Excerpts from Books and Wikipedia
"The Battle of Kosovo was fought on St. Vitus's Day (June 28) 1385, at Kosovo Polje ('field of the Blackbirds') between the armies of the expanding Ottoman Turkish empire under sultan Murad I and a coalition of Christian armies from disparate Balkan states under Serbia's Prince Lazar. . . . The consequences were dire for the Serbs. Their kingdom was absorbed by the Ottomans, leading to four centuries of Turkish rule. The Serbs, however, preserved their Orthodox Christian culture. Kosovo became a cornerstone of Serb nationalism, and its anniversary was marked down the centuries. It is no accident that the assassination of Archduke Franz Ferdinand of Austria by Serb nationalist in 1914 (which began World War I), took place on June 28. Kosovo became the Serbs' spiritual heartland. The Kosovan conflict of the 1990s between Serbs and ethnic Albanians was fueled by the potent issues that Kosovo has come to represent." [1001 Days]