On Sunday, September 20, 1863 Braxton Bragg planned to drive Rosecrans from the Chickamauga battlefield by having General Leonidas Polk assault Rosecrans’ forces on the Confederate right flank. The Confederate assault was delayed until approximately nine-thirty in the morning.
The Union left retreated but held until noon. Then James Longstreet’s corps, just arrived from Virginia, struck the Union right, exploiting the gap in the Union lines accidentally created by Rosecrans’ orders of the previous night.
Longstreet’s assault ultimately forced the Union army to retreat into the safety of Chattanooga. Only Thomas’ Union corps prevented the Confederates from taking Chattanooga, earning for Thomas his famous nickname, the “Rock of Chickamauga.” Casualties for both armies were in the thousands, making Chickamauga—after Gettysburg-- the second most costly battle of the Civil War.