William Rosecrans followed up his successful Tullahoma Campaign with an offensive aimed at forcing the Confederates out of Chattanooga. The three army corps of the Army of the Cumberland split and set out for Chattanooga by separate routes. Clever deception convinced elements of Braxton Bragg's Army of Tennessee to abandon Chattanooga and, in early September, Rosecrans began to consolidate his scattered troops and head south in pursuit of Bragg's army. Determined to reoccupy Chattanooga, Bragg marched his army north on September 17. After skirmishing with Union cavalry and John T. Wilder's Lightning Brigade—mounted infantry armed with Spencer repeating rifles—Bragg opened the battle in earnest on September 19 by hammering the Union left. Though the Federal line held, Bragg renewed his assault the following day. In the rush to send troops to his embattled left, Rosecrans mistakenly believed he had a gap in his line. The Union commander’s efforts to correct the imagined error created real breach, one that was exploited with deadly consequences by James Longstreet’s newly arrived corps. Longstreet's attack created paralyzing confusion in the Union ranks, including Rosecrans himself, who was driven from the field with a third of his force. George H. Thomas took over command and began consolidating forces on Horseshoe Ridge and Snodgrass Hill where the Federals repeatedly fought off the Rebels' determined assaults. Darkness soon ended the contest and Thomas then led the remnant of the Army of the Cumberland men from the field. The Union retired to Chattanooga while the Rebels occupied the surrounding heights.