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September 17, 1862, was the bloodiest day in the history of the United States.
In just 12 hours, 22,726 people were killed, wounded, or went missing just outside of Sharpsburg, Maryland, in one of the largest battles of the US Civil War.
Despite ending in a draw, the Union’s ability to hold off the Confederates was a major win, with significant political ramifications that helped shift the course of the conflict.
Learn about the Battle of Antietam on this episode of Everything Everywhere Daily.
By the late summer of 1862, the Civil War appeared to be going increasingly well for the Confederacy in the eastern theater.
Earlier that year, Union Major General George B. McClellan had launched the Peninsula Campaign, attempting to capture Richmond by advancing up the Virginia Peninsula. McClellan came close to the Confederate capital, but Robert E. Lee, who assumed command of what became the Army of Northern Virginia in June, drove him away during the Seven Days Battles.
Lee then turned north against Major General John Pope’s newly created Army of Virginia. At the Second Battle of Bull Run, fought August 28 through 30, Lee inflicted another serious defeat on the Union.
The Confederates now had the initiative.
Lee believed there were several reasons to carry the war north into Maryland. Northern Virginia had been ravaged by months of campaigning, and moving into Maryland would allow Virginia farms some respite while Confederate forces obtained food and supplies from Northern territory. An invasion could also threaten Baltimore, Washington, and perhaps Pennsylvania.
There were political objectives as well. Maryland was a slave state that had remained in the Union, and Confederate leaders hoped that Confederate troops might attract Marylanders who were sympathetic to secession.
Lee thought Marylanders would welcome the Confederate army. His belief was reasonable given the state’s previous reactions towards President Lincoln. He specifically felt that the Baltimore riots in 1861 and the fact that the president had to travel through Baltimore in disguise for his inauguration were evidence of Southern sympathy.
But the welcome was colder than expected. By 1862, many people in Maryland had changed their tune and supported the Union. This meant that, instead of giving the Confederate armies a warm welcome, the citizens mostly stayed indoors or just watched the army pass by.
More important than their reception, a dramatic Confederate victory on Northern soil might strengthen peace advocates in the North and encourage Britain or France to recognize the Confederacy diplomatically.
The stakes were therefore far greater than control of a few miles of Maryland countryside. Beginning in early September, approximately 40,000 to 50,000 Confederate soldiers crossed the Potomac River into Maryland.
Lee concentrated much of his army near Frederick, Maryland. But an important problem soon emerged behind him. The Union garrison at Harpers Ferry, located where the Shenandoah River meets the Potomac, remained in Confederate lines of communication. Lee could not safely continue north while thousands of Union troops occupied the town.
He therefore made a dangerous decision. He divided his already relatively small army.
On September 9, Lee issued Special Orders No. 191, instructing different portions of the Army of Northern Virginia to move in separate directions. Thomas “Stonewall” Jackson and other Confederate forces would converge on Harpers Ferry, while other commands guarded mountain passes and supply routes. Lee expected the separated portions of his army to reunite afterward.
Dividing an army in enemy territory was risky, but Lee had repeatedly demonstrated a willingness to accept such risks. By splitting the troops, Lee believed he could better defend his supply and communication lines, allowing his invasion of the North to proceed more smoothly.
Harpers Ferry was so important because it was home to a crucial Union garrison. By capturing it, Lee could replenish supplies and secure his supply line.
On September 13, Union soldiers from the 27th Indiana Infantry were resting near Frederick when they discovered an envelope containing three cigars wrapped in a piece of paper.
The paper turned out to be a copy of Special Orders No. 191. The document revealed the disposition and movements of Lee’s divided army.
Corporal Barton W. Mitchell is usually credited with physically finding the orders, although the exact circumstances have generated considerable historical debate.
The document eventually reached McClellan.
For a commander famous for believing that Confederate armies were much larger than they really were, the discovery was an extraordinary intelligence coup. McClellan suddenly possessed information that commanders almost never receive: a detailed description of the enemy’s operational movements while the enemy army was divided.
McClellan reportedly recognized the opportunity immediately.
Between Frederick and the area around Sharpsburg lies South Mountain, an extension of the Blue Ridge Mountains. To reach Lee, McClellan’s Army of the Potomac had to force its way through mountain gaps guarded by Confederate troops.
On September 14, Union and Confederate soldiers fought at several passes, including Turner’s Gap, Fox’s Gap, and Crampton’s Gap. Union forces eventually broke through.
At Crampton’s Gap, the Union VI Corps overwhelmed Confederate defenders and entered Pleasant Valley. Across the day’s fighting, Union losses were roughly 2,300 while Confederate losses exceeded 4,000.
Lee briefly considered abandoning the campaign. Instead, he chose to make a stand near Sharpsburg, a small Maryland town just east of the Potomac River.
Meanwhile, Stonewall Jackson was completing the Confederate operation against Harpers Ferry.
The Union position was dominated by surrounding heights. Confederate artillery positioned on those heights made the garrison’s position untenable.
On September 15, the Union garrison surrendered. More than 12,000 Union soldiers became prisoners in one of the largest Union surrenders of the war.
Jackson then hurried most of his forces toward Lee at Sharpsburg. This development was crucial. Without Jackson’s reinforcements, Lee might have been unable to remain in Maryland.
Lee positioned the Army of Northern Virginia on high ground immediately west of Antietam Creek, with Sharpsburg behind him and the Potomac River farther west.
It was a dangerous position because retreat depended largely upon crossings of the Potomac. But it also offered advantages.
The creek itself was a strong natural barrier because it was deep and fast-moving. Any army trying to cross had to use one of three bridges, making the spot easy to defend.
Lee also joined up with Jackson’s men, which nearly doubled his forces. Still, the Confederates were outnumbered, with only 30,000 men against 53,000 Union troops.
On the afternoon of September 16, McClellan made the first move. He sent General Joseph Hooker’s troops across the creek to find the Confederate left flank. Hooker’s men were intercepted around dusk, and both sides skirmished until nightfall.
The main battle began the next morning, September 17, and lasted twelve hours. McClellan realized that attacking Lee’s center would fail, so he chose to attack the flanks first.
General Hooker took his Union troops and launched an assault against the Confederate left flank, pushing south to gain the high ground. The attack lasted for roughly 2.5 hours.
Ultimately, the initial attack ended when the Confederates ran out of additional reinforcements, causing them to fall back. This allowed the Union to secure the high ground advantage.
A group led by General John Sedgwick, attacked the other Confederate flank. The Union soldiers didn’t know that two Confederate divisions were waiting there, that being the men from the Harpers Ferry attack. The surprise caused heavy Union losses and forced them to retreat.
Another group of Union soldiers was caught up elsewhere. Some Union soldiers got caught fighting in a sunken road. It was a natural trench worn down by years of agricultural work. This allowed 2,600 Confederate soldiers to defend the position against 10,000 Union soldiers for hours. The sunken road at Antietam became known as the Bloody Lane because 5,000 soldiers died along its 800 yards.
While Union soldiers fought at the Sunken Road, a group led by General Ambrose Burnside attacked the Confederate right. Burnside and his men captured a key bridge and got into position to attack the weakened Confederate right flank. After regrouping, Burnside launched an attack that pushed the Confederates out.
However, Burnside’s group couldn’t finish off the Confederate army because Lee’s reinforcements arrived just in time.
Even with heavy attacks, McClellan and the Union army couldn’t break through the Confederate defenses to attack the center. As a result, many Union soldiers didn’t take part in the main fighting, neutering the Union’s numerical advantage.
The Confederates managed to hold their ground until nightfall.
When the battle ended, the scene was horrific. Thousands of bodies lay scattered across the ground. At the end of the day, there were 23,000 casualties on both sides, and neither side was able to claim they won a decisive battle. Local churches, homes, and barns became field hospitals, where people did their best to care for the wounded.
After the main battle, McClellan and Lee fought small skirmishes on September 18. By the 19th, Lee saw that McClellan had stopped chasing him, so he crossed the Potomac River back into Virginia.
As Lee and his men retreated, McClellan sent General Fitz John Porter to follow them. However, Porter moved too cautiously and was pushed back at the Battle of Shepherdstown.
President Lincoln was deeply frustrated that McClellan did not aggressively pursue the retreating Confederates. Lee’s army was exhausted and badly depleted as it crossed the Potomac.
A rapid pursuit might have inflicted further damage. Instead, McClellan remained cautious. His continuing reluctance to move eventually exhausted Lincoln’s patience. On November 5, 1862, Lincoln removed McClellan from command of the Army of the Potomac. Ambrose Burnside replaced him. Within weeks, Burnside would lead the Union army to disaster at Fredericksburg.
Technically, the Battle of Antietam was a draw. But President Lincoln and the Union called it a strategic victory because it stopped the South from invading further and slowed their momentum.
The greatest consequence of Antietam may have had relatively little to do with the battlefield.
For months, Abraham Lincoln had been moving toward emancipation as a war policy. By the summer of 1862, he had prepared a proclamation declaring enslaved people in Confederate-controlled territory free
.
But Secretary of State William Seward advised Lincoln not to announce it following a series of Union military defeats. Doing so could make emancipation appear to be a desperate measure taken by a government losing the war.
Lincoln was waiting for a military success, and Antietam provided it.
Five days after the battle, on September 22, 1862, Lincoln issued the Preliminary Emancipation Proclamation. It announced that on January 1, 1863, enslaved people in states or portions of states still in rebellion against the United States would be declared free.
This also affected European recognition of the Confederacy.
Both France and Britain had strong economic ties to the American South, particularly through cotton, but slavery was deeply unpopular among large parts of the British public. Once the Union openly linked its war effort to emancipation, diplomatic recognition of the Confederacy became much more politically difficult.
A Confederate victory in Maryland might have strengthened the case for European intervention. Instead, Antietam weakened it.
Antietam also changed the way Americans perceived warfare. Photographer Alexander Gardner, working for Mathew Brady’s photographic enterprise, arrived on the battlefield shortly after the fighting.
Gardner photographed dead soldiers where they had fallen. These were not romantic paintings of heroic charges. They were corpses. Gardner’s photographs were subsequently exhibited at Brady’s gallery in New York. For Americans accustomed to newspaper illustrations and idealized depictions of warfare, the images were shocking.
Antietam was a battle without a clear tactical victor, yet its consequences were enormous. Lee’s first invasion of the North had been stopped, but McClellan had allowed the Army of Northern Virginia to escape and fight another day. The price was nearly 23,000 casualties in just twelve hours of combat, making September 17, 1862, the bloodiest single day in American military history.