On the 17th the division was put in motion on a road running up the Potomac.  The march began, at sunrise.  Soon the sounds of battle were heard far in front, and the step was lengthened.  The day was hot, and the road was dusty.  Frequently we went at double-quick.  About one o’clock we waded the Potomac below Shepherdstown.  Beyond the river the march turned northeast—­a rapid march; many men had fallen out before we reached the river; now many more began to straggle.  All the while the roar of a great battle extended across our front, mostly in our left front.  We passed through a village called Sharpsburg.  Its streets were encumbered with wagons, ambulances, stragglers, wounded men, and all the horrid results of war that choke the roads in rear of an army engaged in a great battle.

Beyond the village we turned to the right.  We marched up one side of a hill and down the other side.  On the slope of the opposite hill we halted, some of the troops being protected by a stone fence.  The noise of battle was everywhere, and increasing at our right, almost on our right flank.  Wounded men were streaming by; the litter-bearers were busy.  Nothing is so hard to bear as waiting while in expectation of being called on to restore a lost battle from which the wounded and dead are being carried.  Our time was near.

Thick corn was growing on the hillside above us.  General Gregg dismounted.  His orders reached our ears and were repeated by the colonels and the captains.  We were to advance.

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 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.