Historical ethnic and religious feuds
The violent breakup of Yugoslavia was because of age old prejudices between the Croatian Roman Catholics, Serbian Orthodox Christians, Bosniak Muslims and the conflict between Kosovar Albanian Muslims and Serbian and Macedonian Orthodox Christians.
This is the ethnic and religious map of the Balkans:
(The box is the Balkans)
This, fueled by the resentment over WWII genocide of Serbs committed by the Croatian Ustaše collaborators with the Italians and Germans, fueled Serbia’s fears for the large Serb minority in Croatia and Bosnia of facing another genocide. Hatred of the Bosnian Muslims by the Serbs and Croats over Ottoman-era animosity and the Albanian minority demands in Kosovo and Macedonia also played a huge part in the bloodshed.
This cocktail of feuds ended up starting a violent set of wars that ultimately broke up a once stable state bonded by unity, pan-nationalism and brotherhood and replaced with warring ethno-states.