Croatia (Croatian Hrvatska), republic in southeastern Europe, on the Balkan Peninsula. Formerly a constituent republic of Yugoslavia, Croatia declared its independence in June 1991. Civil war broke out in July and for four years ethnic Serbs, backed by the federal Yugoslav People’s Army, battled with ethnic Croats for territory. By January 1998, when all areas had reverted to Croatian control, hundreds of thousands of ethnic Serbs had fled the country while corresponding numbers of ethnic Croats had immigrated from Serbia and Bosnia and Herzegovina. Croatia is bounded on the north by Slovenia and Hungary, on the east and south by Bosnia and Herzegovina (often referred to simply as Bosnia), and on the east by Serbia. In the south, a 20 km (12 miles) wide section of Bosnia extends to the sea, separating a narrow coastal strip of Croatia from the rest of the country. The southern tip of this coastal strip has a short border with Montenegro. The Adriatic Sea forms Croatia’s long western boundary.