Yugoslavia was never a homogeneous nation, but rather a forced federation of several distinct ethnic groups. Their cultural differences and religious expressions were stifled under the strict socialist system and the policy of bratstvo-jedinstvo , brotherhood and unity, brutally imposed by Yugoslavia's former ruler, Josip Broz Tito. For more than 30 years, people of all ethnicities worked side by side, socialized, intermarried, and lived in relative peace. But the ethnic lines never blurred entirely. History and religion have much to do with today's strife in the Balkans.
People remember bitter, sometimes centuries-old grievances against each other's ethnic groups.
Serbs, who practice the Eastern Orthodox Christian religion, harbor deep resentment toward Muslims because Ottoman Turks oppressed the Serbian people in the 14th century. They also feel threatened by Roman Catholicism, of which Croats are devout followers, since Catholic Austrians pressed the Serbs to convert in the late 1800s. Both Croats and Serbs regard aggression toward Muslims as Christendom's revenge for past Turkish oppression, even though Bosnian Muslims are not Turks, but descendants of Croats and Serbs who converted to Islam during Turkish rule. World War II freshened and deepened the hostility. The Croatian fascist Ustasa regime became a puppet government for the Nazi party, and Croats, along with Muslims, massacred tens of thousands of Serbs.