[60]
Garfield recommended that Rosecrans replace wing commanders Alexander McCook and Thomas Crittenden, as he believed they were ineffective, but Rosecrans ignored the suggestion.[61] With Rosecrans, Garfield devised the Tullahoma Campaign to pursue and trap Confederate General Braxton Bragg in Tullahoma. After initial Union success, Bragg retreated toward Chattanooga, where Rosecrans stalled and requested more troops and supplies.[62] Garfield argued for an immediate advance, in line with demands from Halleck and Lincoln.[62] After a council of war and lengthy deliberations, Rosecrans agreed to attack.[63]
At the ensuing Battle of Chickamauga on September 19 and 20, 1863, confusion among the wing commanders over Rosecrans's orders created a gap in the lines, resulting in a rout of the right flank. Rosecrans concluded that the battle was lost and fell back on Chattanooga to establish a defensive line.[64] Garfield, however, thought part of the army had held and, with Rosecrans's approval, headed across Missionary Ridge to survey the scene. Garfield's hunch was correct.[64] Consequently, his ride became legendary and Rosecrans's error reignited criticism about the latter's leadership.[64] While Rosecrans's army had avoided disaster, they were stranded in Chattanooga, surrounded by Bragg's army. 