My mother grew up in the region during the war and we visited Yugoslavia just as it was falling apart (around 1993), going to Bosnia Herzegovina and Croatia, and stopping briefly in Serbia. It was still communist when we were there, but it was becoming less totalitarian and "western"

The various countries/provinces in Yugoslavia were each part of different ethnic groups, which united under Tito to fight against the invading Nazis. Serbia was primarily Orthodox. CroatIa was more Catholic, influenced more from Europe. Bosnia sat between the two regions and was a blend of Christians, Muslims, and Jews (who fled there from the various pogroms in other parts of Europe - at that time the Ottoman empire welcomed Jews because they were well-educated and industrious - my mother's family included Sephardic Jews escaping the Inquisition in Spain).

The region has a long history, with wars between the ethnic groups causing many grudges, which were set aside when uniting against the Nazis, under Tito's communist banner.

Once Tito was out of the picture, these old grudges re-surfaced (I suspect from prodding from the West) and we had the civil war we saw in the late1990s.

The varied reasons for the country’s breakup ranged from the cultural and religious divisions between the ethnic groups making up the nation, to the memories of WWII atrocities committed by all sides, to centrifugal nationalist forces. However, a series of major political events served as the catalyst for exacerbating inherent tensions in the Yugoslav republic. Following the death of Tito in 1980, provisions of the 1974 constitution provided for the effective devolution of all real power away from the federal government to the republics and autonomous provinces in Serbia by establishing a collective presidency of the eight provincial representatives and a federal government with little control over economic, cultural, and political policy.

https://history.state.gov/milestones/1989-1992/breakup-yugoslavia