Secession, in United States history, term applied to the withdrawal of a state from the Union. The right of secession was usually regarded by seceding states as one of their sovereign powers, as the U.S. Constitution contained no prohibition in this respect nor conferred any power on the federal government to compel a state to remain in the Union against its wishes (see States’ Rights). The legality of the concept was, however, always disputed by the federal government.

The idea of secession appeared in New England about 15 years after the ratification of the Constitution, in connection with the opposition of the Federalist Party to the Louisiana Purchase. The most famous movement for secession was in opposition to the exclusion of slavery from the United States. Following the Compromise Measures of 1850, a group of extremists in South Carolina undertook to secure the cooperation of the slaveholding states in a movement toward secession, but at that time the plan failed. In 1860-61, however, after the election of Abraham Lincoln as U.S. president, 11 Southern states seceded from the Union and formed the Confederate States of America. The act of secession was accomplished in the individual states through a convention either called by the state legislature or, as in the case of Texas, self-assembled. The defeat of the South in the Civil War was considered to have decided the question of secession in favor of the federal government; this decision was confirmed by the U.S. Supreme Court in 1869.