Having forced Bragg to evacuate Chattanooga on September 6 through a series of skillful maneuvers, Rosecrans ordered his army to advance along three different roads into northwestern Georgia in pursuit of the Army of Tennessee. With the Federals divided and unable to support each other due to the mountainous terrain, Bragg chose to concentrate on one Union corps, the XIV Corps, which had advanced through the same gap taken by the 33rd at Pigeon Mountain (and adjacent passes), and was moving toward Lafayette. An abortive assault launched by Cleyburne's and Hindman's divisions at Davis's Cross Roads allowed the Federals to escape to safety, so Bragg now turned north, where Rosecran's main force was rapidly concentrating at Lee and Gordon's Mill along Chickamauga Creek. Having been promised three extra divisions—including two led by the renowned Lieutenant General James Longstreet—Bragg decided to attack Rosecrans' army. Skirmishing commenced on September 18, with the main battle opening on September 19. This engagement would mark one of the very few major contests of the war in which the Confederate force would outnumber the Federals: 65,000 to 60,000.