George Henry Thomas (July 31, 1816 March 28, 1870) was an American general in the Union Army during the American Civil War and one of the principal commanders in the Western Theater.

Thomas served in the MexicanAmerican 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 FranklinNashville 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.

Thomas had a successful record in the Civil War, but he failed to achieve the historical acclaim of some of his contemporaries, such as Ulysses S. Grant and William T. Sherman. He developed a reputation as a slow, deliberate general who shunned self-promotion and who turned down advancements in position when he did not think they were justified. After the war, he did not write memoirs to advance his legacy.

The Battle of Nashville was a two-day battle in the Franklin-Nashville Campaign that represented the end of large-scale fighting west of the coastal states in the American Civil War. It was fought at Nashville, Tennessee, on December 15–16, 1864, between the Confederate Army of Tennessee under Lieutenant General John Bell Hood and the Union Army of the Cumberland (Dept. of the Cumberland) under Major General George H. Thomas. In one of the largest victories achieved by the Union Army during the war, Thomas attacked and routed Hood's army, largely destroying it as an effective fighting force.