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 west of the Mississippi River see Trans-Mississippi Theater of the American Civil War.