In the western theater of the Civil War, during the late summer and autumn of 1863, Union and Confederate forces were struggling over control of the key railroad center of Chattanooga, Tennessee. By mid-September, Union General William Rosecrans had pushed Braxton Bragg’s Army of Tennessee out of Chattanooga and gathered his army of some 60,000 at Chickamauga, Georgia, located 12 miles southwest of Chattanooga. Though Confederate morale in the region was at a low point, the imminent arrival of reinforcements led by James Longstreet helped shore up Bragg’s forces, and the general decided to go on the offensive.

After his subordinates failed to follow through with a series of initial attacks, the first of Longstreet’s troops arrived. With some 65,000 men at his disposal (either on the field or on the way), Bragg was assured that he would enjoy a numerical advantage over Rosecrans. On the early morning of September 19, the two armies met in the woods lining the banks of Chickamauga Creek.

On the first day of battle, Bragg’s men repeatedly attacked the Union left, anchored by a large Union corps led by George Thomas. With reinforcements sent in by Rosecrans, Thomas was able to hold his position for the most part, with heavy losses on both sides. That evening, Longstreet arrived with two more brigades. Bragg decided to split his army into two wings, with Longstreet in command of the left and Leonidas Polk leading the right.

Though Polk frustrated Bragg with his delays, Longstreet advanced around 11:30 am on September 20. In a stroke of luck for the Confederates, the advance occurred just at the point when Rosecrans was shifting his troops. As a result, the rebels were able to burst through a gap in the Federal lines and send the Union troops into a chaotic retreat north towards Chattanooga. Even as Bragg refused Longstreet’s call for reinforcements, Thomas organized the remaining Federals in a desperate Union stand, earning a lasting reputation as the “Rock of Chickamauga” for his efforts. A reserve division arrived in time to aid Thomas, and the last of Rosecrans’ troops were able to make an orderly retreat to Chattanooga that night.

Though Longstreet and his fellow general Nathan Bedford Forrest wanted to pursue the enemy the following morning, Bragg was preoccupied with the toll taken on his army by the battle at Chickamauga. Ten Confederate generals had been killed or wounded, including the fiery Texan John Bell Hood (whose leg was amputated), and overall Confederate casualties numbered close to 20,000. The Union suffered some 16,000 casualties, making the Battle of Chickamauga the costliest one in the war’s western theater.

Bragg’s inaction turned a tactical triumph for the South into a strategic defeat, as Union forces were allowed to get safely to Chattanooga. The Confederates subsequently put that city under siege, but in October General Ulysses S. Grant arrived with reinforcements and took over the Union command in the region. Promoted to brigadier general after his service at Chickamauga, Thomas received command of the Army of the Cumberland, succeeding Rosecrans. In November, Thomas helped Grant’s forces reverse the results of Chickamauga with a decisive victory over the Confederates in the Battle of Chattanooga.