The American Civil War was a civil war in the United States fought between states supporting the federal union and southern states that voted to secede and form the Confederate States of America. The central cause of the war was the status of slavery, especially the expansion of slavery into newly acquired land after the Mexican–American War. On the eve of the Civil War in 1860, four million of the 32 million Americans were black slaves, mostly 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 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. After Confederate forces seized numerous federal forts within territory they claimed, the attempted Crittenden Compromise failed and 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.