The Chickamauga campaign of the American Civil War was a series of battles fought in northwestern Georgia from August 21 to September 20, 1863, between the Union Army of the Cumberland and Confederate Army of Tennessee. The campaign started successfully for Union commander William S. Rosecrans, with the Union army occupying the vital city of Chattanooga and forcing the Confederates to retreat into northern Georgia. But a Confederate attack at the Battle of Chickamauga forced Rosecrans to retreat back into Chattanooga and allowed the Confederates to lay siege to the Union forces.