Longstreet’s attack hit Wood’s and Brannan’s Divisions on their exposed flank and drove them from the immediate field of battle. On the other side of the gap the Confederates struck Brig. Gen. Jefferson C. Davis’ Division, which was marching up to take Wood’s place in the line, and Maj. Gen. Philip H. Sheridan’s Division in flank. In a very short time the entire Union right flank was in disorder and driven from the field. Wilder’s brigade on the extreme right made a valiant stand for a while, employing to good effect the heavy fire power of the Spencer repeating carbine with which it was armed. Nothing, however, seemed to daunt the onrush of the Confederates, and Wilder withdrew for fear of being cut off from escape. The routed divisions from the Union right withdrew northwestward through McFarland’s Gap to Rossville. Generals Rosecrans, Crittenden, and McCook were caught in the breakthrough and fled the field. General Thomas was now in command of the Union forces left there. The altered conditions of the battlefield now dictated a change in Confederate strategy. The original plan of enveloping the Union left changed to a sweep from the Union right to the left. A pause in the fighting enabled Thomas to form a new line quickly to his rear on Snodgrass Hill, almost at a right angle with the Union left. From this vantage point he met the onslaught of Longstreet’s troops with such stubborn and determined resistance on that Sunday afternoon that he earned the name “Rock of Chickamauga.” The Union line on Snodgrass Hill was composed of Brannan’s Division with fragments of Wood’s, Negley’s, and Van Cleve’s Divisions. Longstreet vigorously assaulted the line again and again and nearly succeeded in enveloping Brannan’s right. Confederate success seemed assured as Thomas’ troops were hard hit and were short of ammunition, but at this moment unexpected reinforcements reached General Thomas. [Illustration: _The Battle of Chickamauga. Scene from diorama in the Museum, Park Headquarters Building._] General Granger, without orders and following the sound of battle, had hastened to the aid of Thomas. He arrived at Snodgrass Hill at a very opportune moment and just in time to stop the Confederates from enveloping Brannan’s right. A fierce engagement took place as Brig. Gen. James B. Steedman’s Division of Granger’s Corps forced the southern troops from the crest of the hill.