- The American Civil War (April 12, 1861 – May 26, 1865; also known by other names) was a civil war in the United States between the Union (“the North”) and the Confederacy (“the South”), which had been formed by states that had seceded from the Union. The central cause of the war was the dispute over whether slavery would be permitted to expand into the western territories, leading to more slave states, or be prevented from doing so, which many believed would place slavery on a course of ultimate extinction.