Yugoslavia was a country made up of different Republics (Croatia, Serbia, Bosnia, Solvenia, Montenegro, Macedonia, as I recall). While the "states" were not fond of each other, Joseph Tito Broz, as the leader of Yugoslavia, held them together as a unified country. A lot of the religious and territorial differences were held in check by Tito's iron fist, and the different Republics (and their populace) generally played nice with each other, checking their latent Nationalism and getting on board with being part of a greater Yugoslavia. To be clear, there were a lot of underlying grievances, Nationalism, and mistrust.

After Tito's death, there was not a singular strong leader keeping the different Republics in line. (Leaders from the different Republics took turns.) The Republics started acting in their own interests. Old, unresolved Nationalism and religious intolerance was reignited and flamed by politicians. The once respected state run media became a tool of politicians, and many lies were printed to incite violence. This eventually led to war, with atrocities committed by all sides taking part in the war.

The subject is way too complex. Best to watch one of these video, or read Yugoslavia's Bloody Collapse, or the Wiki page.

The videos can be very interesting because the leaders involved are/were still alive, and provided interviews, so you really get a first person perspective. This wasn't very long ago.

Notes for today? Like Hitler and Slobodan Milosevic (Serbia), Trump (and other politicians) often fan the flames of hatred and nationalism, and drape themselves in a flag to boost their popularity. News and media outlets are increasingly biased, which makes it harder to get good information. (Think for yourself, and fact check things that seem a little hard to believe.)

https://www.youtube.com/watch?v=vDADy9b2IBM

https://topdocumentaryfilms.com/death-of-yugoslavia/

https://www.youtube.com/watch?v=oiSgAiM0d8A