American Civil War was a military conflict between the United States of America (the Union) and 11 Southern states, the Confederate States of America (the Confederacy), who had declared their secession from the U.S. It lasted from 1861 to 1865, and the primary cause of the war was slavery and its expansion into newly acquired territories after the Mexican War. The civil war ended slavery in the U.S. and decisively changed the social, racial, political and economic landscape of the U.S.