The Battle of Antietam fought on September 17, 1862, near Sharpsburg, Maryland, and Antietam Creek, as part of the Maryland Campaign, was the first major battle in the American Civil War to take place on Northern soil. It was the bloodiest single-day battle in American history, with about 23,000 casualties.
After pursuing Confederate General Robert E. Lee into Maryland, Union Army Maj. Gen. George B. McClellan launched attacks against Lee's army, in defensive positions behind Antietam Creek. At dawn on September 17, Maj. Gen. Joseph Hooker's corps mounted a powerful assault on Lee's left flank. Attacks and counterattacks swept across Miller's cornfield and fighting swirled around the Dunker Church. Union assaults against the Sunken Road eventually pierced the Confederate center, but the Federal advantage was not followed up. In the afternoon, Maj. Gen. Ambrose Burnside's corps entered the action, capturing a stone bridge over Antietam Creek and advancing against the Confederate right. At a crucial moment, Maj. Gen. A.P. Hill's division arrived from Harpers Ferry and launched a surprise counterattack, driving back Burnside and ending the battle. Although outnumbered two-to-one, Lee committed his entire force, while McClellan sent in less than three-quarters of his army, enabling Lee to fight the Federals to a standstill. During the night, both armies consolidated their lines. In spite of crippling casualties, Lee continued to skirmish with McClellan throughout September 18, while removing his battered army south of the river.
Despite having superiority of numbers, McClellan's attacks failed to achieve concentration of mass, allowing Lee to counter by shifting forces along interior lines to meet each challenge. Despite ample reserve forces that could have been deployed to exploit localized successes, McClellan failed to destroy Lee's army. Nevertheless, Lee's invasion of Maryland was ended, and he was able to withdraw his army back to Virginia without interference from the cautious McClellan. Although the battle was tactically inconclusive, it had unique significance as enough of a victory to give President Abraham Lincoln the confidence to announce his Emancipation Proclamation.
Near the town of Sharpsburg, Lee deployed his available forces behind Antietam Creek along a low ridge, starting on September 15. While it was an effective defensive position, it was not an impregnable one. The terrain provided excellent cover for infantrymen, with rail and stone fences, outcroppings of limestone, and little hollows and swales. The creek to their front was only a minor barrier, ranging from 60 to 100 feet (18-30 m) in width, and was fordable in places and crossed by three stone bridges each a mile (1.5 km) apart. It was also a precarious position because the Confederate rear was blocked by the Potomac River and only a single crossing point, Boteler's Ford at Shepherdstown, was nearby should retreat be necessary. (The ford at Williamsport, Maryland, was 10 miles (16 km) northwest from Sharpsburg and had been used by Jackson in his march to Harpers Ferry. The disposition of Union forces during the battle made it impractical to consider retreating in that direction.) And on September 15, the force under Lee's immediate command consisted of no more than 18,000 men, only a third the size of the Federal army.
The first two Union divisions arrived on the afternoon of September 15 and the bulk of the remainder of the army late that evening. Although an immediate Union attack on the morning of September 16 would have had an overwhelming advantage in numbers, McClellan's trademark caution and his belief that Lee had over 100,000 men caused him to delay his attack for a day. This gave the Confederates more time to prepare defensive positions and allowed Longstreet's corps to arrive from Hagerstown and Jackson's corps, minus A.P. Hill's division, to arrive from Harpers Ferry. Jackson defended the left (northern) flank, anchored on the Potomac, Longstreet the right (southern) flank, anchored on the Antietam, a line that was about 4 miles (6 km) long. (As the battle progressed and Lee shifted units, these corps boundaries overlapped considerably.)
On the evening of September 16, McClellan ordered Hooker's I Corps to cross Antietam Creek and probe the enemy positions. Meade's division cautiously attacked the Confederates under Hood near the East Woods. After darkness, artillery fire continued as McClellan continued to position his troops. McClellan's plan was to overwhelm the enemy's left flank. He arrived at this decision because of the configuration of bridges over the Antietam. The lower bridge (which would soon be named Burnside Bridge) was dominated by Confederate positions on the bluffs overlooking it. The middle bridge, on the road from Boonsboro, was subject to artillery fire from the heights near Sharpsburg. But the upper bridge was 2 miles (3 km) east of the Confederate guns and could be crossed safely. McClellan planned to commit more than half his army to the assault, starting with two corps, supported by a third, and if necessary a fourth. He intended to launch a simultaneous diversionary attack against the Confederate right with a fifth corps, and he was prepared to strike the center with his reserves if either attack succeeded. The skirmish in the East Woods served to signal McClellan's intentions to Lee, who prepared his defenses accordingly. He shifted men to his left flank and sent urgent messages to his two commanders who had not yet arrived on the battlefield: Lafayette McLaws with two divisions and A.P. Hill with one division.
McClellan's plans were ill-coordinated and were executed poorly. He issued to each of his subordinate commanders only the orders for his own corps, not general orders describing the entire battle plan. The terrain of the battlefield made it difficult for those commanders to monitor events outside of their sectors, and McClellan's headquarters were more than a mile in the rear (at the Philip Pry house, east of the creek), making it difficult for him to control the separate corps. Therefore, the battle progressed the next day as essentially three separate, mostly uncoordinated battles: morning in the northern end of the battlefield, mid-day in the center, and afternoon in the south. This lack of coordination and concentration of McClellan's forces almost completely nullified the two-to-one advantage the Union enjoyed and allowed Lee to shift his defensive forces to meet each offensive.
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