The Atlantic slave trade between Africa and the Americas was caused by the enormous demand for labor in the plantations of the America and Africa's already extant slave markets. It took little time for the demand to outstrip the supply of slaves.
Plantations in the Americas were initially worked by indigenous American peoples who were virtually enslaved to the Spanish encomienda system. However, by Spanish law, these laborers could only be forced to work if they were entirely indigenous by birth, which encouraged a high rate of intermarriage. In addition to the Native American susceptibility to European disease, this soon made laborers scarce. Portuguese and Spanish traders looked to the African slave markets for a ready supply of able-bodied labor.