it necessary for him to first clear out the Federal garrison at Harper's Ferry and establish connection with Winchester before he could engage in a contest with McClellan west of the Blue ridge or make an offensive movement into Pennsylvania. After a conference with Jackson, at Frederick City, he issued a general order on the 9th of September, for the movements of his troops, for the two-fold purpose of capturing the Federal stronghold at Harper's Ferry and for the concentration of his army in the vicinity of Hagerstown.
Jackson was perfectly familiar with the topographical conditions at Harper's Ferry, and knew, from his late experience in threatening but not capturing that place, the strategic and tactic movements that would be necessary to successfully invest and secure possession of it. Therefore, with good reason, Lee had taken Jackson into his councils and provided to put in his hands the execution of the plan of campaign decided on.
Harper's Ferry, located in the fork at the junction of the Shenandoah and the Potomac, just above where the united rivers break through the Blue ridge, cannot be held and defended unless Loudoun heights on the south, across the Shenandoah, the northeastern end of the double Blue ridge, and Maryland heights, across the Potomac, the southwestern end of the Blue ridge in Maryland, are both occupied and defended at the same time; for each of these positions overlooks and thoroughly commands the fronts and flanks of the defenses of Harper's Ferry proper. The Federals had not occupied Loudoun heights, but they had Maryland heights, with formidable batteries placed to command the approaches to Harper's Ferry from Virginia, and with defensive works to protect in the rear from Maryland.
The instructions of Lee's order were, that Jackson should march westward in the early morning of September 10th, along the great National road leading from Frederick across the Blue ridge (South mountain) to Boonsboro, with his fourteen brigades, then take the macadam road leading to Williamsport, on the Potomac, and there, having turned the flank of the Federal outpost at Martinsburg, to cross the Potomac, break the Federal line of communication from the west by the Baltimore & Ohio railroad, then move upon the garrison at Martinsburg, and either capture or drive it in toward
