Yugoslavia was a country in Southeast Europe that existed in various forms throughout the 20th century. At its largest, it covered about 255,804 square kilometers and had a population of over 23 million people. Formed after World War I in 1918 as the Kingdom of Serbs, Croats, and Slovenes, it was later renamed Yugoslavia. After World War II, it became a socialist federation made up of six republics: Bosnia and Herzegovina, Croatia, Macedonia, Montenegro, Serbia, and Slovenia.