At Chickamauga, Palmer's regiment was detailed as headquarters guard for the Army of the Cumberland with many troopers doled out to the various corps as couriers and scouts. When Longstreet unexpectedly attacked the union right near Rosecrans' headquarters, Palmer gathered all the men of his regiment available and prepared to counterattack with a saber charge. The Union right flank dissolved, however, and after attempting to rally the panicked infantry, his regiment crossed the battlefield in good order under Confederate artillery fire to protect the Union artillery. During the army's retreat to Chattanooga, the 15th Pennsylvania provided escort for the army's supply train. Not easily impressed, Major General George H. Thomas (the "Rock of Chickamauga") recommended that Palmer receive a brigadier's star for his success at turning a highly demoralized group of men to an effective group of soldiers.