The Northern Army of the Cumberland, some 58,000 strong, was moving toward Chattanooga under the command of General William Rosecrans in the early fall of 1863. General Braxton Bragg, the Southern commander, removed his forces from the city and marched southward. Rosecrans reasoned that the Confederates were headed for Atlanta, but he badly misunderstood the situation. Bragg’s forces received reinforcements and managed to spring a trap on their opponents in an encounter along the Chickamauga Creek, about 10 miles south of Chattanooga.
Neither Rosecrans nor Bragg demonstrated memorable military skills at Chickamauga, but a Confederate charge led by elements of the First Corps of the Army of Northern Virginia under Lt. General James Longstreet, attached the day before to Bragg`s command, broke through the Northern line. Distinguished service was provided by General George H. Thomas, whose forces held their part of the line until an orderly Union retreat could be organized. Thomas would become known at “the Rock of Chickamauga,” and Longstreet the "Bull of the Woods" for their respective roles in the battle.
Heavy Southern losses prevented Bragg from pressing his advantage as the Union soldiers headed north to Chattanooga. Union casualties numbered 16,000 and the Confederates, 18,000.
Following this disappointment, President Lincoln replaced Rosecrans with U.S. Grant, who would command the western armies. Grant assigned Thomas to hold Chattanooga.