Because of the high demands of the transatlantic slave trade, African coastal nations warred against nations on the interior for the sake of capturing humans. Over time, this devastated much of Africa, weakening once-wealthy nations and plunging the entire continent into easily exploited chaos.
Prior to the transatlantic slave trade, slavery was an endemic part of most African cultures. People commonly sold themselves into slavery to pay off debt or support their families, or they were forcibly enslaved for crimes or in war. When Dutch, Portuguese and other European traders came to the West African coast, they quickly saw the value of able-bodied young men and women as slaves in the New World. The trade that developed between these merchants and the rich and powerful rulers and merchants of West African nations drained the interior nations of productive young men and fertile young women, the very people nations need to prosper and grow.
As time went on, the slave trade continued out of necessity. If the coastal nations did not provide slaves to their European partners, they understood that those partners would simply attack the coastal cities and take what they wanted.
Europeans and North African slave traders were regular customers to Africa's slave markets. However, their Middle Eastern customers primarily purchased women and children and not young men. White Europeans were also in demand by these traders, diluting the burden on Africa.