The American Civil War (1861–1865), also known as the War Between the States and several other names, was a civil war in the United States of America. Eleven Southern slave states declared their secession from the United States and formed the Confederate States of America (the Confederacy). Led by Jefferson Davis, they fought against the United States (the Union), which was supported by all the free states and the five border slave states. Union states were loosely referred to as "the North".

In the presidential election of 1860, the Republican Party, led by Abraham Lincoln, had campaigned against the expansion of slavery beyond the states in which it already existed. The Republican victory in that election resulted in seven Southern states declaring their secession from the Union even before Lincoln took office on March 4, 1861. Both the outgoing and incoming US administrations rejected the legality of secession, considering it rebellion.

Hostilities began on April 12, 1861, when Confederate forces attacked a US military installation at Fort Sumter in South Carolina. Lincoln responded by calling for a volunteer army from each state, leading to declarations of secession by four more Southern slave states. Both sides raised armies as the Union assumed control of the border states early in the war and established a naval blockade. In September 1862, Lincoln's Emancipation Proclamation made ending slavery in the South a war goal, and dissuaded the British from intervening. Confederate commander Robert E. Lee won battles in the east, but in 1863 his northward advance was turned back after the Battle of Gettysburg and, in the west, the Union gained control of the Mississippi River at the Battle of Vicksburg, thereby splitting the Confederacy. Long-term Union advantages in men and material were realized in 1864 when Ulysses S. Grant fought battles of attrition against Lee, while Union general William Sherman captured Atlanta, Georgia, and marched to the sea. Confederate resistance collapsed after Lee surrendered to Grant at Appomattox Court House on April 9, 1865.

The American Civil War was the deadliest war in American history, resulting in the deaths of 620,000 soldiers and an undetermined number of civilian casualties. Its legacy includes ending slavery in the United States, restoring the Union, and strengthening the role of the federal government. The social, political, economic and racial issues of the war decisively shaped the reconstruction era that lasted to 1877, and brought changes that helped make the country a united superpower.