Following his victories in the Sevens Days' Battles and Second Manassas, Robert E. Lee launched his first northern invasion. Maj. Gen. George McClellan and the Army of the Potomac pursued. Union soldiers discovered a copy of Lee's Special Orders No. 191 which outlined his plans in Maryland. McClellan moved west and on September 14, clashed with elements of the Army of Northern Virginia in the passes of South Mountain. After a day-long engagement, the Confederates withdrew toward Antietam Creek.