Why were Africans enslaved?
Channels of wealth flowing from the New World to the Old were based largely on the cultivation of crops like sugar, tobacco, and coffee. These crops figured importantly in European rituals of conspicuous consumption. Profits for a plantation owner depended on the number of labourers he controlled. The key to wealth for plantation owners was employing many labourers, while keeping costs low. Free European labourers demanded high wages. In the long run, the cost of enslaved labourers was lower.
Initially Europeans tried enslaving Native Americans. But Native peoples lacked genetic immunities to Old-World diseases like smallpox and malaria and suffered catastrophic mortality - over 90% of the native population of the Americas perished from Old-World diseases after contact. Africans, on the other hand, possessed the required immunities. Europeans discovered this by the early 1500s and the transatlantic slave trade developed rapidly to fill the demand.
In the notorious triangular trade, ships departed from Bristol, Liverpool and other ports in England carrying trade goods, such as beads, cloth and guns, to West Africa, where they exchanged their goods for enslaved Africans who were then transported to the Caribbean, South America, and American colonies to work on the plantations. The vessels returned home with sugar, tobacco and cotton, the produce of the enslaved workforce. Before 1820, more than 80% of the people arriving in the New World were enslaved Africans. It is estimated that 12 million enslaved Africans were transported to the Americas.
Further information is available in the history of the transatlantic slave trade section of the International Slavery Museum website.