The answer is - war.
Both the Belgian withdrawal of Rwanda and the fall of Yugoslavia led to war, wars in which was also performed genocide. In Rwanda it was between the Hutu and the Tutsi, where the Hutu majority government performed a genocide over the Tutsi ethnicity. In Yugoslavia there was war between Serbia and Slovenia, Serbia and Croatia with Bosnia and Herzegovina in the middle as a battle ground. There was genocide from Serbians over the Croatians, from the Croatians over the Serbians, and from the Serbians over the Muslims in Bosnia and Herzegovina.
