I don’t mean to pick on you, but I hear this all the time. Please name all the wars between Serbs and Croats prior to WWII.
I was told by a Hungarian-American that the recent ethnic violence in Yugoslavia was stirred up by Marxist dictators, which tends to support your position. However, Yugoslavia, like Czechoslovakia and Belgium, was an artificial country with no prior history of unification. All those countries were created by "great powers" in the wake of wars which destroyed the empires which once controlled them.