From Wikipedia

The Civil War
Episode Seven

Most Hallowed Ground

The Western Theater was an area defined by both geography and the sequence of campaigning. It originally represented the area east of the Mississippi River and west of the Appalachian Mountains. It excluded operations against the Gulf Coast and the Eastern Seaboard, but as the war progressed and William Tecumseh Sherman's Union armies moved southeast from Chattanooga, Tennessee, in 1864 and 1865, the definition of the theater expanded to encompass their operations in Georgia and the Carolinas. For operations in the Southwest see Trans-Mississippi Theater of the American Civil War.

The West was by some measures the most important theater of the war. The Confederacy was forced to defend with limited resources an enormous land mass, which was subject to Union thrusts along multiple avenues of approach, including major rivers that led directly to the agricultural heartland of the South. Capture of the Mississippi River was one of the key tenets of Union General Winfield Scott's Anaconda Plan.