Rosecrans launched the Chickamauga Campaign in late August, staging demonstrations near Chattanooga and upstream of the city along the Tennessee River. This convinced Bragg that Rosecrans was crossing the river to the north; however, Union forces were actually crossing to the south of the city. This forced Bragg to fall back into northern Georgia, abandoning the important railroad hub of Chattanooga on September 8. Over the course of the next several days, Bragg attempted to launch several attacks on isolated parts of the Union army but each attempt failed. On the evening of September 18, Bragg concentrated the army near Chickamauga Creek; he thought that only part of the Union army was nearby but Rosecrans had concentrated his army faster than Bragg had expected. During September 19 at Chickamauga, both sides fed in reinforcements as the day progressed. Longstreet arrived on the battlefield during the night of the 19 and 20; Bragg divided the army into two wings, with Polk commanding one division of his corps along with Hill's and Walker's corps on the right, and Longstreet commanding the other division of Polk's corps with Buckner's corps and his own corps (commanded by John B. Hood), on the left. Polk was ordered to attack at daylight on September 20, with Longstreet attacking immediately afterwards, but Polk didn't launch his attack until midmorning. The left wing failed to dislodge the Union army but Longstreet's wing attacked a gap in the Union army which routed the Union right flank. A portion of the Union army rallied on Horseshoe Ridge and held off multiple Confederate attacks until evening, when it followed the rest of Rosecrans' army into Chattanooga.