The central Balkans were part of the Roman and Byzantine Empires before ethnic Serbs migrated to the territories of modern Kosovo in the 7th century. During the medieval period, Kosovo became the center of a Serbian Empire and saw the construction of many important Serb religious sites, including many architecturally significant Serbian Orthodox monasteries. The defeat of Serbian forces at the Battle of Kosovo in 1389 led to five centuries of Ottoman rule during which large numbers of Turks and Albanians moved to Kosovo. By the end of the 19th century, Albanians replaced Serbs as the dominant ethnic group in Kosovo. Serbia reacquired control over the region from the Ottoman Empire during the First Balkan War of 1912. After World War II, Kosovo's present-day boundaries were established when Kosovo became an autonomous province of Serbia in the Socialist Federal Republic of Yugoslavia (S.F.R.Y.). Despite legislative concessions, Albanian nationalism increased in the 1980s, which led to riots and calls for Kosovo's independence. The Serbs - many of whom viewed Kosovo as their cultural heartland - instituted a new constitution in 1989 revoking Kosovo's autonomous status. Kosovo's Albanian leaders responded in 1991 by organizing a referendum declaring Kosovo independent. Serbia undertook repressive measures against the Kosovar Albanians in the 1990s, provoking a Kosovar Albanian insurgency.