In Rwanda the problem related to the colonial boundaries drawn by European powers at the Berlin Conference of 1884-85. These boundaries were drawn to delineate spheres of influence of colonial powers, with no reference to tribal, cultural or linguistic boundaries among the native populations. These arbitrary boundaries became the national boundaries of African nations in the post-colonial world, and have been a source of great friction. Many ethnic groups are divided by national boundaries, and many which are traditional enemies are within the same country. The colonial governments did little or nothing to help the locals to become politically educated in ways which could help them govern themselves in a post-colonial world, since the European powers didn't even imagine such a thing until after World War II. The rise of independance movements and the inability of the colonial powers to economically continue their occupations in the aftermath of the war inevitably led to independance. Upon independance from Belgium in 1962 the Hutu tribe seized control of the government and systematically oppressed the minority Tutsis ever since. Although the Hutus were forced into a power-sharing agreement by the Rwandan Patriotic Front in 1990, in 100 days in 1994 something like 80,000 Tutsis were killed.
In Bosnia the situation was more complex. Tito managed to keep the ethnic tensions in Yugoslavia between ethnic Albanians and the Croat-Serbian-Bosnian population under control, largely by the force of his own character. After his death the usual ethnic tensions of the Balkans drove the country apart. Albanians were a small ethnic minority in the country, but the other three groups are the same ethnically and in language. The difference is religious; Croats are mostly Roman Catholic, Serbians Orthodox and Bosnians Muslim. The split between the two ostensibly Christian groups dates back to the first schism in the church, the Bosnians were largely compelled to convert under the rule of the Turks. It wasn't so much the collapse of Communism per se which drove those groups apart, but the death of Tito and the rise of Milosevic and his cronies. The conflicts between these groups and other ethnic groups in the Balkans are complex, interrelated, and date back centuries (a thousand years in the case of the Albanians). Unfortunately for the Croats and Bosnians, most of the former Yugoslav arms production was centered in Serbia.
The fighting broke out in 1991 in Croatia and the next year in Bosnia. These groups believed the Yugoslav National Army, fourth largest in Europe, would protect them, but soon realized it was in the hands of Milosevic and the nationalist Serbs. The Bosnian and Croats created their own army, which later broke down into a unified force and a breakaway Croat army. Sarajevo soon looked like something from World War II, and the fighting spread across the entire country. Eventually there were at least four different major local armies involved. There followed massacres of Bosnians by Serbs, Bosnians by Croats, Croats by Serbs, Serbs by Croats and Bosnians, etc. The Bosnians were pretty much the worst handled, and the Serbs pretty much the worst offenders, but no one is innocent. The collapse of Communism was not the real problem, only the excuse used by Milosevic and company to launch the war.