The Battle of Antietam: Bloodiest Day in the American History


By Gary W. Gallagher, Ph.D., University of Virginia

In 1862, Robert Lee sent out orders for the Confederate Army to gather at Sharpsburg. By September 17, Lee had almost 35,000 troops in place. One division was back at Harper’s Ferry overseeing the parole of the 12,000 Federals who had surrendered and also looking after all the supplies that had been captured. But why did Union Army’s George McClellan let precious time slip by and not hit Lee when he was really vulnerable?

Picture of the maps depicting the battle fields of Antietam.
The Battle of Antietam broke down into three very separate battles.
(Image: steve estvanik/Shutterstock)

Battle of Antietam

On September 17, 1862, McClellan had more than 70,000 troops in position at Antietam, a two-to-one advantage over Lee. His plan was to apply pressure to both Confederate flanks, weaken the center, and then punch through the center and cut Lee off from the Potomac River, a few miles away. There was only one ford across the Potomac. If anything serious had happened to Lee’s army at Sharpsburg, it would have been an utter disaster. That’s what McClellan hoped to do, but he didn’t apply simultaneous pressure against the line.

Segregated Battle of Antietam

The Battle of Antietam broke down into three very separate battles. It began on the northern end of the field with very heavy Union attacks that were held off by the slimmest of margins on the Confederate side. Then it shifted to the center part of the field, with continued heavy attacks. McClellan in fact broke through in the center, and the officers in command of his troops begged for reinforcements to exploit the break.

However, though McClellan wanted to do so, he decided not to commit his reserves as he considered it to be a dangerous move. So the fighting died out in the center part of the field, and shifted to the left in the afternoon, the southern end of the field. The Federals literally pushed the Confederates to within a few dozen yards of the key road that led to the fords over the Potomac River

However, just when the Federals were about to win, the last division from Harper’s Ferry, marching a 17-mile march, came onto the battlefield, immediately deployed and plowed into the Union left flank, and stopped the final attacks. The battle sputtered to a halt late in the afternoon of the seventeenth.

It had been a series of near disasters for Lee and his army. Lee had been very active, moving back and forth along the line from heavy fighting in a cornfield on the northern end of the field, watching the fighting at what was called the Sunken Road or the Bloody Lane in the middle of the field, and watching anxiously in the afternoon as his right flank seemed about to crumble. Time and again he had plugged in troops at just the last moment to stave off disaster. Good luck and effective management had allowed him to keep his army intact.

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Bloodiest Day in American History

A photograph showing multiple dead bodies of Confederate troops lying on the ground during the Battle of Antietam.
During the Battle of Antietam, thousands of soldiers lost their lives in just one day. (Image: Alexander Gardner/Public domain)

As for McClellan, fully a quarter of his army didn’t fire a shot during the Battle of Antietam, while Lee mustered every man he had. McClellan didn’t use a good part of his army, and, in the end, Lee was able to hold on.

However, it was the bloodiest day in American history. 10,500 Confederates, a third of Lee’s army, were shot down at Antietam in one day, as were 12,500 Federals, a total of more than 23,000.

There were more than 8,000 casualties in a 23 acre cornfield. One Confederate regiment, the First Texas, suffered more than 80 percent casualties in about ten minutes. In a small country lane in the center of the battlefield, the sunken lane, later called Bloody Lane, Confederate bodies lay so thickly packed that a Union officer said that he walked for more than 100 yards without ever touching the ground in that lane, just going from body to body.

Painting by Captain James Hope which shows the dead troops on the battlefield at the Bloody Lane.
A small sunken country lane in the center of the battlefield, later called Bloody Lane, was filled with bodies of Confederate soldiers.
(Image: Captain James Hope (d.1892)/Public domain)

A Pennsylvania solider said, “No tongue can tell, no mind conceive, no pen portray the horrible sights I witnessed.” Photographers got to the battlefield in time to take pictures of the corpses of the dead Confederates. The Union dead had been buried. Those pictures caused a sensation in the North as no one had seen what a battlefield really looked like before. They’d seen heroic woodcuts and so forth. Here was the true human debris of a battlefield.

Learn more about how modern Americans view the people and events of 1861-65.

Withdrawal of Military Campaign

Lee remained on the field through most of September 18, but McClellan did not press him anymore. It was a very risky move on Lee’s part, but he got away with it because McClellan chose not to renew the attacks. Lee withdrew that night toward the fords over the Potomac. McClellan let him go and that was the end of the military side of the campaign.

Mixed Impact of Military Campaign

The consequences and impact of all this were immense. The military consequences were mixed. The battle itself was a tactical standoff. Neither side drove the other from the field. It was very bloody, but there was no real decisive decision on the battlefield. It was not interpreted as a great defeat in the Confederacy at the time, but seen as a drawn battle.

Lee didn’t retreat until a full day later, taking up a position along the Potomac River, and the military frontier remained where it had been following the Battle of Second Manassas. McClellan chose not to pursue Lee, which frustrated Lincoln. He visited McClellan and the army, but could not get McClellan to move. Eventually, in early November, the day after the elections in the North, Lincoln removed McClellan.

So though militarily it was a murky picture, the fact that Lee did retreat and lost the momentum that he had generated with the Seven Days and Second Manassas meant that it was seen as a Union victory.

Consequences on Diplomatic Front

On the diplomatic front, Great Britain edged toward intervention in the war. In fact, on the day of the battle, key British leaders had said that if Lee won another victory, they would mediate an end to this war. When they found out that Lee had retreated, they pulled back from that idea to see what the Confederates would do in the next campaign. It was harder for them to intervene because of another thing that happened right after Antietam, and that was Lincoln’s issuance of the preliminary Proclamation of Emancipation.

Learn more about what made the young men bear the war’s awful dangers and hardships.

Proclamation of Emancipation

There was enough of victory for Lincoln to issue his preliminary Proclamation of Emancipation. This was far more important than just the diplomatic side of what had happened. Once the proclamation was issued, it became difficult for France and Great Britain to come in on the side of the Confederacy because they had ended slavery, and it would be hard for them to support an overtly slaveholding republic in its contest against a nation that was partly on record against the institution of slavery.

What Preliminary Proclamation Meant?

The preliminary proclamation meant that Northern war aims had changed. The war for the Union had become a war for union and freedom because, wherever Union forces marched now, they would be taking the possibility of freedom with them. The stakes were much higher in the war. The whole social fabric of the South was now on the table. If the Confederacy lost the war, they would lose slavery, turning their whole social system upside down. For the white South, there was no more status quo antebellum after the Emancipation Proclamation. It changed the whole nature of the war.

Common Questions about the Battle of Antietam

Q: Who won the battle of Antietam?

Though there were heavy causalities on both the sides, Robert Lee lost the momentum and retreated. It was seen in that light as a Union victory.

Q: What was one major result of the Battle of Antietam?

Since Lee retreated and lost the momentum that he had generated with the Seven Days and Second Manassas, Battle of Antietam was seen as a Union victory. This victory encouraged Lincoln to issue his preliminary Proclamation of Emancipation.

Q: What was the bloodiest event in the American History?

Battle of Antietam is considered to be the bloodiest event in the American History. 10,500 Confederates and 12,500 Federals died in one day.

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