The Battle of Antietam – Significance and Impact

Following their victory at Second Bull Run on August 30, the Confederate leadership wanted to take the war into enemy territory. They had cogent reasons for doing so and a general strategic plan. The invasion started just 3 weeks after that southern victory. Lee brought 55,000 men into Maryland to face McClellan with about 85,000 men.

Campaign Concept

The idea for the invasion was well conceived. The Confederacy hoped to gain a number of advantages by invading the North. The leadership thought that they had the upper hand militarily, and a victory in the north might lead to foreign recognition. The South hoped that Maryland, a border state that Lincoln had to resort to questionable constitutional measures to keep in the Union, would arise and assist in its attack. The Baltimore riots of 1861 seemed to suggest a strong Confederate sentiment. This was true: in Baltimore, but not in the west of the state. It was also necessary for logistical reasons, as northern Virginia’s farms had been stripped bare of food.

Many modern-day civil war enthusiasts consider this a terrible idea and suggest that the Confederacy should have stayed on defense, not fritter away their resources. In fact, they had a bold plan that few civil war buffs recognize in large part because it didn’t work. Lee’s invasion of Maryland was intended to run simultaneously with an invasion of Kentucky by the armies of Braxton Bragg and Edmund Kirby Smith. 

Frederick was chosen as the starting point because, as the map shows, strategically it is centrally placed between Washington and Baltimore. The B& O RR sets up a supply line to that town. It is also well located to Harper’s Ferry. And, it was the temporary capital of Maryland.

The First Stage

As shown on the map, Lee started off September 3 and crossed the Potomac at two fords west of Washington.  His army moved to Frederick, camping in a field 2 miles south of the town at Best’s Farm

McClellan knew that Lee was in his northwest and moved, slowly, to that region. By the time he arrived on September 13, Lee has been gone for 4 days.

Where was Lee going? What was his objective? Lee was moving to attack Harpers Ferry. That town is west of Frederick. He was not moving to advance on the big eastern cities. He was looking to draw McClellan out into a place to have a large fight. Jackson was to take Harpers Ferry while the rest of Lee’s army was posted at Boonsboro under command of Maj. Gen. James Longstreet. Lee hoped that after taking Harper’s Ferry to secure his rear, he could carry out an invasion of the Union, wrecking the Monocacy aqueduct, before turning his attention to Baltimore, Philadelphia, or Washington, D.C. itself. Unlike the Pennsylvania Campaign, Lee had a real plan. 

Once at Frederick, Lee then informed his subordinates what the initial plan was. In Special Order #191, General Lee outlined the routes to be taken and the timing for the attack of Harpers Ferry. It provided specific details of the movements his army would take during the invasion of Maryland. The crucial point was that Lee divided his army, which he planned to regroup later.  The order directed Major General Stonewall Jackson to move his corps to Martinsburg while McLaws’s and Walker’s divisions “endeavored to capture Harpers Ferry.” Major General James Longstreet was to move his corps northward to Boonsborough. Major General DH Hill’s division was to act as rearguard on the march from Frederick.

The Special Orders were written in the farm field shown in the photo. Lee bivouacked in this field on the Best Farm shown. Famously, the order was lost and later found by the Union army about a half mile north and east of this location(see Who Lost the “Lost Order”?).

Photo by Author

When General McClellan came into the possession of Special Order #191, he had an accurate and timely picture of exactly where the components of the Confederate army were located and what routes they were going to be using in the next several days. He knew that the Confederate army was divided and he knew exactly where they were. Instead, McClellan waited about 18 hours before deciding to take advantage of this intelligence and reposition his forces, thus squandering an opportunity to defeat Lee conclusively. Some historians believe that for this reason, the loss of Special Order #191 wasn’t as decisive as history makes it out to be. I disagree, in that McClellan otherwise had no clue where Lee was going.

South Mountain and Harpers Ferry

There were two significant engagements in the Maryland campaign prior to the battle of Antietam: 1) Maj. Gen. Thomas J. “Stonewall” Jackson captured Harpers Ferry on September 12. This is important because Jackson’s Corps, a large portion of Lee’s army was absent from the start of the battle of Antietam, attending to the surrender of the Union garrison. And 2)

McClellan’s assault through the Blue Ridge Mountains resulted in the Battle of South Mountain on September 14. In this battle, stout Confederate defenses at two passes through the mountains delayed McClellan’s advance enough for Lee to concentrate the remainder of his army at Sharpsburg. Lee did not expect to be attacked by McClellan at this vulnerable moment. He was hiding at Boonsboro precisely to keep McClellan guessing as to his whereabouts. He could not have known that McClellan knew where he was and had possession of his written orders.

In the battle, the Union army attacked at 3 gaps as shown on the map. Crampton’s Gap was taken; Turner’s and Fox’s Gap were held by the Confederates at the end of the day but were not considered defensible for longer. Consequently, Lee retreated.



McClellan was now in a position to destroy Lee’s divided army. But astonishingly, McClellan displayed no urgency to follow up on September 15 after his victory at South Mountain. This condemned the garrison at Harpers Ferry to capture and gave Lee time to unite his scattered divisions at Sharpsburg. 

Knowing they were being pursued slowly, Lee’s army set up defensive positions behind Antietam Creek (See map, left panel). Lee expected a battle, but half of his army was still in Harper’s Ferry. Antietam Creek is pretty shallow in most places, ranging from 60 to 100 feet in width, and was fordable in places. It was crossed by three stone bridges each a mile apart. The first two Union divisions arrived on the afternoon of September 15 and the bulk of the remainder of the army late that evening (see map, right panel).

McClellan’s army was there on the morning of September 16. If he had attacked at that time, the war might have ended that day. He outnumbered Lee by as much as 2:1. He somehow didn’t know that Jackson was still in Harpers Ferry, even though he had the order saying where he was likely to be and knowing that the town had fallen. So, he was facing just half of Lee’s divided army.

Instead, that evening he determined to attack and overwhelm Lee’s left flank. The bridges were the key. The lower bridge (soon to 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 (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

He had ordered Meade to skirmish near the East Woods on the afternoon of September 16. This skirmish signaled 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.

The Battle

The Battle of Antietam began at dawn on September 17 as the fog lifted. Longstreet’s and Hill’s units were placed on the Confederate right and center, just to the west of Antietam Creek. Union forces outnumbered Confederates by two to one, although McClellan thought Lee’s forces were much larger.

Morning Phase

At dawn, Hooker’s corps assaulted Lee’s left flank. Numerous attacks and counter-attacks occurred across a 30-acre cornfield owned by David Miller. Confederate troops fought off multiple offensives, turning the cornfield into a scene of destruction. 

The battle continued around the Dunker Church. Again, numerous waves of attacks took place. Lee brilliantly moved his limited troops across the battlefield to defend his lines.

Second Phase

At the center of the battlefield, a farm lane that became known as the Sunken Road was the site of a series of Union assaults. Hill’s division of 2,600 men created a fortification composed of piled fence rails along the road’s embankment. Union Major General William H. French’s 5,500 troops made multiple assaults at various locations. 

The ferocity of these numerous waves ultimately crushed the Confederate center with huge casualties, as the road was a great defense when the attackers rushed it but a detriment when it was used to flank those inside.  The fighting ultimately continued at close range. 

After three hours, Union troops had pushed the Confederates back with over 5,000 casualties However, the resulting advantage was squandered. McClellan did not follow up the attacks of his right flank, allowing Lee to move men over to his center as reinforcements.

Afternoon Phase

For the entire preceding portions of the battle, fewer than 500 Confederate soldiers held the Lower Bridge. They held out against multiple assaults by Union General Ambrose Burnside’s Ninth Corps. 

In the afternoon, Burnside’s corps finally captured the stone bridge over Antietam Creek and advanced against the Confederate right. It appeared that he would wrap up this weal right flank. At this critical moment, Jackson’s subordinate, Maj. Gen. A. P. Hill arrived from Harpers Ferry. Hill’s counterattack saved the position just in the nick of time.

Command Analysis

Antietam is the battle with the highest casualties of any American battle. There were 24,000 casualties, including over 5000 deaths.

Strategically, the battle was a disaster for Lee. The casualties were catastrophic for an army of his size.

Army of the Potomac: 87,164 total troops

Casualties and losses: 12,410 total (2,108 killed, 9,549 wounded, 753 captured/missing)

Army of Northern Virginia: 38,000 troops

Casualties and losses: 12,051 total (3,281 killed, 7,752 wounded, 1,018 captured/missing)

The ANV retreated to Virginia. Lee was fortunate that he had not lost his entire army that day. The entire Union army had attacked just half of his men. McClellan did not pursue Lee to follow up on his victory. He allowed Lee to retreat without resistance, and that was his final error.

McClellan has been criticized ever since for advancing portions of his line at different times rather than at once and holding back an entire corps entirely. McClellan far outnumbered Lee but consistently sent in his reserves too late and held back too many men for fear that Lee outnumbered him. 

The notorious story is that the reserve troops belonging to McClellan’s most trusted corps commander, Gen. Fitz-John Porter were ready to attack the Sunken Road. McClellan was ready to commit these reserves under Porter, his most trusted corps commander. Porter supposedly slowly shook his head with disapproval and said to McClellan, “Remember, General! I command the last reserve of the last army of the Republic.” For this reason, McClellan did not commit these troops, which would have assuredly carried the day. Modern scholarship suggests that this never happened and that Porter was blamed after the fact. 

Two months later he was court-martialed for his actions at 2nd Bull Run and dismissed from the army. Then 16 years later, a special commission exonerated him and in 1886, his commission was restored to an infantry colonel. He enjoyed a very successful post-war career in New York City.

Burnside took many more hours than required to cross the creek. He might have found several places where his men could have waded across the river and not be concerned with the bridge, which was heavily covered by sharpshooters.

Lincoln urged McClellan to immediately follow up on his victory but he demurred once again. Lincoln met with him on the battlefield, then removed him from command. McClellan never again held a battle command commission. He went on to the Democratic nomination for president in 1864, losing in a landslide to Lincoln.

Also, the Emancipation Proclamation that followed and London’s decision not to get involved were the direct consequence of Lee’s failed invasion.

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