The American Civil War (April 12, 1861 – May 9, 1865, also known by other names) was a civil war in the United States fought between the Union states (—states remaining in the federal union— or "the North") and the Confederate states (—southern states that voted to secede— "the Confederacy" 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 (nearly 13%) were enslaved black people, almost all in the South.