By now, Lee’s scouts were bringing reports of the great Federal army slowly pushing out from Rockville toward Frederick.
Lee’s proposed route into Pennsylvania was dictated by geography. West of Frederick—beyond South Mountain—is the Cumberland Valley. This is the northern half of the Great Valley that sweeps northeastward through Virginia, Maryland, and Pennsylvania. That part of the Great Valley immediately south of the Potomac is called the Shenandoah Valley.
Lee planned to concentrate his army west of the mountains near Hagerstown, Md. There he would be in direct line with his supply base at Winchester in the Shenandoah Valley. After replenishing his supplies and ammunition, he could strike northeast through the Cumberland Valley toward Harrisburg, Pa., where he could destroy the Pennsylvania Railroad bridge across the Susquehanna River. Once loose in the middle of Pennsylvania he could live off the country and threaten Philadelphia, Baltimore, and Washington.
Before launching this daring maneuver, Lee must first clear his line of communications through the Shenandoah Valley to Winchester and to Richmond. Blocking it were strong Federal garrisons at Harpers Ferry and Martinsburg. Unaccountably, they had remained at their posts after the Confederate army crossed the Potomac. Now they must be cleared out.
Lee decided to accomplish this mission by boldly dividing his army into four parts. On September 9, he issued Special Order 191. Briefly, it directed Maj. Gen. James Longstreet and Maj. Gen. D. H. Hill to proceed across South Mountain toward Boonsboro and Hagerstown. Three columns cooperating under Maj. Gen. Thomas J. “Stonewall” Jackson were ordered to converge on Harpers Ferry from the northwest, northeast, and east. En route, the column under Jackson’s immediate command was to swing westward and catch any Federals remaining at Martinsburg. Maj. Gen. Lafayette McLaws, approaching from the northeast, was to occupy Maryland Heights, which overlooks Harpers Ferry from the north side of the Potomac. Brig. Gen. John Walker, approaching from the east, was to occupy Loudoun Heights, across the Shenandoah River from Harpers Ferry. Maj. Gen. J. E. B. Stuart’s cavalry was to screen these movements from McClellan by remaining east of South Mountain.