The European Slave Trade
There has always been slavery in Africa (slaves were often the by-products of intertribal warfare, and the Arabs and Shirazis who dominated the East African coast took slaves by the thousands), but it was only after Portuguese ships arrived off the African coast in the fifteenth century that slaving turned into an export industry. The Portuguese in West Africa, the Dutch in South Africa and other Europeans who came after them were initially searching for lucrative trade routes, but they soon saw how African slavery worked and were impressed with how slaves helped fuel agricultural production. They figured that slaves would be just the thing for their huge American sugar plantations. At the same time, African leaders realized they could extend their kingdoms by waging war, and get rich trading with Europeans, whose thirst for slaves (and gradual insistence that slaves be exchanged for guns) created a vicious circle of conflict.
Exact figures are impossible to establish, but from the end of the 15th century until around 1870, when the slave trade was abolished, up to 20 million Africans were enslaved. Perhaps half died en route to the Americas; millions of others perished in slaving raids.
The trans-Atlantic slave trade gave European powers a huge economic boost, while the loss of farmers and tradespeople, as well as the general chaos, made Africa an easy target for colonialism.