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 U.S. and formed the Confederate States of America (the Confederacy). Led by Jefferson Davis, they fought against the U.S. federal government (the "Union"), which was supported by all the free states and the five border slave states.

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 U.S. administrations rejected secession, regarding it as rebellion.

Hostilities began on April 12, 1861, when Confederate forces attacked a U.S. 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 at 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 as 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 war, the deadliest in American history, caused 620,000 soldier deaths and an undetermined number of civilian casualties, ended slavery in the United States, restored the Union, and strengthened 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 continued into the 20th century.

Causes of the war

main: Timeline of events leading to the American Civil War The coexistence of a slave-owning South with an increasingly anti-slavery North made conflict likely. Lincoln did not propose federal laws against slavery where it already existed, but he had, in his 1858 House Divided Speech, expressed a desire to "arrest the further spread of it, and place it where the public mind shall rest in the belief that it is in the course of ultimate extinction". Much of the political battle in the 1850s focused on the expansion of slavery into the newly created territories. All of the organized territories were likely to become free-soil states, which increased the Southern movement toward secession. Both North and South assumed that if slavery could not expand it would wither and die.