September 19-20, 1863. About ten days after taking Chattanooga, Union, General Rosecrans, thinking erroneously he was pursuing a beaten enemy, sent his army south, into Georgia, where it clashed with Confederate General Braxton Bragg's army near Chickamauga Creek.

The result was the second bloodiest engagement of the war, after Antietam, and the biggest battle fought in Georgia; there were a total of 35,000 casualties, including 4,000 killed. On the second day of the battle, writes historian Geoffrey C. Ward, "Rosecrans made a near-fatal mistake, ordering his troops to close a gap in the Union line -- that wasn't there. In the process, he opened up a new one and [Confederate General James] Longstreet's troops stormed through it, routing two Union corps, and sending Rosecrans and most of his army staggering back to Chattanooga. Rosecrans, Lincoln said, was 'confused and stunned like a duck hit on the head.'

Union "[t]roops under George Henry Thomas, a Unionist from Virginia known to his men as 'Pap,' managed a stubborn, staged, last-minute withdrawal that kept Chickamauga from being worse for the Union than it was -- and earned Thomas a new nickname, 'the Rock of Chickamauga.'"

"The Confederates occupied the field at day's end, but Bragg refused to follow up on his advantage. His officers were livid. Longstreet formally demanded Bragg's removal." Later, every corps commander under Bragg's command told the Confederacy's president, Jefferson Davis, that Bragg should be replaced, but Davis, who disliked the two leading candidates to replace him -- [P.G.T.] Beauregard and Joseph E. Johnson -- kept Bragg in charge.

"While the battle [of Chickamauga] was considered a Confederate victory because it pushed the Union army back to Chattanooga rather than letting them proceed into Georgia (it would be the next year before the Union army tried again), Rosecrans achieved his [principal] objective for the campaign [as a whole], the capture of Chattanooga... [T]he staggering losses sustained in both field armies produced few immediate tangible results."

Battle of Chickamauga, Library of Congress