After the Tullahoma Campaign, Rosecrans renewed his offensive, aiming to force the Confederates out of Chattanooga. The three army corps comprising Rosecrans's army split and set out for Chattanooga by separate routes. In early September, Rosecrans consolidated his forces scattered in Tennessee and Georgia and forced Bragg's army out of Chattanooga, heading south. The Union troops followed it and brushed with it at Davis' Cross Roads.
Bragg was determined to reoccupy Chattanooga and decided to meet a part of Rosecrans's army, defeat them, and then move back into the city. On September 17, he headed north, intending to meet and defeat the XXI Corps. As Bragg marched north on September 18, his cavalry and infantry fought with Union cavalry and mounted infantry armed with Spencer repeating rifles. Fighting began in earnest on the morning of September 19, and Bragg's men hammered but did not break the Union line.
The next day, Bragg continued his assault on the Union line on the left, and in late morning, Rosecrans was informed that he had a gap in his line. In moving units to shore up the supposed gap, Rosecrans created one, and General James Longstreet's men promptly exploited it, driving one-third of the Union army, including Rosecrans himself, from the field. General George Thomas took over command and began consolidating forces on Horseshoe Ridge and Snodgrass Hill. Although the Rebels launched determined assaults on these forces, they held until after dark. Thomas then led these men from the field,leaving it to the Confederates. His defense earned him the nickname, "Rock of Chickamauga." The Union retired to Chattanooga while the Confederates occupied the surrounding heights.
Adapted from a United States National Park Service battle summary