(At this point a fateful event occurred—one which was destined to change the subsequent course of the campaign. D. H. Hill, Jackson’s brother-in-law, had until this time been under Jackson’s command. Unaware that a copy of Lee’s order had already been sent to Hill, Jackson now prepared an extra copy for that officer. Hill kept the copy from Jackson; the other was to provide the script for much of the drama that followed.)
Lee was courting danger by thus dividing his force in the face of McClellan’s advancing army. Against a driving opponent, Lee probably would not have done it. But he felt certain that McClellan’s caution would give Jackson the margin of time needed to capture Harpers Ferry and reunite with Longstreet before the Federal army could come within striking distance. That margin was calculated at 3 or 4 days. By September 12, Jackson’s force should be marching north toward Hagerstown. As soon as the army reconcentrated there, Lee could begin his dash up the Cumberland Valley into Pennsylvania.
So confident was Lee of the marching capacities of the Harpers Ferry columns, and so certain was he that McClellan would approach slowly, that he made no provision for guarding the gaps through South Mountain.
[Illustration: _Maj. Gen. James Longstreet._ Courtesy, Library of Congress.]
[Illustration: _Maj. Gen. Thomas J. “Stonewall” Jackson._ From photograph by George W. Minnes. Courtesy, Library of Congress.]
Lee’s army departed Frederick on September 10. Two days later leading elements of McClellan’s army entered that city. On September 13, came McClellan himself with his usual cavalcade of staff officers.
That same afternoon a copy of Lee’s Special Order 191 was discovered in the encampment grounds previously used by the Confederate army. Quickly it was passed to McClellan. The handwriting was recognized as that of Col. R. H. Chilton, Lee’s assistant adjutant general; the document’s authenticity could not be doubted.
The fate of Lee’s army literally lay in McClellan’s hands. If he slashed swiftly through the South Mountain gaps and planted his army squarely between Longstreet’s force near Hagerstown and Jackson’s columns at Harpers Ferry, he could overwhelm the Confederate detachments in turn.