William Swinton, Campaigns of the Army of the Potomac

The uncertainty at first overhanging Lee's intentions caused the advance from Washington to be made with much circumspection; and it might, perhaps, be fairly chargeable with tardiness, were there not on record repeated dispatches of the time from the general-in-chief, charging McClellan with too great a precipitancy of movement for the safety of the capital. The van of the army entered Frederick on the 12th of September, after a brisk skirmish at the outskirts of the town with the Confederate troopers left behind as a rearguard. It was found that the main body of Lee's army had passed out of Frederick two days before, heading westward towards Harper's Ferry.

It is now necessary, for a just appreciation of the events of the Maryland campaign, that I should give an outline of the plan of operations which the Confederate commander had marked out for himself. This plan was simple, but highly meritorious. Lee did not propose to make any direct movement against Washington or Baltimore with the Union army between him and these points, but aimed so to manoeuvre as to cause McClellan to uncover them. With this view, he designed, first of all, to move into Western Maryland and establish his communications with Richmond through the Shenandoah Valley. Then, by a northward movement, menacing Pennsylvania by the Cumberland Valley, he hoped to draw the Union army so far towards the Susquehanna as to afford him either an opportunity of seizing Baltimore or Washington, or of dealing a damaging blow at the army far from its base of supplies. His first movement from Frederick was, therefore, towards the western side of that mountain range which, named the Blue Ridge south of the Potomac, and the South Mountain range north of the Potomac, forms the eastern wall of the Shenandoah and Cumberland valleys—the former