It's important to note that, throughout the existence of the Yugolslav state, there was resistance to it from the periphery. The reality is that, both the Kingdom of Yugoslavia pre-WW2, and the Socialist Federal Republic of Yugoslavia were dominated by Serbia. Although the 'Brotherhood and Unity' program of the later was designed to integrate the various ethnicity into the governing beauracracy, the reality was that Yugoslavia was a dictatorship under the iron rule of Tito, and once he died that iron control began to unravel slowly but surely. In this way, it was not much different than the resistance to the centralized monarchy of the original Kingdom. Throughout the history of both states, agitators for indepence, sometimes even resorting to terrorism, existed. There's a reason that the Third Reich found Croatia such fertile ground for cooperation, as they were granted freedom from the Serbs. Other aspects of this are based heavily on religious differences which the Yugoslav state in the post-WW2 period attempted to slowly stamp out. However, they were largely unsuccessful.