After World War II, Bosnia, Serbia, Montenegro, Croatia, Slovenia, and Macedonia unified to form the Federal Republic of Yugoslavia, comprised of a number of ethnic groups, including: Serbs (Orthodox Christians), Croats (Catholics), Bosniaks (Muslims) and ethnic Albanians (Muslims).
There had long been a history of tensions in the Balkans between these groups. During World War II several armed forces committed abuses. For example, Croats collaborating with Nazi Germany killed several hundred thousand Serbs, Roma ("Gypsies"), and Jews.
Yugoslavia’s president, Josip Tito, governed with an iron hand and was able to keep ethnic tensions in check. But when he died in 1980, Yugoslavia spiraled into chaos, and some of the republics and ethnic groups expressed the desire for independence. Yugoslav republics began declaring their independence in 1990. During the Croatian war of independence (1991), the Serb-dominated Yugoslav army supported Serb separatists in Croatia and committed atrocities against Croatian civilians.
There was also a growing sense of nationalism among some of the republics’ leaders, which gained momentum in the mid-1980s after the rise of Serbian leader Slobodan Milosevic. Milosevic seized these nationalist feelings to engineer changes that strengthened Serbia's position in the Yugoslav constitution. He transformed the military so that it became 90 percent Serbian and extended his control over the country's financial, mass media, and security structures to support Serbian nationalists in Serbia, Croatia, and Bosnia. He and Serbian separatists in Croatia and Bosnia used their influence to foment ethnic tensions by convincing Serbian civilians across the former Yugoslavia that their Croatian, Bosniak, and Albanian neighbors would threaten their rights.