A clerical error opened a gap in the Union line at exactly the wrong moment — and Longstreet's corps poured through it, routing a third of the Union army in minutes.
The Battle of Chickamauga (September 18–20, 1863) was the most significant Union defeat in the Western Theater and the second-bloodiest battle of the Civil War, with over 34,000 combined casualties in three days of brutal fighting in the north Georgia woods.
Union General Rosecrans had just pulled off a brilliant maneuver, forcing the Confederates out of Chattanooga without a major battle. But he then pursued too aggressively, spreading his army across 40 miles of mountain terrain — exactly the kind of vulnerability that Bragg, reinforced by Longstreet's corps from Virginia, was waiting to exploit.
On September 20, a staff officer misread the Union line and reported a gap that didn't exist. Rosecrans, trying to close it, ordered a division to shift — and accidentally created the very gap the report described, right as Longstreet was launching his assault with eight brigades.
Longstreet's attack was one of the most devastating single blows of the war. His corps smashed through the hole in the Union line and routed an entire third of the Union army, including Rosecrans himself, who fled the field toward Chattanooga. It looked like total Confederate victory.
One Union general didn't run: George Thomas rallied the remaining forces on Snodgrass Hill and held against relentless Confederate assaults for the rest of the day, allowing the shattered Union army to escape. His stand earned him the nickname 'The Rock of Chickamauga' — and arguably saved the Union's entire Western campaign.
The Confederate victory trapped Rosecrans's army in Chattanooga under siege. But Bragg failed to aggressively pursue or exploit his win, a decision that infuriated Confederate leadership. Within weeks, Grant arrived to take command, and the strategic picture reversed dramatically.