How did the history of imperialism contribute to the ethnic conflict in Rwanda, Burundi, and the democratic republic of the congo?
The British, actually, were a little more deliberate than the above answer suggests. They did pay attention to which tribes lay within the boundaries they selected for their colonies. Through a very successful and ingenious process they called indirect rule, they found minority tribes that had been largely oppressed by majority tribes in the past and they empowered them. They gave them education, jobs, positions in government, and most importantly, military training. The Belgians copied this model, as did the French to some degree. In exchange for the privileges, the minority tribes were loyal to the empire and maintained the steady flow of resources to the mother country.
When colonialism ended, the same tribal resentments existed, but now one side was armed and trained, while the other side had numbers and a grudge. It has been a recipe for disaster in post-colonial Africa.
The history of imperialism has helped cause conflict within the independent African countries that were created out of the colonies. The way that this tended to happen was that the imperial powers set up colonies without any regard to what ethnic groups they were sticking together. So they would take the territories of a bunch of different groups and slap them together into one colony.
When the colonies became independent, they generally kept the boundaries that the imperialists had set. When this happened, they ended up with multi-ethnic societies. In many cases, like that of Rwanda, the ethnic groups ended up in conflict with one another.