After Tito died, which kept the country together, the ethnic groups started fighting each other. Serbs against Kroat, Muslims and a mix between all of them. Ethnic cleansing was practiced during the civil war by all sides.
Yugoslavia broke up up into many countries because after WW2 Serbs, Croats, Bosnian Muslims, Albanians, and Slovenes, were pushed from their home countries that no longer existed and put into one country. Eventually fighting broke out and the nation split.
