In United States' history, a slave state was a U.S. state in which slavery was legal at a particular point of time, and a free state was one in which slavery was prohibited or being phased out at that point of time. Slavery was a divisive issue and was one of the causes of the American Civil War. The Thirteenth Amendment to the United States Constitution, ratified in 1865, abolished slavery throughout the United States, and the distinction ended.