In the post-World War Il period, which issue is most closely associated with the boundaries created by Europeans for newly independent African countries?

If you look at the map of Africa, many borders are straight lines. (compare it to the borders in Europe, for example). However, Africa was not an empty land, but full of cultures and ethnic groups, and those groups were split when the straight lines were drawn and some groups hostile to each other were put in the same country. This created ethnic tensions