US Secretary of State William Seward proclaims the adoption of the Thirteenth Amendment, prohibiting slavery throughout the USA.

The legal institution of human chattel slavery, comprising the enslavement primarily of Africans and African Americans, was prevalent in the United States of America from its founding in 1776 until 1865, predominantly in the South. Slavery was established throughout European colonization in the Americas. From 1526, during early colonial days, it was practiced in what became Britain's colonies, including the Thirteen Colonies that formed the United States. Under the law, an enslaved person was treated as property that could be bought, sold, or given away. Slavery lasted in about half of U.S. states until abolition. In the decades after the end of Reconstruction, many of slavery's economic and social functions were continued through segregation, sharecropping, and convict leasing.

By the time of the American Revolution (17751783), the status of enslaved people had been institutionalized as a racial caste associated with African ancestry. During and immediately following the Revolution, abolitionist laws were passed in most Northern states and a movement developed to abolish slavery. The role of slavery under the United States Constitution (1789) was the most contentious issue during its drafting. Although the creators of the Constitution never used the word "slavery", the final document, through the three-fifths clause, gave slave owners disproportionate political power by augmenting the congressional representation and the Electoral College votes of slaveholding states. All Northern states had abolished slavery in some way by 1805; sometimes, abolition was a gradual process, a few hundred people were enslaved in the Northern states as late as the 1840 Census. Some slaveowners, primarily in the Upper South, freed their slaves, and philanthropists and charitable groups bought and freed others. The Atlantic slave trade was outlawed by individual states beginning during the American Revolution. The import trade was banned by Congress in 1808, although smuggling was common thereafter. It has been estimated that about 30% of congressmen who were born before 1840 were, at some time in their lives, owners of slaves.The rapid expansion of the cotton industry in the Deep South after the invention of the cotton gin greatly increased demand for slave labor, and the Southern states continued as slave societies. The United States became ever more polarized over the issue of slavery, split into slave and free states. Driven by labor demands from new cotton plantations in the Deep South, the Upper South sold more than a million slaves who were taken to the Deep South. The total slave population in the South eventually reached four million. As the United States expanded, the Southern states attempted to extend slavery into the new western territories to allow proslavery forces to maintain their power in the country. The new territories acquired by the Louisiana Purchase and the Mexican Cession were the subject of major political crises and compromises. By 1850, the newly rich, cotton-growing South was threatening to secede from the Union, and tensions continued to rise. Bloody fighting broke out over slavery in the Kansas Territory. Slavery was defended in the South as a "positive good", and the largest religious denominations split over the slavery issue into regional organizations of the North and South.

When Abraham Lincoln won the 1860 election on a platform of halting the expansion of slavery, seven slave states seceded to form the Confederacy. Shortly afterward, on April 12, 1861, the Civil War began when Confederate forces attacked the U.S. Army's Fort Sumter in Charleston, South Carolina. Four additional slave states then joined the Confederacy after Lincoln, on April 15, called forth in response "the militia of the several States of the Union, to the aggregate number of seventy-five thousand, in order to suppress" the rebellion. During the war some jurisdictions abolished slavery and, due to Union measures such as the Confiscation Acts and the Emancipation Proclamation, the war effectively ended slavery in most places. After the Union victory, the Thirteenth Amendment to the United States Constitution was ratified on December 6, 1865, prohibiting "slavery [and] involuntary servitude, except as a punishment for crime."

William Henry Seward (May 16, 1801 – October 10, 1872) was an American politician who served as United States Secretary of State from 1861 to 1869, and earlier served as governor of New York and as a United States Senator. A determined opponent of the spread of slavery in the years leading up to the American Civil War, he was a prominent figure in the Republican Party in its formative years, and was praised for his work on behalf of the Union as Secretary of State during the Civil War.

Seward was born in 1801 in the village of Florida, in Orange County, New York, where his father was a farmer and owned slaves. He was educated as a lawyer and moved to the Central New York town of Auburn. Seward was elected to the New York State Senate in 1830 as an Anti-Mason. Four years later, he became the gubernatorial nominee of the Whig Party. Though he was not successful in that race, Seward was elected governor in 1838 and won a second two-year term in 1840. During this period, he signed several laws that advanced the rights of and opportunities for black residents, as well as guaranteeing fugitive slaves jury trials in the state. The legislation protected abolitionists, and he used his position to intervene in cases of freed black people who were enslaved in the South.

After many years of practicing law in Auburn, he was elected by the state legislature to the U.S. Senate in 1849. Seward's strong stances and provocative words against slavery brought him hatred in the South. He was re-elected to the Senate in 1855, and soon joined the nascent Republican Party, becoming one of its leading figures. As the 1860 presidential election approached, he was regarded as the leading candidate for the Republican nomination. Several factors, including attitudes to his vocal opposition to slavery, his support for immigrants and Catholics, and his association with editor and political boss Thurlow Weed, worked against him, and Abraham Lincoln secured the presidential nomination. Although devastated by his loss, he campaigned for Lincoln, who appointed him Secretary of State after winning the election.

Seward did his best to stop the southern states from seceding; once that failed, he devoted himself wholeheartedly to the Union cause. His firm stance against foreign intervention in the Civil War helped deter the United Kingdom and France from recognizing the independence of the Confederate States. He was one of the targets of the 1865 assassination plot that killed Lincoln and was seriously wounded by conspirator Lewis Powell. Seward remained in his post through the presidency of Andrew Johnson, during which he negotiated the Alaska Purchase in 1867 and supported Johnson during his impeachment. His contemporary Carl Schurz described Seward as "one of those spirits who sometimes will go ahead of public opinion instead of tamely following its footprints".