While Jackson had marched south from Maryland in order to effect the capture of Harper's Ferry, Longstreet had retired before McClellan, who had collected an immense army and had advanced. The North had risen at the first news that Lee had crossed the Potomac and McClellan's army, vast as it was, yet continued to receive reinforcements almost daily; his army was perhaps stronger than it had been before his disastrous campaign of the Chickahominy, his troops on James River had marched down the Peninsula and had been taken in transports to Fredericksburg and Alexandria. Porter's and Heintzelman's corps of McClellan's army had fought under Pope in the second battle of Manassas. Now McClellan had his own army, Pope's army, Burnside's corps, and all other troops that could be got to his help. To delay this army until Jackson could seize Harper's Ferry had been the duty intrusted to Longstreet and his lieutenants. But Longstreet with his twenty thousand were now in danger of being overwhelmed. On the 15th, in the afternoon of the surrender at Harper's Ferry, two of Jackson's divisions had marched to reënforce Longstreet. Had not time been so pressing, Hill's division would not have been ordered to assault the works at Harper's Ferry--an assault which was begun and which was made unnecessary by the surrender. McClellan knew the danger to Harper's Ferry and knew of the separation of the Confederate forces. A copy of General Lee's special order outlining his movements had fallen into General McClellan's hands. This order was dated September 9th; it gave instructions to Jackson to seize Harper's Ferry, and it directed the movements of Longstreet. With this information, General McClellan pressed on after Longstreet; he ordered General Franklin to carry Crampton's Gap and advance to the relief of Harper's Perry. On Sunday, the 14th, McClellan's advanced divisions attacked D.H. Hill's division in a gap of South Mountain, near Boonsboro, and Franklin carried Crampton's Gap, farther to the south. Though both of these attacks were successful, the resistance of the Confederates had in each case been sufficient to gain time for Jackson. On the 15th Harper's Ferry surrendered, and McClellan continued to advance; Longstreet prepared for battle. The next day, at nightfall, the Federals were facing Lee's army, the Antietam creek flowing between the hostile ranks. At 3 P.M. of the 17th, A.P. Hill's division, after a forced march of seventeen miles, and after fording the Potomac, found itself in front of the left wing of the Federal army,--consisting of Burnside's corps,--which had already brushed away the opposition in its front, and was now advancing to seize the ford at Shepherdstown and cut off Lee from the Potomac.