As Union forces advanced to repel the Confederate invasion of Maryland, the Army of the Potomac under Major General Geo. B. McClellan moved northward from Washington with its front extending from near the Baltimore and Ohio Railroad to the Potomac River. On September 12th, Cox Kanawha Division of the Ninth Corps occupied Frederick. On the 13th, Pleasonton's Union cavalry, moving from Frederick on the National Road, forced the passage of Catoctin Mountain, and Stuart's cavalry retired to Catoctin Creek and then to the east foot of this pass. Cox's Division moved to Middletown, while Willcox's and Sturgis' divisions bivouacked at the west base of the Catoctin and Rodman's at Frederick. The First Corps under Hooker bivouacked on the south side of the Monocacy near the crossing of the National Road, the Sixth Corps under Franklin at Buckeystown, and Couch's Division between that place and the Potomac. The Second Corps under Sumner, the Twelfth Corps under Mansfield, and Sykes' Division of the Fifth Corps concentrated at Frederick. Informed there of the position of the Confederate army and the intentions of its commander, McClellan on the evening of the 13th ordered a crossing of South Mountain on the 14th, with the main body moving by this pass to attack D.H. Hill and Longstreet, while the Sixth Corps and Couch's Division moved at Crampton's Pass, six miles south, to attack McLaws and relieve Harper's Ferry.