one regimental commander. Though Pickett's division was only one element of the attack the whole has usually been named Pickett's Charge. The army fell back to Virginia, reaching it after a costly retreat 10 days later. Chickamauga Note: see Chickamauga order of battle for the command structure of the Army of Tennessee at this time. On September 9, the First Corps was transferred to the Department of Tennessee, except for Pickett's division and the brigade of "Tige" Anderson. The Tennessee rail hub at Chattanooga was the primary goal of both armies in the West, these being the Army of the Cumberland under Union Maj. Gen. Rosecrans and numbering nearly 60,000 men, and the Army of Tennessee led by Confederate Gen. Braxton Bragg and the First Corps along with additional reinforcements would constitute over 65,000 troops. To get to and reinforce Bragg's army, the First Corps would use 16 railroads on a nearly 800-mile (1,350 km) route through North and South Carolina to reach the Army of Tennessee, stationed in northern Georgia. This round-about route was necessary due to the different gauges of the surviving Southern rail system between the forces, and would take three weeks to complete. The lead elements of Longstreet's corps, three brigades of Maj. Gen. John Bell Hood's division, were grouped with a western division under Brig. Gen. Bushrod Johnson and opened the battle on September 18 by crossing Chickamauga Creek at Reed's Bridge. Since Longstreet had not yet arrived, Hood assumed command of the corps while Brig. Gen. Evander Law assumed command of Hood's division. The First Corps would fight in the same formation the next day on the Confederate left. On September 20, Longstreet arrived along with two brigades from McLaws's division. Since McLaws had not yet arrived, the brigades were led by Brig. Gen. Joseph B. Kershaw. Bragg used the arrival of Longstreet to reorganize his army into wings, with Longstreet commanding the left wing consisting of his own corps under Hood, Buckner's corps, and Hindman's division while Leonidas Polk assumed command of the right. The troops of the First Corps played an important role in the breakthrough Longstreet achieved on the 20th. This fight is considered the most significant Union defeat in the Western Theater, with total casualties on both sides exceeding 34,000. Chattanooga Knoxville Campaign Note: see for the command structure of Longstreet's Corps at this time. November 4 - Longstreet leaves