The Battle of Chickamauga marked the end of a Union offensive in the American Civil War in Catoosa County and Walker County, Georgia from September 18, 1863 to September 20, 1863. Considered a Confederate victory (for halting the Union advance), it was a costly one. It claimed an estimated 34,624 casualties (16,170 for the Union; 18,454 for the Confederates).

The battle was fought between the Army of the Cumberland and the Army of Tennessee and was so-named for the Chickamauga Creek, which flows into the Tennessee River about 12 miles southwest of Chattanooga. The respective armies were led by Union Major General William Rosecrans and Confederate General Braxton Bragg.

After his successful Tullahoma Campaign, Rosecrans renewed the offensive, aiming to force the Confederates out of Chattanooga by threatening their supply lines to the south. The three army corps comprising Rosecrans's army split and advanced on Chattanooga by separate routes. In early September, Rosecrans consolidated his forces scattered in Tennessee and Georgia and forced Bragg's army out of Chattanooga, heading south. The Union troops followed it and brushed with it at Davis' Cross Roads.

Bragg was determined to reoccupy Chattanooga and decided to meet a part of Rosecrans's army, defeat them, and then move back into the city. Through disinformation provided by Confederate 'deserters', Bragg managed to convince Rosecrans that the Confederate army was in full retreat towards Dalton. In fact, on September 17, Bragg headed north, intending to meet and defeat the XXI Corps . As Bragg marched north on September 18, his cavalry and infantry fought with Union cavalry and mounted infantry armed with Spencer repeating rifles. Although Bragg had achieved some degree of surprise, he failed to strongly exploit it. Fighting began in earnest on the morning of September 19, and Bragg's men hammered but did not break the Union line.

The next day, Bragg was reinforced with Longstreet's Corps from the Army of Northern Virginia and continued his assault on the Union line on the left, and in late morning, Rosecrans was incorrectly informed that he had a gap in his line. In moving units to shore up the supposed gap, Rosecrans actually created one, and General James Longstreet's men promptly exploited it, driving one-third of the Union army, including Rosecrans himself, from the field. General George Thomas took over command and began consolidating forces on Horseshoe Ridge and Snodgrass Hill. Although the Rebels launched determined assaults on these forces, they held until after dark. Thomas then led these men from the field,leaving it to the Confederates. His defense earned him the nickname, "Rock of Chickamauga." The Union retired to Chattanooga while the Confederates occupied the surrounding heights.

Adapted from a United States National Park Service battle summary