Gregg's South Carolina brigade marched with Jackson's corps from the vicinity of Boonsboro on the 11th and camped at Williamsport on the Potomac. On the 12th, crossing the Potomac, Jackson marched upon Martinsburg, occupied by a Federal force under Brigadier-General White. Gregg was in front and deployed for battle, but White retired upon Harper's Ferry. Jackson entered the town and the inhabitants rejoicingly received him and his troops. His hungry men were feasted, their general caressed and honored, and the sutler's stores and army provisions left by the enemy duly appropriated. Marching on for Harper's Ferry, Jackson was in position before that place, on Bolivar heights, by noon of the 13th. Next day Gregg was sent to Jackson's right (with Branch's brigade) to take position on the Shenandoah, move along its north bank, and be ready on the morning of the 15th to assault from that point. Early on the 15th all the batteries opened on the defenses of Harper's Ferry, among them McIntosh's South Carolina battery. McLaws' rifled guns from Maryland heights, Walker's batteries from the Loudoun hills, and Jackson's from Bolivar heights poured their shot and shell into every opposing fort and battery, and the signal was about to be given to "cease firing" to give chance for the concerted assault of Jackson's infantry, when the banner of surrender was raised and Harper's Ferry was captured. The enemy replied from every one of his batteries with vigor, and kept up his defense until he saw his doom. Gregg had not lost a man, and remaining with A. P. Hill's division to secure the spoil of battle, his brigade reaped a harvest of good things at Harper's Ferry. The situation is thus described by Captain Caldwell:
We fared sumptuously. In addition to meat, crackers, sugar, coffee, shoes, blankets, underclothing, etc., many of us captured horses, of which the quartermaster, however, duly deprived us.
Jackson was the great theme of conversation. The Federals seemed never weary of extolling his genius and inquiring for particulars of his history. They were extremely anxious to see him. He came up from the riverside late in the afternoon. The intelligence spread like electricity. Almost the whole mass of prisoners broke over us, rushed to the road, threw up their hats, cheered, roared, bellowed, as even Jackson's own troops had scarcely ever done. We, of course, joined in with them. The general gave a stiff acknowledgment of the compliment, pulled down his hat, drove spurs into his horse, and went clattering down the hill away from the noise.
The garrison of Harper's Ferry, surrendered, gave Jackson over 11,000 prisoners, 73 pieces of artillery, 13,000 small-arms, and a large supply of military stores.
General McClellan did not push his advantage gained at Boonsboro gap. It was 8 o'clock on the morning of the 15th before his troops appeared on the west of the mountain, and General Lee had the columns of D. H. Hill and Longstreet beyond his reach by that time. Marching all the night of the 14th, these commands were in front of Sharpsburg early on the morning of Monday, the 15th. Jackson left Harper's Ferry on the night following, with McLaws', Walker's and Anderson's divisions, marched up to Shepherdstown, and crossed the river and reported to General Lee on the battlefield early on the 16th. He had left A. P. Hill's division at Harper's Ferry to parole the prisoners, secure the property captured, and hold the place. As will be seen, McLaws and R. H. Anderson did not reach the field of battle until it had been raging for hours, but they came up, as did A. P. Hill, in time to reinforce Lee at critical moments.
In writing of Sharpsburg there are particular features of that battlefield to which reference must frequently be made in order to comprehend the struggle, and these will first be noted.
The town of Sharpsburg is about a mile from the southward bend of the Potomac. A straight line running due east from the Potomac and passing through Sharpsburg would cross the Antietam river about 1½ miles from the town. The general direction of the Antietam in front (east) of Sharpsburg is a little west of south. And this, too, is the general direction of the Potomac in the vicinity of the battlefield. About 3 miles below Sharpsburg the Potomac makes a sweeping bend to the east and the Antietam to the west, the latter entering the former just below the point where the river turns sharply to the south. Lee's line was in front of Sharpsburg and behind the Antietam, which was easily forded, and crossed by good stone bridges in Lee's front and on each flank.
Two main roads gave direction to the battle, one running north to Hagerstown, and the other a little north of east to Boonsboro. About 1¼ miles from the town, on the Hagerstown road, was a church known as the Dunker's chapel, with a heavy wood north, south and west of it. The hills along the Antietam, on both sides, were high and commanding, and gave the best positions for artillery. The country between the Antietam and the Hagerstown road was undulatory, with good elevations for artillery, and south of Sharpsburg very much the same. The Antietam makes a very long bend to the west about 1½ miles below the town and then bends south again. General Lee's right rested on this bend, the hills being high and steep on the Sharpsburg side. Lee formed two lines of battle on the hills described, its direction parallel with the Antietam, bending toward the Potomac on the left.
On the 15th, Longstreet was posted on the south of the Boonsboro road, and D. H. Hill north of it. Hood's division prolonged the line on Hill's left bending west until it touched the Hagerstown road. Jackson, early on the 16th, was put on Hood's left, with his right on the Hagerstown road. Stuart with cavalry and horse artillery guarded the extreme left next the Potomac. Walker, with his two brigades, came up from Harper's Ferry by afternoon, and was posted on the extreme right and immediately on the Antietam bluffs. As the divisions slept on arms, on the night of the 16th, they stood for battle, from right to left, in the following order: Walker, D. R. Jones, Evans (brigade), D. H. Hill, Hood, Lawton, J. R. Jones, cavalry. The artillery opened the great battle at dawn on the 17th, and before the sun had risen Jackson was hotly engaged with Hooker's corps on the Confederate left.
Jackson's and Hood's troops held their ground with great courage and firmness, sometimes advancing in triumph and then repulsed by the front lines of the enemy. The history of Jackson's battle is a history of violent and bloody contention, advances and retirements, with ground lost, gained, relost and regained, until at last the enemy was forced to the defensive and the Confederate battle held on nearly its chosen line. The three corps of Hooker, Mansfield and Sumner were engaged in these battles with Jackson and Hood, while the latter were reinforced from time to time by three brigades from D. H. Hill, one from D. R. Jones, and two with Walker. These forces, with Jackson's two small divisions and Hood's two brigades, had forced the battle beyond the Hagerstown road, and were on the successful offensive, as against Hooker's and Mansfield's corps, when Sumner entered the battle. His advance was against Jackson's right and center, two of his divisions (Richardson's and French's) operating east of the turnpike and south of the church, and one (Sedgwick's) moving against the woods just north of the church. Sumner's line operated at once to check the tide of his retreating friends, and to stem that on his advancing foes. Fresh, strong and admirably handled, the divisions of Richardson, French and Sedgwick moved to renew the waning battle. Richardson, supported by French, moved against D. H. Hill's left center, and Sedgwick attacked in front and north of the church.
Sumner's account of affairs on the battlefield when he reached it shows the work which had been done by the troops of Jackson, Hood, D. H. Hill and the brigade from D. R. Jones. He said: "On going upon the field, I found that General Hooker's corps had been dispersed and routed. I passed him some distance in the rear, where he had been carried wounded, but I saw nothing of his corps as I was advancing on the field. There were some troops lying down on the left which I took to belong to Mansfield's command. In the meantime, General Mansfield had been killed, and a portion of his corps thrown into confusion."