In October 1862, Rosecrans became commander of the Army of the Cumberland. His first battle as army commander was at the Battle of Stones River from December 31, 1862 to January 2, 1863. The Union army won the battle. During the summer of 1863, Rosecrans then captured the city of Chattanooga, Tennessee. But he lost the Battle of Chickamauga in September, which forced him to retreat to Chattanooga. The Confederates surrounded the city and started a siege. Union president Abraham Lincoln and Major General U.S. Grant, who commanded all Union armies in the Western Theater, didn't think Rosecrans could defend the city. So Rosecrans was fired as commander of the Army of the Cumberland and sent to Missouri.