The Campaign and Battle of Antietam

Just after dawn on September 17, 1862, the Union Army of the Potomac and the Confederate Army of Northern Virginia collided in the farm fields and wood lots near Sharpsburg, Maryland. By evening, over 25,000 men lay dead or wounded. It remains the bloodiest single day in American military history.

The battle marked the culmination of the Maryland Campaign (September 4-20, 1862), the first of two major efforts by General Robert E. Lee to carry the war onto northern soil. The other would be the Gettysburg Campaign (June 5-July 14, 1863). His rationale for the campaign was basically fourfold: first, to retain the initiative seized during the Seven Days Battles and Second Manassas Campaigns; second, to remove the Union army from Virginia soil so that Virginia farmers could harvest their crops without interruption; third, to offer Marylanders an opportunity to leave the Union and join the Confederacy; and fourth, to fight a decisive battle on Northern soil.

Lee believed that if the Confederacy were to win the war, it must do so quickly. The longer the war continued, the more the North could bring to bear its substantial advantages in materiel and population. Lee also considered Northern public opinion to be fragile. If the Confederate army could achieve decisive victory, especially on Northern soil, Northerners might conclude that the war was unwinnable and make peace.

On September 4, the first elements of his army crossed the Potomac River into Maryland. By September 7, his whole army was across, and concentrating at the market town of Frederick.

Strictly speaking, Lee’s invasion was a raid. That is to say, he had neither the desire nor the ability to occupy Northern soil for any length of time, and his troops and horses drew much of their food and forage from the countryside. However, he still needed some sort of connection with Richmond for purposes of communication and at least limited resupply. He supposed he could get it by way of the Shenandoah Valley, which terminated at Harpers Ferry, Virginia (now West Virginia) at the confluence of the Potomac and Shenandoah Rivers. Some 12,500 Union troops garrisoned Harpers Ferry and nearby Martinsburg, but Lee anticipated that, once he had interposed his army between them and Washington, they would withdraw to the north rather than risk capture. He was wrong. They stayed in place, and on September 9 Lee issued Special Orders No. 191, an elaborate plan to surround and capture them.

The plan called for the division of his army into four parts. Three divisions under Major General Thomas Jonathan “Stonewall” Jackson would make a wide sweep to the west, re-cross the Potomac, and move toward Harpers Ferry like an encircling arm. A division under Major General John George Walker would also re-cross the Potomac in order to seize Loudoun Heights, a high ridge that overlooked Harpers Ferry. Meanwhile another division under Major General Lafayette McLaws would capture Maryland Heights, the southern terminus of a high ridge that also overlooked Harpers Ferry. The “main body” - three remaining divisions under Major General James Longstreet - would cross South Mountain and halt at the village of Boonsboro.

Things did not go as planned. Soon after the operation began on September 10, Lee got word that Union troops were gathering at Greencastle, Pennsylvania, just across the state line. To guard against them, Lee had Longstreet continue his march to Hagerstown, leaving just one division under Major General Daniel Harvey Hill at Boonsboro to function as the army’s rear guard. Lee hoped that Harpers Ferry would be in Confederate hands no later than September 13, but only late on that day did Jackson, Walker, and McLaws even get into position. The Army of Northern Virginia was now divided into five parts, and until Harpers Ferry capitulated it was theoretically in a highly risky posture. If the Federals in Washington conducted a rapid pursuit of Lee, they could strike his army while it was divided and destroy it one piece at a time.

The posture was theoretically risky only because Lee assumed that it would take at least a week for Major General George Brinton McClellan to take the field, that he would advance cautiously, and that he would not know how badly Lee’s army was divided. Lee turned out to be wrong on all counts.

McClellan rapidly consolidated and reorganized the Army of the Potomac, incorporating elements of the now defunct Army of Virginia. He took the field on September 7, the same day the last of Lee’s troops crossed the Potomac, and although his advance was cautious, by September 13 his army had reached Frederick, occupied by Lee’s army until just three days before.

There he learned just how badly Lee’s army was divided from the best possible source: Lee himself. Two Union soldiers discovered a mislaid copy of Special Order 191 and it swiftly made its way into McClellan’s hands. “I have all the plans of the Rebels,” McClellan informed President Abraham Lincoln, “and will catch them in their own trap if my men are equal to the emergency.” On September 14, McClellan advanced against three key passes through South Mountain: Turner’s Gap, Fox’s Gap, and Crampton’s Gap. The first two would give McClellan access to Boonsboro, the location, according to Special Orders 191, of Lee’s main body. In fact the only Confederate unit at Boonsboro was of course D. H. Hill’s division, and for much of the day Hill defended Turner’s and Fox’s Gaps against the Union II Corps, under Major General Edwin Vose Sumner, and the IX Corps under Major General Jesse Lee Reno (who was killed early in the battle). Reinforcements arrived from Hagerstown during the afternoon and enabled Lee to hang on to these two gaps.

Crampton’s Gap, close to the Potomac River, offered the Federals access to the rear of McLaws’ division, which had reached Maryland Heights and begun to besiege Harpers Ferry. McLaws could spare only a small force to defend the gap. Fortunately for him, Major General William Buell Franklin, commanding the Union VI Corps, took most of the day to approach and attack the gap. The Confederates held off the Federals until nightfall and withdrew to a less formidable position where they girded themselves for a renewal of Franklin’s attack the next morning.

Although the Confederate army had managed to fend off the Union army on the 14th, Lee realized he could not hold the mountain gaps for a second day. And since the Federal garrison of Harpers Ferry had not yet surrendered, his army remained badly divided. Initially Lee concluded that he must abandon the campaign and re-cross Potomac. But when he informed Jackson of this, Jackson responded that he expected Harpers Ferry to capitulate on the 15th. He proved correct. The garrison did indeed surrender, and with it 12,500 Union troops became prisoners of war - The surrender of Harpers Ferry did not mean that the Confederate army could immediately reunite. A large amount of valuable supplies had fallen into their hands at Harpers Ferry; it took time to do a quick inventory of these and start them on their journey south. The thousands of Federal prisoners were another problem. Until late 1863 the Union and Confederacy maintained few prison camps but instead had an agreement to exchange prisoners. According to this system, prisoners were released - “paroled” - after signing a written pledge not to take arms again until properly exchanged. It took almost three days to process the 12,500 Union captives.

In the meantime, Lee looked for a defensible position on the Maryland side of the Potomac. He found it at Sharpsburg, on the west side of Antietam Creek. His decision to make a stand at that location has long been controversial. Its advantages included the creek itself: although only about waist deep, it had steep banks that made it almost impossible for large numbers of troops to cross under enemy fire. The high ground west of the creek made for a strong position. The terrain also had numerous knolls and ravines that would give the Confederates good concealment, so that the Federals would be unable to get a good look at Confederate dispositions. But the Sharpsburg position had one huge drawback: Lee’s army would have to fight with its back to the Potomac River, with no bridge available in case of defeat and only a single ford.

Since Lee never explained his reasons for selecting Sharpsburg, historians have been left to guess. Some have speculated that for political reasons, he could not afford to abandon the “oppressed” state of Maryland without a major battle. Some have believed that Lee did not realize the extent of the troops he had lost in combat or from straggling and overestimated the force at his disposal. And some have believed that until Jackson consolidated the Harpers Ferry victory, Sharpsburg placed Lee on the flank of any Union bid to recapture the town. Historians are equally divided about the wisdom of the decision. While some have given Lee the benefit of the doubt, many - probably most - have maintained that in standing at Sharpsburg, Lee made a serious mistake.

Historians have been equally critical of McClellan. The advance elements if his army reached the high ground east of Antietam Creek late on the afternoon of September 15 and on the 16th McClellan had a clear opportunity to strike Lee’s army while much of it was still near Harpers Ferry. McClellan, however, contented himself with shifting his I and XII Corps across the creek to a point north of Lee’s position. He intended these units to attack the Confederate left flank the next morning. The rest of his plan is a bit murky since McClellan offered several after-the-fact rationales, but as events unfolded it took the form of an engagement in which McClellan committed his forces piecemeal. One problem was that this policy was to some extent deliberate. Another was that McClellan’s headquarters, which were at the Pry House east of Antietam Creek, was too remote from the battlefield for McClellan to influence the action. Even a courier riding hard from headquarters usually required the better part of an hour to reach his destination. Further, McClellan intentionally held in reserve the entire V Corps under Major General Fitz John Porter, as an insurance policy against a counterstroke by Lee’s supposedly superior numbers.

McClellan’s I Corps, under Major General Joseph Hooker, opened the attack at 7:30 a.m. on September 17. His forces surged through a large cornfield that was the scene of desperate fighting until a Confederate division under Major General John Bell Hood counterattacked, losing most of its troops but stopping Hooker’s attack in its tracks. The Union XII Corps advanced at about the same time, but only one division made much progress. Brigadier General George Sears Greene’s Division managed to reach a position near the Dunker Church, where it halted and awaited reinforcements. Although no reinforcements were deliberately sent to support Greene, one division of the Union II Corps under Sumner advanced into the West Woods, immediately north of the Dunker Church - only to be shredded by a massive Confederate counterattack that struck it on the flank and rear.

Sumner’s remaining two divisions lost contact with the first and lurched to the southwest, eventually running into Confederate forces posted within a wagon road whose trace had over the years been worn several feet below the surrounding terrain. This “sunken road” proved to be a natural trench. The two Union divisions fought the single Confederate division with scant success until a Confederate error in communication resulted in an untimely retreat by some of the division’s troops. This gave the two Union divisions the chance to seize the Sunken Road, and by 11:00 a.m. it was in northern hands.

At about that hour, the Union IX Corps under Major General Ambrose Everett Burnside successfully stormed a stone bridge across Antietam Creek, since known as Burnside’s Bridge. This was almost two hours after McClellan instructed him to do so, a delay attributable in part to several failed attempts to secure the bridge. Using the bridge and a ford discovered several hundred yards downstream, Burnside got his entire corps across by early afternoon, but paused for a considerable period and did not launch a major attack until 3:00 p.m. This attack was foiled by a furious counterattack by the division of Major General Ambrose Powell Hill, which arrived on the field in the nick of time after a seventeen-mile forced march from Harpers Ferry. With Hill’s success the battle essentially ended for the day.

That evening Lee conferred with his senior commanders and debated whether to launch an attack of his own the following day. As he eventually realized, this was misplaced aggressiveness, and in fact Lee overestimated the force he had in hand. Most estimates place it at no more than 40,000, of whom at least a third had become casualties. Nonetheless, Lee defiantly held his position for a second day, correctly gambling that McClellan would not renew the offensive. Only then did he withdraw his army back across the Potomac. A sharp rear guard action at Shepherdstown, Virginia (now West Virginia), closed out the campaign for good.

Casualties during the campaign were severe. In addition to 12,500 Union troops captured at Harpers Ferry, the Federals lost 443 killed, 1,807 wounded, 75 missing at South Mountain out of total of 28,000+ troops engaged. Confederate losses at Harpers Ferry were negligible, but at South Mountain they amounted to an estimated 325 killed, 1,560 wounded, 800 missing out of about 18,000 engaged.

Casualties in the battle of Antietam were horrific. The Army of the Potomac suffered 2,010 killed, 9,416 wounded, and 1,043 missing - 12,469 out of about 87,000 troops available. The Army of Northern Virginia lost 2,700 killed, 9,024 wounded, and 2,000 missing - 13,724 out of about 40,000 engaged.

The result of the campaign was a tactical stalemate. Confederates could point to their great success at Harpers Ferry. Federals could point to the fact that they had repelled Lee’s invasion. Tactically the battle was only a marginal success for McClellan, since Lee escaped intact. Strategically, however, it was a major Confederate defeat, and five days after Antietam Lincoln used the marginal victory on that day to issue, from a newly acquired position of strength, the preliminary Emancipation Proclamation.