The Battle of Chickamauga was the second deadliest battle in the entire Civil War, and the most significant defeat for Union forces in the Western Theater. The Western Theater of the Civil War was the group of battles that occurred in Alabama, Georgia, Florida, Mississippi, North Carolina, Kentucky, South Carolina and Tennessee, as well as Louisiana east of the Mississippi River.

After his victories over the Confederates in middle Tennessee, Union General William Rosecrans continued through Tennessee in an attempt to drive the Confederate Army from Chattanooga in the southeastern part of the state. Controlling Chattanooga would open up much of Georgia, including Atlanta, to Union assaults and would result in the capture of essential Southern railroad lines. In converging on Chattanooga, Rosecrans divided his army into three divisions, each of which would approach Chattanooga from different directions. As Rosecrans' army threatened his rear, Confederate General Braxton Bragg temporarily withdrew from Chattanooga with his Army of the Tennessee to Lafayette, Georgia. Bragg planned to attack the Union divisions in isolation, before they could consolidate, but such plans never made it to fruition. By the 17th of September, two of Rosecrans' division had united and moved north toward Chickamauga Creek to unite with the third. Bragg planned to attack the Union force as they crossed the creek to drive them back to the mountains, away from Chattanooga.

Fighting began in earnest on the morning of September 19. Union forces under George Thomas engaged Confederate cavalry in what turned out to be a massive back and forth battle through a setting of thick forests that spanned the duration of the afternoon. By the end of the night, neither side had gained the advantage and Union forces built defensive works along Lafayette Road.

On September 20th, Confederate General Braxton Bragg divided his force into a right wing under the command of Leonidas Polk and into a left wing commanded by James Longstreet, who had brought reinforcements from Virginia. Bragg's attack strategy however was foiled by Polk, who began his assault two hours after schedule. In the meantime, Union lines were reinforced, and Confederate assaults were turned back. At around 11:00, Union General Rosecrans began shifting his troops to fill a perceived gap in the defensive line. In doing so, he created a real gap that was exploited by Longstreet and his men. Longstreet's division crushed the center and right wings of the Union army causing uthinkable casualties as well as a disorganized Union retreat toward Chattanooga. Despite Longstreet's success, and the subsequent retreat of the Union Army, Confederate forces were unable to follow-up or pursue the enemy. While Rosecrans was unable to proceed into Georgia, Union forces maintained Chattanooga, despite a Confederate siege and the ensuing Battle of Chattanooga. In all, nearly 34,000 casualties were recorded at Chickamauga, making in the second bloodiest battle in the Civil War.