist republic and a member of the Warsaw Pact. With the fall of the Iron Curtain and the 1989 Revolution, Romania began its transition towards democracy and a capitalist market economy. After a decade of post-revolution economic problems and living standards decline, extensive reforms fostered economic recovery. As of 2010, Romania is an upper middle-income country with high human development. Romania joined NATO on 29 March 2004, the European Union on 1 January 2007 and is also a member of the Latin Union; the Francophonie; the OSCE; the WTO; the BSEC; and the United Nations. Today, Romania is a unitary semi-presidential republic, in which the executive branch consists of the President and the Government. The name România is a derivative of the Latin romanus, meaning "citizen of Rome". The first mention of the appellation was made in the 16th century by Italian humanists travelling in Transylvania, Moldavia and Wallachia. The oldest surviving document written in Romanian, a 1521 letter known as the "Letter of Neacșu from Câmpulung", is also notable for having the first documented occurrence of the country's name: Wallachia is mentioned as Țeara Rumânească ("The Romanian Land", țeara from the Latin terra, "land"; current spelling: Țara Românească). Two spelling forms: român and rumân were used interchangeably until sociolinguistic evolutions in the late 17th century led to a process of semantic differentiation: the form rumân received the meaning of "bondsman", while the form român kept an ethnolinguistic meaning. After the abolition of serfdom in 1746, the form rumân gradually disappeared and the spelling stabilised to the form român. Tudor Vladimirescu, a revolutionary leader of the early 19th century, used the term Rumânia to refer exclusively to the principality of Wallachia. The name România as common homeland of all Romanians is documented in the early 19th century. The name has been officially in use since 11 December 1861. English-language sources still used the terms Rumania or Roumania, derived from the French spelling Roumanie, as recently as World War II, but the name has since been replaced with the official spelling Romania.