Thomas served in the Mexican–American War and later chose to remain with the U.S. Army for the Civil War as a Southern Unionist, despite his heritage as a Virginian (whose home state would join the Confederate States of America). He won one of the first Union victories in the war, at Mill Springs in Kentucky, and served in important subordinate commands at Perryville and Stones River. His stout defense at the Battle of Chickamauga in 1863 saved the Union Army from being completely routed, earning him his most famous nickname, "the Rock of Chickamauga." He followed soon after with a dramatic breakthrough on Missionary Ridge in the Battle of Chattanooga. In the Franklin–Nashville Campaign of 1864, he achieved one of the most decisive victories of the war, destroying the army of Confederate General John Bell Hood, his former student at West Point, at the Battle of Nashville.