In addition, the British used a system called indirect rule, whereby they gave authority, training and weapons to the long oppressed minority tribes, then allowed them to run the country in exchange for the resources and loyalty.
Later, when African colonies declared independence from the empires (17 in 1961 alone), the borders remained, as did the guns, and the tribal disagreements and grudges. Many of the civil wars we see now are the result of these colonial legacies.
Colonization contributed to civil wars in a couple of major ways.
First, colonization caused relatively artificial lines to be drawn -- countries' borders were drawn based on where the colonizers wanted them to be. What often ensued was that various ethnic groups were lumped together in one country even though they did not want to be. In addition, the same ethnic group could be split between a number of countries. Both of these things made for conflict.
Second, the colonizers often favored one ethnic group over the others. That led to hatred or jealousy along ethnic lines.
These factors caused civil wars to be much more likely in newly-independent African countries.
In answering a textbook question, I would always suggest that checking through your text would be of vital importance. Any answer you receive here might be profoundly different than what is in this source and if it is a part of your classroom setting, you might be assessed against what it says over what others say. Having said this, I would argue that Colonialism's impact on emerging nations in Africa was that it created an external enemy which was able to create a lid or a type of check on competing ethnic and racial rivalries. This was not necessarily a good thing, thought it has been argued to do so. In creating this cap on tensions, an artificial element of repression was introduced into the atmosphere and the result of this was that when the Colonial nations left, little was done to transition the newly formed nations into the ways of pluralist social orders. The result was extreme tension and proliferation of anger towards one another in an unchecked and unrestrained manner.