The Army of the Cumberland was one of the principal Union armies in the Western Theater during the American Civil War. It was originally known as the Army of the Ohio.

The western theater of the American Civil War encompassed major military operations in the states of Alabama, Georgia, Florida, Mississippi, North Carolina, Kentucky, South Carolina and Tennessee, as well as Louisiana east of the Mississippi River. Operations on the coasts of these states, except for Mobile Bay, are considered part of the Lower Seaboard Theater. Most other operations east of the Appalachian Mountains are part of the eastern theater. Operations west of the Mississippi River took place in the trans-Mississippi theater.