Slavery was a system of coerced labor, primarily agricultural labor in North America, in which slaves-and the children of slave women — were legally the chattle property of their masters for their entire lives. Save for a few Indians during the seventeenth century, in North America it was confined exclusively to Africans and their African-American descendants. Slaves' race and their life-long legal status as property is what primarily distinguished slaves from white European indentured servants, who were also coerced laborers for fixed terms and who formed the vast proportion of the agricultural labor supply in the tobacco-growing colonies of Virginia and Maryland for most of the seventeenth century. Only after 1680, when the supply of white indentured servants sharply declined, did labor-hungry planters in the Chesapeake region and the new colony of South Carolina begin to import African slaves on a large scale. By 1750 black slaves formed 44 percent of Virginia's population, 31 percent of Maryland's, and 61 percent of South Carolina's.

While slavery was always more dominant in the southern than northern colonies, at the time of the Declaration of Independence, when slaves formed about one-fifth of the total colonial population, slavery was legal and existed in every northern colony, although only in New York did the proportion of slaves exceed 10 percent of the population. After the Revolution, however, the history of slavery followed a different course in the North and South. All of the original northern states abolished slavery, although usually very gradually by post-nati laws that freed no slaves alive at the time of their passage but stipulated that slave children born after their passage were to be freed when they reached their twenties. This meant that well into the third and fourth decades of the nineteenth century a few slaves still lived in some northeastern states, but the course of slavery's extinction there was clear.

The eradication of slavery in western areas north of the Ohio River bequeathed to the United States by the Treaty of Paris in 1783 came much sooner. The Northwest Ordinance of 1787 prohibited slavery from the Northwest Territory that encompassed the later states of Ohio, Indiana, Illinois, Michigan, and Wisconsin. Even so, five years after Illinois's admission as a free state in 1818, there was a concerted effort, that came remarkably close to succeeding, to admend its state constitution in order to legalize slavery within the state.

South of the Ohio River, however, the story was dramatically different. Southern areas west of the Appalachian mountains inherited the same laws regarding slavery as the South Atlantic states of Virginia, North Carolina, and Georgia which ceded them to the United States in the 1780s. Thus Kentucky in 1792 and Tennessee in 1796 entered the Union as slave states, and slavery was also legal in the area from which the states of Mississippi and Alabama would eventually be formed. Slaveholders did not move in great numbers into those areas, however, until the invention of the cotton gin in 1793 and the end of the War of 1812. The cotton gin allowed the profitable growth of short-staple cotton which would become the antebellum South's pre-eminent market crop. That invention ignited a massive migration of slaveholders away from the Atlantic Coast into Up-Country South Carolina, central and western Georgia, Mississippi, which gained statehood in 1817, and Alabama which became a state in 1819. The result was a dramatic shift in the distribution of the slave population within the South. In 1790 the upper South states of Delaware, Maryland, Virginia, North Carolina, Kentucky, and Tennessee contained four-fifths of the South's slaves. By 1860 the seven lower South states contained three-fifths of a total slave population of 3,953,760.

By 1860, the size of the United States had increased considerably since the boundaries established in 1783. Thus the question arose as to whether slavery should be extended to new territorial acquisitions. At first, this question aroused little controversy, for most Americans agreed to perpetuate the laws regarding slavery of the nations from which the United States acquired new territories. Thus, because the Spanish had legalized slavery in Florida, slavery existed there after the United States acquired it from Spain in 1819, and in 1845 Florida entered the Union as a slave state. Similarly, because the Spanish and French allowed slavery in New Orleans, slavery was presumed to be legal there and in the huge, if still vaguely defined, Louisiana Territory which Americans acquired in the Louisiana Purchase of 1803. Thus Louisiana entered as a slave state in 1812, slaveholders moved confidently into what became Arkansas, and, more important, into Missouri, most of which was west of the free state of Illinois.

Sectional controversy over slavery extension, indeed, first empted when residents of Missouri applied to Congress for entry as a slave state in late 1818. Northerners tried to bar slavery from the new state, setting off a heated debate that was only settled by passage of the Missouri Compromise in 1820. This allowed Missouri's admission as a slave state in 1821, called for Maine's admission as a free state in 1820, and, most important, "forever prohibited" slavery from the remainder of the Louisiana Purchase territory north of the 36°30' line, the southern border of the new state of Missouri.

Controversy over slavery extension west of the Mississippi River broke out again in 1844 over annexation of the proslavery Republic of Texas. Southern slaveholders had begun moving into Texas when Mexico was still controlled by Spain and slavery was legal, but after Mexico's successful rebellion from Spain in 1821, Mexicans declared slavery abolished throughout the new nation. Thus slaveholders in Texas lived in defiance of Mexican law, an anomaly that helped provoke Texans' war of independence from Mexico in 1836. Immediately after this revolution, Texans applied to the United States for annexation, but abolitionist groups in the North, appalled by the possible extension of slave territory, agitated so intensively against it that annexation was shelved until President John Tyler reopened the issue. Because of Tyler's initiative and the Democratic party's embrace of immediate annexation, Texas became a central issue in the presidential campaign of 1844. After Democrats won that contest, they passed a joint-resolution in Congress, over nearly unanimous Whig opposition, that offered Texas immediate entry as a slave state. Texas formally accepted the offer in July 1845, and it was admitted as a slave state in December 1845.

Northern opposition to slavery expansion became even more intense when the outbreak of war with Mexico in the spring of 1846 raised the possibility of acquiring Mexican territory into which slavery might expand still further westward. Northerners opposed slavery extension for a combination of reasons — antipathy to the spread of an institution they considered morally intolerable; a racist insistence that western areas be preserved for the exclusive settlement of whites; and a growing determination that the South's political power in Washington not increase with the addition of still more slave states. But unquestionably the disregard of slaveholders in Texas and the older slave states for Mexico's antislavery laws, by demonstrating that Southerners, or at least Southern Democrats, would not be bound by the laws regarding slavery of the nation from which the United States acquired new lands, if those laws were antislavery, also intensified their concern. Thus in August 1846, twenty months before the United States actually acquired the Mexican Cession in March 1848, David Wilmot introduced his famous Proviso barring slavery from any land acquired from Mexico as a result of the war. Wilmot's Proviso set off four years of sectional wrangling over the Mexican Cession because even Southerners who believed that slavery could never be profitably established in the arid Southwest considered a federal ban on slavery to be an intolerable deprivation of white Southerners' equal rights in the nation. So heated did this quarrel over the Proviso and the Cession grow that it would not be resolved until passage of the Compromise of 1850.

Yet Northerners' antipathy to the possible spread of slavery into the Mexican Cession paled in terms of its ferocity and extent next to the outrage provoked by passage of the Kansas-Nebraska Act in 1854. This statute effectively repealed the Missouri Compromise's ban on slavery north of the 36 30' line in the remainder of the Louisiana Purchase territory, and it ignited a chain of political events that led to the formation of the Republican party in the North, its election of Abraham Lincoln as President in 1860, southern secession, and Civil War.