The slave trade had many effects on Africa. Most of them were, of course, negative, though we can argue that the slave trade was positive for some African states at least in the short term.
One negative about the slave trade was that it tended to increase the amount of war that occurred in West Africa. The reason for this is that European (and American) slave traders did not simply go out into the African countryside and kidnap their own slaves. Instead, they bought slaves from the coastal kingdoms. Those kingdoms generally got slaves to sell through war and through raiding against inland tribes. Because the slave traders wanted more slaves, the coastal kingdoms were encouraged to wage more wars and conduct more raids against their neighbors. In addition, those kingdoms were provided with things like guns in exchange for slaves. This helped those kingdoms have more of a capability of waging war.
