Disunion came after Abraham Lincoln won the 1860 United States presidential election on an anti-slavery expansion platform. 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. 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. 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. The American Civil War (Ap– May 9, 1865), also known by other names was a civil war fought between the United States ( the Union - states remaining in the federal union or "the North") and the Confederate states ( southern states that voted to secede - "the Confederacy" or "the South").