The Civil War in the United States (April 12, 1861 — May 26, 1865; also known by other names) was a civil war that was fought between the Union ("North") and the Confederacy ("South"). The Confederation was formed by the seceding states. The central cause of the war was a dispute over whether to allow slavery to spread to the Western territories, which would lead to an increase in the number of slave-owning states, or not to allow it, which, according to general opinion, would put slavery on the path of final extinction.

The Civil War is one of the most widely studied and described episodes in the history of the United States. The war remains the subject of cultural and historiographical debates, such as the myth of the "Lost Cause" of the Confederacy. The Civil War in the United States was one of the first wars in which industrial warfare was used. Railways, telegraphs, steamships, armored warships and serial weapons were widely used during the war. In total, between 620,000 and 750,000 soldiers died as a result of the war, and there were an unspecified number of civilian casualties, which made the Civil War the deadliest military conflict in American history.