The Western Theater of the American Civil War encompasses 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 Mississippi are part of the Eastern Theater.

Operations west of the Mississippi River take place in the Trans-Mississippi Theater.

The Western Theater serves as an avenue of military operations by Union armies directly into the agricultural heartland of the South via the major rivers of the region (the Mississippi, the Tennessee, and the Cumberland).

The Confederacy is forced to defend an enormous area with limited resources.

Most railroads run north to south, as opposed to east to west, making it difficult to send Confederate troops and supplies to troops further from the more heavily populated and industrialized areas of the eastern Confederacy.

Union operations begin with securing Kentucky in Union hands in September 1861.

Maj. Gen. Ulysses S. Grant's Army of the Tennessee has early successes in Kentucky and western Tennessee in 1861 and 1862, capturing the important strategic locations of forts Henry and Donelson

The Army of Tennessee and the Army of the Ohio defeat the Confederate Army of Mississippi, commanded by General Albert Sidney Johnston, at the Battle of Shiloh, driving it out of western Tennessee, subsequently marching into Mississippi and capturing Corinth

Grant's troops march towards and capture Vicksburg in 1862–63.

Meanwhile, the Army of the Ohio, later known as the Army of the Cumberland, experiences success, blocking a Confederate invasion of Kentucky and gaining control over large amounts of Tennessee through the Battle of Stones River and the 1863 Tullahoma Campaign while fighting against the Confederate Army of Tennessee, whose commander, Braxton Bragg, is often criticized for his alleged lack of military skill.

The Union army is briefly checked in its invasion of Georgia at the Battle of Chickamauga, and besieged at Chattanooga.

Grant, now commanding the newly created Military Division of the Mississippi, takes command, and received reinforcements from the Army of Tennessee, as well as from the eastern Army of the Potomac.

The siege of Chattanooga is lifted in November 1863

Following his elevation by Abraham Lincoln to General-in-Chief, Grant put Maj. Gen. William Tecumseh Sherman in charge of the combined armies.

Chattanooga serves as a launching pad for Sherman to capture the Confederate rail-hub of Atlanta and to march to the Atlantic, inflicting a major logistical and psychological blow to the Confederacy.

After reaching the ocean, Sherman invades the Carolinas.

Operations in the Western Theater conclude with the surrender of Southern forces to the Union armies in North Carolina and Florida in May 1865 following General Robert E. Lee's surrender to Grant at Appomattox Court House.

The Western Theater typically receives less attention than the Eastern Theater.

This has much to do with the greater proximity of action in the east to capitals and to major population centers.

However, some historians consider it the war's most important theater.

While the Eastern Theater essentially remains in stalemate until 1864, Union troops in the west, beginning in 1861, are able to steadily surround and drive back the Confederate troops, forcing them into eventual capitulation.

This is done through a steady series of Union victories in major battles, interrupted by only a single defeat, which takes place at Chickamauga.