In September 1863, Confederate General Braxton Bragg, commander of the Army of Tennessee, set out to destroy the army of Union General William S. Rosecrans near Chattanooga, the vital railroad center. The armies clashed at Chickamauga on September 19-20. Fewer men were engaged than at "Bloody Shiloh" a year before, but even more were lost at Chickamauga, in the costliest battle of the West.

Led by General Leonidas Polk, the five divisions of Polk's Corps mounted savage assaults through heavily wooded terrain, marching under their distinctive St. George's Cross colors. On the first day at Chickamauga, Polk's most aggressive general, Patrick R. Cleburne, smashed through Union defenses, then had to fall back when Federal reinforcements counterattacked. Through the night, Union troops strengthened breastwork defenses preparing for an all-out Confederate: attack to come the next morning. After dawn, Polk's battle-hardened men surged forward again, division after division, as the Union fortifications exploded in flames. When word arrived of a breakthrough by a brigade led by Polk's nephew, Cleburne roused his troops to exploit the gain: "Forward, boys, and give them hell!" As Polk's storming forces pushed nearer to breaking Union lines on the right, General George Thomas asked for reinforcements from the center of the Federal stand to stave off the relentless pressure. As a Union division went to Thomas' aid, Confederate General James Longstreet's divisions began to pour through the hole opened in the center, and routed over a third of the Union defenders into full retreat.
Both Polk's and Longstreet's corps combined in a two-pronged assault on the position gallantly held by Thomas ("the Rock of “Chickamauga"). After furious fighting, Thomas finally pulled out his troops, and the two Confederate corps linked up amid wild cheers of triumph which continued into the night. It was "a tremendous swell of heroic harmony," as described by Longstreet. Chickamauga would remain the greatest Confederate victory in the West, although each side lost one-third of its army in casualties.

Leonidas Polk was the roommate of Jefferson Davis at West Point. Choosing the ministry over a military career, he had become the Episcopal Bishop of Louisiana by the time the Civil War began. But when Polk visited President Jefferson Davis in Richmond n June 1861, he was summarily commissioned a major general with a command in the Mississippi Valley. Polk, mown as "the fighting bishop," did not give up his bishopric, saying he was "like a man who dropped his business when his house was on fire." On occasion he would wear his vestments over his uniform, as he did when he performed the marriage ceremony for one of his officers before the Battle of Murfreesboro. His corps' St. George's Cross flag stood out in battle from the St. Andrew's Cross on the standard Confederate flag.
From his headquarters in Memphis, Polk marched his forces to Columbus, Kentucky, to establish a Confederate presence in the neutral state. When General Ulysses S. Grant attacked his troops across the river at Belmont, Polk skillfully repulsed Grant and forced a Union retreat is Brigadier General Gideon Pillow helped surround Grant's troops. After Polk's victory at Belmont, confederate General Braxton Bragg assigned 10,000 troops to Polk's Corps. Their powerful assaults went on to push back General William Tecumseh Sherman's defenses at Shiloh. Polk's Corps again fought magnificently at Perryville, when Confederate forces drove off Union troops three times their number. In the course of the fighting, Polk himself captured a Union brigadier general. Later in the skirmish, he bluffed his way out of Federal lines when his dark-gray uniform was mistaken for blue in the dusk.

Polk's Corps again plunged into the thickest fighting at Murfreesboro. In an early breakthrough, Polk's men penetrated three miles beyond the Union line, capturing 3,000 prisoners. Their determined fighting spirit was later a key at Chickamauga, the greatest victory of the Army of Tennessee. Among Polk's favorite commanders was the Irish-born General Patrick R. Cleburne, "the Stonewall Jackson of the West." When Cleburne urged his troops to give the Yankees hell, Polk concurred:
"Yes, boys, do what General Cleburne says!"
In the spring of 1864, Polk resisted Sherman's advances into Alabama and Georgia, until he was killed by a Federal artillery shell at Pine Mountain. The Confederacy lost "a guiding spirit" and one of its most effective corps commanders.