Lee had assumed, quite as a matter of course, that upon his passage of the Potomac, Martinsburg and Harper's Ferry would be evacuated, being obviously untenable. But in fact they were not abandoned. So Lee was compelled to pause and to send Jackson back to the south side of the river to secure control of positions that commanded his own only secure line of retreat in case of disaster.
This caused a very serious delay in Lee's operations, and in such a campaign of aggression, promptitude and swiftness are all important to the accomplishment of desired results.
Jackson went back across the river to assail Harper's Ferry from the South. In the meanwhile McLaws, Walker and D. H. Hill seized and held respectively Maryland Heights, Loudon Heights and Boonesboro Pass, while Lee with the remainder of his now dangerously divided army advanced to Hagerstown in search of food supplies.
Jackson did his part of the work perfectly, as it was his custom to do. He drove his enemy out of Martinsburg and captured Harper's Ferry with 11,500 prisoners, seventy-three serviceable guns and important stores.
But in the meanwhile Lee's army had been scattered in a very perilous way, and in his anxiety for its reconcentration, he wrote out an order, giving in detail his instructions to his several subordinates.
A copy of this order somehow fell into McClellan's hands. It clearly revealed to him Lee's divided and scattered condition, and for once in his life McClellan hurried. If he, with 70,000 men, could manage to attack in detail the several widely separated fragments of Lee's army which had now been reduced by casualties to less than a total of 40,000, surely he must win.
Accordingly he hurriedly pushed forward, hoping to carry Turner's and Crampton's Gaps in the South Mountain before Lee could concentrate for their defense.
He was a trifle too late, however, and a stubborn defense was made there on the fourteenth, giving Lee time to bring up the remainder of his forces for the decisive battle at Sharpsburg or Antietam, as the action is variously called at the South and at the North. McClellan finally carried the gaps at cost of a loss of 2,000 men—the Confederates losing a like number.
But in the meanwhile McClellan had lost all the strategic advantage that he was striving for. It had been his hope to push his columns through the gaps—as he might have done twenty-four hours earlier without serious resistance—and to occupy commanding positions between Lee's widely scattered forces, from which, with his vastly superior numbers he might conquer them in detail, probably compelling Lee's surrender as a part of the price exacted.
But McClellan was twenty-four hours late. He therefore had to fight all day in order to force his way through passes that a day earlier had been practically open to him.