 Despite this show of bravado , McClellan continued his cautious line . After telegraphing to the president at noon on September 13 , rather than ordering his units to set out for the South Mountain passes immediately , he ordered them to depart the following morning . The 18 hours of delay allowed Lee time to react , because he received intelligence from a Confederate sympathizer that McClellan knew of his plans . ( The delay also doomed the federal garrison at Harpers Ferry because the relief column McClellan sent could not reach them before they surrendered to Stonewall Jackson . ) In the Battle of South Mountain , McClellan 's army was able to punch through the defended passes that separated them from Lee , but also gave Lee enough time to concentrate many of his men at Sharpsburg , Maryland . The Battle of South Mountain presented McClellan with an opportunity for one of the great theatrical moments of his career , as historian Sears describes : 
