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1530s, from Latin secessionem (nominative secessio) "a withdrawal, separation; political withdrawal, insurrection, schism," noun of action from past participle stem of secedere "secede," from se- "apart" (see secret) + cedere "to go" (see cede). Originally in a Roman historical context, "temporary migration of plebeians from the city to compel patricians to address their grievances;" modern use in reference to religious or political unions dates from 1650s.
The withdrawal from the United States of eleven southern states in 1860 and 1861. The seceding states formed a government, the Confederacy, in early 1861. Hostilities against the remaining United States, the Union, began in April 1861 (see Fort Sumter), and the Civil War followed.