Despite its brutality and inhumanity, the slave system aroused little protest until the 18th century, when rationalist thinkers of the Enlightenment began to criticize it for its violation of the rights of man, and Quaker and other evangelical religious groups condemned it for its un-Christian (Christianity) qualities. By the late 18th century, moral disapproval of slavery was widespread, and antislavery reformers won a number of deceptively easy victories during this period. In Britain, Granville Sharp (Sharp, Granville) secured a legal decision in 1772 that West Indian planters could not hold slaves in Britain, since slavery was contrary to English law. In the United States, all of the states north of Maryland abolished slavery between 1777 and 1804. But antislavery sentiments had little effect on the centres of slavery themselves: the great plantations of the Deep South, the West Indies, and South America. Turning their attention to these areas, British and American abolitionists began working in the late 18th century to prohibit the importation of African slaves into the British colonies and the United States. Under the leadership of William Wilberforce (Wilberforce, William) and Thomas Clarkson (Clarkson, Thomas), these forces succeeded in getting the slave trade to the British colonies abolished in 1807. The United States prohibited the importation of slaves that same year, though widespread smuggling continued until about 1862.

The situation in the United States was more complex because slavery was a domestic rather than a colonial phenomenon, being the social and economic base of the plantations of 11 Southern states. Moreover, slavery had gained new vitality when an extremely profitable cotton-based agriculture developed in the South in the early 19th century. Reacting to abolitionist attacks that branded its “peculiar institution” as brutal and immoral, the South had intensified its system of slave control, particularly after the Nat Turner (Turner, Nat) revolt of 1831. By that time, American abolitionists realized the failure of gradualism and persuasion, and they subsequently turned to a more militant policy, demanding immediate abolition by law.

American abolitionism laboured under the handicap that it threatened the harmony of North and South in the Union, and it also ran counter to the U.S. Constitution, which left the question of slavery to the individual states. Consequently, the Northern public remained unwilling to adopt abolitionist policy and was distrustful of abolitionist extremism. But a number of factors combined to give the movement increased momentum. Chief among these was the question of permitting or outlawing slavery in new Western territories, with Northerners and Southerners taking increasingly adamant stands on opposite sides of that issue throughout the 1840s and '50s. There was also revulsion at the ruthlessness of slave hunters under the Fugitive Slave Law (Fugitive Slave Acts) (1850), and the far-reaching emotional response to Harriet Beecher Stowe (Stowe, Harriet Beecher)'s antislavery novel Uncle Tom's Cabin (1852) further strengthened the abolitionist cause.

Jolted by the raid (1859) of the abolitionist extremist John Brown (Brown, John) on Harpers Ferry, the South became convinced that its entire way of life, based on the cheap labour provided by slaves, was irretrievably threatened by the election to the presidency of Abraham Lincoln (Lincoln, Abraham) (November 1860), who was opposed to the spread of slavery into the Western territories. The ensuing secession of the Southern states led to the American Civil War (1861–65). The war, which began as a sectional power struggle to preserve the Union, in turn led Lincoln (who had never been an abolitionist) to emancipate the slaves in areas of the rebellion by the Emancipation Proclamation (1863) and led further to the freeing of all other slaves in the United States by the Thirteenth Amendment to the Constitution in 1865.