In the advance of the Union forces to repel the invasion of Maryland by the Confederates, the Army of the Potomac commanded by 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, Stuart's cavalry retired to Catoctin Creek and then to the east foot of this Pass. Cox's Division moved to Middletown. Willcox's and Sturgis' Division bivouacked at the west base of the Catoctin, and Rodman's at Frederick. The First (Hooker's) Corps bivouacked on the South side of the Monocacy near the crossing of the National Road, the Sixth (Franklin's) Corps at Buckeystown and Couch's Division between that place and the Potomac. The Second (Sumner's) and Twelfth (Mansfield's) Corps and Sykes' Division of the Fifth Corps concentrated at Frederick. Informed at Frederick of the position of the Confederate Army and the intentions of the Confederate commander, General McClellan, on the evening of the 13th, gave orders to cross South Mountain on the 14th, the main body by this pass to attack D.H. Hill and Longstreet, the Sixth Corps and Couch's Division at Crampton's Pass, six miles south, to attack McLaws and relieve Harper's Ferry.