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.] _The Lost Order_ 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. But again McClellan was methodical. Not until the next morning, September 14, did his heavy columns get underway. This crucial delay was to give Lee the chance to pull his army together at the small town of Sharpsburg. _Fighting for Time at South Mountain_ By September 12, Lee had begun to worry. Stuart’s scouts had reported the Federal approach to Frederick. McClellan was moving too fast. Next evening things looked worse. Jackson had not yet captured Harpers Ferry, and already McClellan’s forward troops were pushing Stuart back toward the South Mountain gaps. Delay at Harpers Ferry made these passes through South Mountain the key to the situation. They must be defended. [Illustration: _The Battle of South Mountain._ From lithograph by Endicott. Courtesy, Library of Congress.]