The trans-Mississippi theater of the American Civil War was the scene of the major military operations west of the Mississippi River. The area is often thought of as excluding the states and territories bordering the Pacific Ocean, which formed the Pacific coast theater of the American Civil War (1861–1865).

During the American Civil War, Missouri was a hotly contested border state populated by both Union and Confederate sympathizers. It sent armies, generals, and supplies to both sides, maintained dual governments, and endured a bloody neighbor-against-neighbor intrastate war within the larger national war.