The American Civil War was a war between the North and the South in the United States. Eleven Southern "slave states"(where slavery was legal) declared themselves separate from the United States and formed the Confederate States of America, also known as the Confederacy. They fought against the "free states," (where slavery was prohibited) and "border slave states" (states bordering the free states, but where slavery was still legal). The war lasted four years, beginning in 1861 and ending in 1865. During that time, the casualties exceeded more than 600,000 people. This death toll is more than the American casualties in all its other wars combined, from the American Revolution to today.