The Army of the Tennessee was a Union army in the Western Theater of the American Civil War, named for the Tennessee River. A 2005 study of the army states that it "was present at most of the great battles that became turning points of the war—Fort Donelson, Vicksburg, and Atlanta" and "won the decisive battles in the decisive theater of the war."

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.