The formal withdrawal of a state from the Union is known as secession. Secession is the act of breaking away from a political entity, such as a country or a federation. During the American Civil War, 11 southern states seceded from the United States and formed the Confederate States of America. They did so because they opposed the federal government's policies on slavery and tariffs. The secession of the southern states triggered the war between the North and the South, which lasted from 1861 to 1865.