tomac 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 reenforce
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 Shepherdsto