After delaying for several weeks in Tullahoma, Rosecrans planned to flush Bragg out of Chattanooga by crossing the Tennessee River, heading south, and interdicting the Confederate supply lines from Georgia. He began operations on August 18 and used a two-week bombardment of Chattanooga as a diversion. The Confederate high command reinforced Bragg with a division from Mississippi as well as a corps previously of the Army of Northern Virginia commanded by James Longstreet. Rosecrans pursued Bragg into the rugged mountains of northwestern Georgia, only to find that a trap had been set. Bragg started the Battle of Chickamauga (September 19–20, 1863) when he launched a three-division assault against Rosecrans's army. A command misunderstanding allowed a major gap to appear in the Union line as reinforcements arrived, and Longstreet was able to drive his corps into that gap and send the Union Army into retreat. If not for the defensive stand by a portion of the line led by the Union XIV Corps, commanded by Major General George H. Thomas ("The Rock of Chickamauga"), the Union Army would have been completely routed. Rosecrans, devastated by his defeat, withdrew his army to Chattanooga, where Bragg besieged it, occupying the high ground dominating the city.