September 20, 1863 was the last day of the Battle of Chickamauga, which was the last chance Braxton Bragg had to take out the Army of the Cumberland and recover Chattanooga, Knoxville, and some portion of middle Tennessee. The battle began 2 days earlier as Bragg sent some of his troops to seize Alexander's and Reed's bridges to cross over to the Chickamauga and flank the Yankees from the north. The day's delay brought by the Spencer repeating rifles of Wilder's Lightning Brigade (mounted infantry) meant that the battle instead was a slog of frontal attacks.
The only reason the Confederates won a major victory was a series of events on the final day. Rosecrans, a choleric commander nettled by difficulties getting his units to move, dressed down division commander Thomas J. Wood in front of his staff for his "damnable disobedience" in waiting for someone to show up to replace him before making a move. Later a staff officer, passing along the line, got the impression that there were no troops between Wood's division and J. J. Reynold's division further north (in reality, John N. Brannan's division was between them). James Garfield, chief of staff for Rosecrans, kept track of movements and no doubt knew better, but he was momentarily unavailable and a different staff officer wrote an order to Wood to "close up on and support" Reynolds.
Having no desire to be chewed out again for failing to obey orders promptly, Wood pulled his division out of the line to move to Reynolds's support. Within moments, by pure chance, James Longstreet's main column -- 8 brigades led by John Bell Hood, divided into 3 small divisions under Bushrod Johnson, Evander Law, and Joseph Kershaw. They broke through, and led to the collapse of the Union right. Although much additional hard fighting remained, the rout of the divisions of Jefferson C. Davis and Philip H. Sheridan ("Little Phil was rarely at his best when the odds were even") as well as other scattered forces enabled the Confederates to prevail in the end.
Bragg followed Rosecrans to Chattanooga and decided to besiege him there. This came fairly close to success, but in the end it failed and Bragg was left facing greatly superior forces, eventually getting routed.