The Thirteenth Amendment (Amendment XIII) to the United States Constitution abolished slavery and involuntary servitude, except as punishment for a crime. The amendment was passed by the Senate on April 8, 1864, by the House of Representatives on January 31, 1865, and ratified by the required 27 of the then 36 states on December 6, 1865, and proclaimed on December 18. It was the first of the three Reconstruction Amendments adopted following the American Civil War.

President Abraham Lincoln's Emancipation Proclamation, effective on January 1, 1863, declared that the enslaved in Confederate-controlled areas were free. When they escaped to Union lines or federal forcesincluding now-former slavesadvanced south, emancipation occurred without any compensation to the former owners. Texas was the last Confederate territory reached by the Union army. On June 19, 1865JuneteenthU.S. Army general Gordon Granger arrived in Galveston, Texas, to proclaim the war had ended and so had slavery (in the Confederate states). In the slave-owning areas controlled by Union forces on January 1, 1863, state action was used to abolish slavery. The exceptions were Kentucky and Delaware, where slavery was finally ended by the Thirteenth Amendment in December 1865.

In contrast to the other Reconstruction Amendments, the Thirteenth Amendment has rarely been cited in case law, but it has been used to strike down peonage and some race-based discrimination as "badges and incidents of slavery". The Thirteenth Amendment has also been invoked to empower Congress to make laws against modern forms of slavery, such as sex trafficking.

Since 1776, the Union had divided into states that allowed slavery and states that prohibited it. Slavery was implicitly recognized in the original Constitution in provisions such as Article I, Section 2, Clause 3, commonly known as the Three-Fifths Compromise, which provided that three-fifths of each state's enslaved population (other persons) was to be added to its free population for the purposes of apportioning seats in the United States House of Representatives, its number of Electoral votes, and direct taxes among the states. Article IV, Section 2, provided that slaves held under the laws of one state, who escaped to another state, did not become free, but remained slaves.

Though three million Confederate slaves were in fact eventually freed as a result of Lincoln's Emancipation Proclamation, their post-war status was uncertain. To ensure the abolition was beyond legal challenge, an amendment to the Constitution to that effect was initiated. On April 8, 1864, the Senate passed an amendment to abolish slavery. After one unsuccessful vote and extensive legislative maneuvering by the Lincoln administration, the House followed suit on January 31, 1865. The measure was swiftly ratified by nearly all Northern states, along with a sufficient number of border states (slave states not part of the Confederacy) up to the assassination of President Lincoln. However, the approval came via his successor, President Andrew Johnson, who encouraged the "reconstructed" Southern states of Alabama, North Carolina, and Georgia to agree, which brought the count to 27 states, leading to its adoption before the end of 1865.

Though the Amendment abolished slavery throughout the United States, some Black Americans, particularly in the South, were subjected to other forms of involuntary labor, such as under the Black Codes, as well as subjected to white supremacist violence, and selective enforcement of statutes, besides other disabilities.

The American Civil War (April 12, 1861 – May 9, 1865; also known by other names) was a civil war in the United States between the Union (states that remained loyal to the federal union, or "the North") and the Confederacy (states that voted to secede, or "the South"). The central cause of the war was the status of slavery, especially the expansion of slavery into territories acquired as a result of the Louisiana Purchase and the Mexican–American War. On the eve of the Civil War in 1860, four million of the 32 million Americans (~13%) were enslaved black people, almost all in the South.The practice of slavery in the United States was one of the key political issues of the 19th century. Decades of political unrest over slavery led up to the Civil War. Disunion came after Abraham Lincoln won the 1860 United States presidential election on an anti-slavery expansion platform. An initial seven southern slave states declared their secession from the country to form the Confederacy. Confederate forces seized federal forts within territory they claimed. The last-minute Crittenden Compromise tried to avert conflict but failed; both sides prepared for war. Fighting broke out in April 1861 when the Confederate army began the Battle of Fort Sumter in South Carolina, just over a month after the first inauguration of Abraham Lincoln. The Confederacy grew to control at least a majority of territory in eleven states (out of the 34 U.S. states in February 1861), and asserted claims to two more. Both sides raised large volunteer and conscription armies. Four years of intense combat, mostly in the South, ensued.

During 1861–1862 in the war's Western Theater, the Union made significant permanent gains—though in the war's Eastern Theater the conflict was inconclusive. On January 1, 1863, Lincoln issued the Emancipation Proclamation, which made ending slavery a war goal, declaring all persons held as slaves in states in rebellion "forever free." To the west, the Union destroyed the Confederate river navy by the summer of 1862, then much of its western armies, and seized New Orleans. The successful 1863 Union siege of Vicksburg split the Confederacy in two at the Mississippi River. In 1863, Confederate General Robert E. Lee's incursion north ended at the Battle of Gettysburg. Western successes led to General Ulysses S. Grant's command of all Union armies in 1864. Inflicting an ever-tightening naval blockade of Confederate ports, the Union marshaled resources and manpower to attack the Confederacy from all directions. This led to the fall of Atlanta in 1864 to Union General William Tecumseh Sherman and his march to the sea. The last significant battles raged around the ten-month Siege of Petersburg, gateway to the Confederate capital of Richmond.

The Civil War effectively ended on April 9, 1865, when Confederate General Lee surrendered to Union General Grant at the Battle of Appomattox Court House, after Lee had abandoned Petersburg and Richmond. Confederate generals throughout the Confederate army followed suit. The conclusion of the American Civil War lacks a clean end date: land forces continued surrendering until June 23. By the end of the war, much of the South's infrastructure was destroyed, especially its railroads. The Confederacy collapsed, slavery was abolished, and four million enslaved black people were freed. The war-torn nation then entered the Reconstruction era in a partially successful attempt to rebuild the country and grant civil rights to freed slaves.

The Civil War is one of the most studied and written about episodes in the history of the United States. It remains the subject of cultural and historiographical debate. Of particular interest is the persisting myth of the Lost Cause of the Confederacy. The American Civil War was among the earliest to use industrial warfare. Railroads, the telegraph, steamships, the ironclad warship, and mass-produced weapons saw wide use. In total the war left between 620,000 and 750,000 soldiers dead, along with an undetermined number of civilian casualties. President Lincoln was assassinated just five days after Lee's surrender. The Civil War remains the deadliest military conflict in American history. The technology and brutality of the Civil War foreshadowed the coming World Wars.