The Army of the Cumberland was a Union army active from 24 October 1862 to 1 August 1865 during the American Civil War. The army's first commander was William S. Rosecrans, and the army's first battle was at Stones River from 31 December 1862 to 2 January 1863, fighting against the Confederacy in Kentucky, Tennessee, Mississippi, and Alabama. On 19 October 1863, after the crippling defeat at Chickamauga, George H. Thomas replaced Rosecrans as the army's commander, and the army would win a decisive victory at Chattanooga before it was incorporated into William T. Sherman's army group. The army was split into two formations after the Atlanta Campaign of 1864, with half of the army accompanying the Army of the Ohio into Tennessee to fight against John Bell Hood's Army of Tennessee and the other half assisting Sherman during his March to the Sea. In August 1865, the army was mustered out of service following the war's end.