 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 . Garfield , however , thought that part of the army had held and , with Rosecrans 's approval , headed across Missionary Ridge to survey the scene . Garfield 's hunch was correct . His ride became legendary , while Rosecrans ' error reignited criticism about his leadership . While Rosecrans 's army had avoided disaster , they were stranded in Chattanooga , surrounded by Bragg 's army . Garfield sent a telegram to Secretary of War Edwin M. Stanton alerting Washington to the need for reinforcements to avoid annihilation , and Lincoln and Halleck delivered 20 @,@ 000 troops by rail within nine days . In the meantime , Grant was promoted to command of the western armies , and quickly replaced Rosecrans with George H. Thomas . Garfield was ordered to report to Washington , where he was promoted to major general , a commission he would resign before taking a seat in the House of Representatives . According to historian Jean Edward Smith , Grant and Garfield had a " guarded relationship " , since Grant promoted Thomas to command of the Army of the Cumberland , rather than Garfield , after Rosecrans was dismissed . 
