mcclellan remained in command of the army of the potomac even though he had failed to support major general john pope at the second battle of bull run. when lee entered maryland, in a rare exception to his usual slowness, mcclellan moved more quickly to cut him off. unknown to lee at the time, a union soldier had found a copy of special order 191. this was lee's plan to split up his army. mcclellan learned lee had sent general stonewall jackson to attack and hold the harpers ferry armory. the information gave mcclellan the chance to destroy lee's army while it was still weakened by jackson's attack on harpers ferry. mcclellan learned of lee's plan on september 13. he boasted "here is a paper with which if i cannot whip bobbie lee, i will be willing to go home." he also sent a telegram to president lincoln where he wrote, "i have all the plans of the rebels, and will catch them in their own trap if my men are equal to the emergency." but mcclellan waited another 18 hours before acting. this delay allowed lee more time to collect all the elements of his army. as lee had moved into maryland, he left detachments to guard two of the passes through south mountain, crampton's gap and turner's gap. these were the two most important routes through the 50-mile long south mountain. had mcclellan moved a little faster, he would have caught lee's army scattered on the other side of the mountain.