For nearly two and a half centuries preceding the Civil War and general emancipation, slavery critically defined the economy and government; black life and culture; and race, ethnic, class, and gender relations in America.

Slavery on the North American mainland evolved with the Atlantic Slave Trade (1502–1870) and European slave systems in the Western Hemisphere. Enduring “physical and psychic hardships” on the 40–69-day crossing known as the Middle Passage, countless Africans died; 10–12 million lived. Europeans also enslaved native Indians and exploited white servants, usually indentured 5–7 years to repay costs of transport and rations. Indians escaped and rebelled; white-borne malaria, measles, and other diseases killed millions of them. Yet, as wages improved in Europe, importations of servants declined. Colonists thus imported more and more blacks from West Africa, where Portugal built its first slave fort in 1482.

Africans’ enslavement braced western economies. Thought by whites to be heathen, plentiful, and strong, they were laborers on voyages of Spanish explorers. They supplied Spain's short-lived settlement near the Cape Fear River (1526) and permanent colony at St. Augustine, Florida (1565). Before 1600 about 75,000 Africans slaved in the fields and mines of Brazil, Hispaniola, Mexico, and Peru alone. British Caribbean colonies imported 2,339,000 Africans from 1630 to 1780, beside 650,000 in North America. Here, mostly as a result of rising births, slaves were 25% of inhabitants after 1740. Owner manumissions, escapes, and gradual emancipation in the North (1777–1846) increased the number of free blacks. Freedpeople were 8% of all blacks in 1790 and 11% by 1860. Slaves totaled 697,897 or 22% of the nation's population in 1790; 1,538,125 or 20% in 1820; 3,204,313 or 16% in 1850, when approximately 400,000 were urban dwellers; and 4,000,000 or 15% by 1860.

Slaving emerged along regional lines, creating color and class oppression by custom and law. Slaveholder and merchant elites accumulated wealth and controlled political power. A nonplantation economy developed in the North: New England, New York, New Jersey, and Pennsylvania, where farms and households had small slave holdings and close living conditions. Crews of bondmen tended iron forges, shipyards, and tanneries in cities (Philadelphia, New York, and Boston). Plantations spread in the South. Gang labor was the rule on the tobacco and wheat plantations of the Chesapeake (Maryland, Virginia, and North Carolina).