The Union had not anticipated meeting the Confederates at Chickamauga
It is a fact that the Union had not anticipated meeting the Confederates at Chickamauga. Rosecrans repeated his onslaught after his victorious Tullahoma Campaign, seeking to drive the Confederates out of Chattanooga. Rosecrans gathered his forces scattered over Tennessee and Georgia in early September and drove Bragg's army out of Chattanooga, going south. Union troops pursued it and collided with it at Davis' Crossroads. Bragg was determined to retake Chattanooga, so he resolved to meet with a portion of Rosecrans' force, defeat it, and then return to the city. On September 17, he set out for the north with the intention of attacking the isolated XXI Corps. On September 18, Bragg's cavalry and infantry clashed with Union cavalry and mounted infantry armed with Spencer repeating rifles. As the Confederates attempted to cross West Chickamauga Creek, the two armies clashed at Alexander's Bridge and Reed's Bridge.
On September 19, the fighting began in earnest. Bragg's soldiers launched a fierce assault but were unable to breach the Union barrier. Bragg repeated his assault the next day. Rosecrans was alerted late in the morning that he had a gap in his line. Rosecrans inadvertently created a real gap by transferring men to shore up the apparent gap, precisely in the line of Confederate Lt. Gen. James Longstreet's eight-brigade assault on a narrow front, whose corps had been separated from the Army of Northern Virginia. Longstreet's onslaught drove one-third of the Union forces, including Rosecrans, from the field, culminating in a rout.
