Battle of Chickamauga, one of the major engagements of the American Civil War (1861-1865), in which the Union's quick regrouping after the Confederate victory ultimately led to the Union victory at Chattanooga. The battle was fought on September 19-20, 1863, near Chickamauga Creek, in northern Georgia, about 20 km (about 12 mi) south of Chattanooga, Tennessee. The battle was fought between the Army of the Cumberland, numbering about 55,000 men, commanded by the Union General William Starke Rosecrans, and a Confederate army, about 70,000 strong, commanded by General Braxton Bragg. At stake was the possession of Chattanooga, which Rosecrans had occupied on September 9, following Bragg's withdrawal from the city. Rosecrans met the Confederates on the upper Chickamauga at about 9 am on September 19. The battle began with a heavy Confederate assault on the Union left, held by troops under General George Henry Thomas. By sunset, Rosecrans had engaged his entire army in the action, while Bragg held in reserve three divisions, later reinforced by several brigades commanded by General James Longstreet.

The battle was resumed, with a Confederate attack, early on September 20. Confederate forces under Longstreet broke through the Union line, rolling back its right flank. This section of the Union line retreated to Chattanooga. Only Thomas, later known as the “Rock of Chickamauga,” was able to withstand repeated attacks on the Union left, thus providing cover for the Union retreat. The Union army fell back to Chattanooga on the following day. This battle was a great success for the Confederates, but Bragg did not consider it a victory and did not pursue the demoralized Union troops and make it decisive. His error led ultimately to the Confederate defeat at Chattanooga and his resignation. Union casualties at Chickamauga were 16,179 killed, wounded, and missing, and Confederate casualties were about 18,000. The battle site was established as the Chickamauga and Chattanooga National Military Park in 1890. The park is administered by the National Park Service. See Chattanooga, Battle of.