The American Civil War (1861–1865), in the United States often referred to as simply the Civil War and sometimes called the "War Between the States", was a civil war fought over the secession of the Confederate States. Eleven southern slave states declared their secession from the United States and formed the Confederate States of America ("the Confederacy"); the other 25 states supported the federal government ("the Union"). After four years of warfare, mostly within the Southern states, the Confederacy surrendered and slavery was abolished everywhere in the nation. Issues that led to war were partially resolved in the Reconstruction Era that followed, though others remained unresolved.