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As Grant was conducting operations against Vicksburg, the American Civil War in the West continued in Tennessee. In June, after pausing in Murfreesboro for nearly six months, Maj. Gen. William Rosecrans began moving against Gen. Braxton Braggs Army of Tennessee at Tullahoma, TN. Conducting a brilliant campaign of maneuver, Rosecrans was able to turn Bragg out of several defensive positions, forcing him to abandon Chattanooga and driving him from the state.

Reinforced by Lt. Gen. James Longstreets corps from the Army of Northern Virginia and a division from Mississippi, Bragg laid a trap for Rosecrans in the hills of northwestern Georgia. Advancing south, the Union general encountered Braggs army at Chickamauga on September 18, 1863. Fighting began in earnest the following day when Union Maj. Gen. George H. Thomas attacked Confederate troops on his front. For most of the day, fighting surged up and down the lines with each side attacking and counterattacking.

On the morning of the 20th, Bragg attempted to flank Thomas position at Kelly Field, with little success. In response to the failed attacks, he ordered a general assault on the Union lines. Around 11:00 AM, confusion led to a gap opening in the Union line as units were shifted to support Thomas. As Maj. Gen. Alexander McCook was attempting to plug the gap, Longstreets corps attacked, exploiting the hole and routing the right wing of Rosecrans army.

Retreating with his men, Rosecrans departed the field leaving Thomas in command. Too heavily engaged to withdrawal, Thomas consolidated his corps around Snodgrass Hill and Horseshoe Ridge. From these positions his troops beat off numerous Confederate assaults before falling back under the cover of darkness.

This heroic defense earned Thomas the moniker The Rock of Chickamauga. In the fighting, Rosecrans suffered 16,170 casualties, while Braggs army incurred 18,454.

Stunned by the defeat at Chickamauga, Rosecrans retreated all