American Civil War: Battle of Antietam

Casualties near the Dunker Church, Battle of Antietam
Casualties near the Dunker Church, Battle of Antietam.

Library of Congress

The Battle of Antietam was fought September 17, 1862, during the American Civil War (1861-1865). In the wake of his stunning victory at the Second Battle of Manassas in late August 1862, General Robert E. Lee began moving north into Maryland with the goal of obtaining supplies and cutting the rail links to Washington. This move was endorsed by Confederate President Jefferson Davis who believed that a victory on Northern soil would increase the likelihood of recognition from Britain and France. Crossing the Potomac, Lee was slowly pursued by Major General George B. McClellan who had recently been reinstated to overall command of Union forces in the area.

Armies & Commanders



Battle of Antietam - Advancing to Contact

Lee's campaign was soon compromised when Union forces found a copy of Special Order 191 which laid out his movements and showed that his army was split into several smaller contingents. Written on September 9, a copy of the order was found at the Best Farm south of Frederick, MD by Corporal Barton W. Mitchell of the 27th Indiana Volunteers. Addressed to Major General D.H. Hill, the document was wrapped around three cigars and caught Mitchell's eye as it lay in the grass. Quickly passed up the Union chain of command and recognized as authentic, it soon arrived at McClellan's headquarters. Assessing the information, the Union commander commented, "Here is a paper with which, if I cannot whip Bobby Lee, I will be willing to go home." 

Despite the time-sensitive nature of the intelligence contained in Special Order 191, McClellan displayed his characteristic slowness and hesitated before acting on this critical information. While Confederate troops under Major General Thomas "Stonewall" Jackson were capturing Harpers Ferry, McClellan pressed west and engaged Lee's men in the passes through the mountains. In the resulting Battle of South Mountain on September 14, McClellan's men attacked the out-numbered Confederate defenders at Fox's, Turner's, and Crampton's Gaps. Though the gaps were taken, fighting lasted through the day and bought time for Lee to order his army to reconcentrate at Sharpsburg.

McClellan's Plan

Bringing his men together behind Antietam Creek, Lee was in a precarious position with the Potomac at his back and only Boteler's Ford to the southwest at Shepherdstown as an escape route. On September 15, when the lead Union divisions were sighted, Lee only had 18,000 men at Sharpsburg. By that evening, much of the Union army had arrived. Though an immediate attack on September 16 likely would have overwhelmed the scrambling Lee, the ever-cautious McClellan, who believed Confederate forces to number around 100,000, did not begin probing the Confederate lines until late that afternoon. This delay allowed Lee to bring his army together, though some units were still en route. Based on the intelligence gathered on the 16th, McClellan decided to open the battle the next day by attacking from the north as this would allow his men to cross the creek at the undefended upper bridge. The assault was to be mounted by two corps with an additional two waiting in reserve.

This attack would be supported by a diversionary attack by Major General Ambrose Burnside's IX Corps against the lower bridge south of Sharpsburg. Should the assaults proved successful, McClellan intended to attack with his reserves over the middle bridge against the Confederate center. Union intentions became clear on the evening of September 16, when Major General Joseph Hooker's I Corps skirmished with Lee's men in the East Woods north of town. As a result, Lee, who had placed Jackson's men on his left and Major General James Longstreet's on the right, shifted troops to meet the anticipated threat (Map).

The Fighting Begins in the North

Around 5:30 AM on September 17, Hooker attacked down the Hagerstown Turnpike with the goal of capturing the Dunker Church, a small building on a plateau to the south. Encountering Jackson's men, brutal fighting began in the Miller Cornfield and the East Woods. A bloody stalemate ensued as the outnumbered Confederates held and mounted effective counterattacks. Adding Brigadier General Abner Doubleday's division into the fight, Hooker's troops began to push the enemy back. With Jackson's line near collapse, reinforcements arrived around 7:00 AM as Lee stripped his lines elsewhere of men.

Counterattacking, they drove Hooker back and the Union troops were forced to cede the Cornfield and West Woods. Badly bloodied, Hooker called for aid from Major General Joseph K. Mansfield's XII Corps. Advancing in columns of companies, XII Corps was hammered by Confederate artillery during their approach and Mansfield was mortally wounded by a sniper. With Brigadier General Alpheus Williams in command, XII Corps renewed the assault. While one division was halted by enemy fire, Brigadier General George S. Greene's men were able to break through and reach the Dunker Church (Map).

While Greene's men came under heavy fire from the West Woods, Hooker was wounded as he tried to rally men to exploit the success. With no support arriving, Greene was forced to pull back. In an effort to force the situation above Sharpsburg, Major General Edwin V. Sumner was directed to contribute two divisions from his II Corps to the fight. Advancing with Major General John Sedgwick's division, Sumner lost contact with Brigadier General William French's division before leading a rash attack into the West Woods. Quickly taken under fire on three sides, Sedgwick's men were forced to retreat (Map).

Attacks in the Center

By mid-day, fighting in the north quieted as Union forces held the East Woods and the Confederates the West Woods. Having lost Sumner, French spotted elements of Major General D.H. Hill's division to the south. Though only numbering 2,500 men and tired from fighting earlier in the day, they were in a strong position along a sunken road. Around 9:30 AM, French began a series of three brigade-sized attacks on Hill. These failed in succession as Hill's troops held. Sensing danger, Lee committed his final reserve division, led by Major General Richard H. Anderson, to the fight. A fourth Union assault saw the famed Irish Brigade storm forward with its green flags flying and Father William Corby shouting words of conditional absolution. 

The stalemate was finally broken when elements of Brigadier General John C. Caldwell's brigade succeeded in turning the Confederate right. Taking a knoll that overlooked the road, Union soldiers were able to fire down the Confederate lines and force the defenders to retreat. A brief Union pursuit was halted by Confederate counterattacks. As the scene quieted around 1:00 PM, a great gap had been opened in Lee's lines. McClellan, believing that Lee had over 100,000 men, repeatedly refused to commit the over 25,000 men he had in reserve to exploiting the breakthrough despite the fact that Major General William Franklin's VI Corps was in position. As a result, the opportunity was lost (Map).

Blundering in the South

In the south, Burnside, angered by command rearrangements, did not begin moving until around 10:30 AM. As a result, many of the Confederate troops that had originally been facing him were withdrawn to block the other Union attacks. Tasked with crossing the Antietam to support Hooker's actions, Burnside was in position to cut off Lee's retreat route to Boteler's Ford. Ignoring the fact that the creek was fordable at several points, he focused on taking Rohrbach's Bridge while dispatching additional troops downstream to Snavely's Ford (Map)

Defended by 400 men and two artillery batteries atop a bluff on the western shore, the bridge became Burnside's fixation as repeated attempts to storm it failed. Finally taken around 1:00 PM, the bridge became a bottleneck which slowed Burnside advance for two hours. The repeated delays permitted Lee to shift troops south to meet the threat. They were supported by the arrival of Major General A.P. Hill's division from Harpers Ferry. Attacking Burnside, they shattered his flank. Though possessing greater numbers, Burnside lost his nerve and fell back to the bridge. By 5:30 PM, the fighting had ended.

Aftermath of the Battle of Antietam

The Battle of Antietam was the bloodiest single day in American military history. Union losses numbered 2,108 killed, 9,540 wounded, and 753 captured/missing while the Confederates suffered 1,546 killed, 7,752 wounded, and 1,018 captured/missing. The next day Lee prepared for another Union attack, but McClellan, still believing he was out-numbered did nothing. Eager to escape, Lee crossed the Potomac back into Virginia. A strategic victory, Antietam allowed President Abraham Lincoln to issue the Emancipation Proclamation which freed enslaved people in Confederate territory. Remaining idle at Antietam until late October, despite requests from the War Department to pursue Lee, McClellan was removed command on November 5 and replaced by Burnside two days later.

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Your Citation
Hickman, Kennedy. "American Civil War: Battle of Antietam." ThoughtCo, Apr. 5, 2023, Hickman, Kennedy. (2023, April 5). American Civil War: Battle of Antietam. Retrieved from Hickman, Kennedy. "American Civil War: Battle of Antietam." ThoughtCo. (accessed May 28, 2023).