September 17, 1862. One day. 22,717 casualties. The Battle of Antietam remains the bloodiest single day in American military history.
Why Lee Invaded the North
By the late summer of 1862, Robert E. Lee's Army of Northern Virginia had won a series of stunning victories — the Seven Days Battles, Second Bull Run — and had taken the strategic initiative from the Union. Lee proposed an invasion of the North for several reasons: to feed his army off the Virginia countryside he'd been exhausting, to influence the 1862 midterm elections in the North, to potentially capture a northern city and demonstrate Confederate strength, and to generate the European recognition that Jefferson Davis believed would force a negotiated peace.
The Lost Order
One of the most consequential accidents in American military history occurred on September 13, 1862, when Union soldiers found a copy of Lee's battle plan — Special Orders No. 191 — wrapped around three cigars in a field where Confederate soldiers had camped. General George McClellan, commanding the Union Army of the Potomac, now knew exactly how Lee had divided his army and where each element was. He failed to act on the intelligence with sufficient speed, giving Lee time to concentrate his forces. If McClellan had moved faster, Antietam might never have been fought on those terms.
The Battle: Three Attacks, One Catastrophic Day
The battle of September 17 can be divided into three phases, each fought in a different part of the field at roughly sequential times.
In the morning, Union forces under General Joseph Hooker attacked Lee's left flank in the area of the Cornfield and the East Woods. The fighting was ferocious and inconclusive — the Cornfield changed hands fifteen times in a single morning. By noon, both sides had suffered thousands of casualties and the Union had gained little ground.
In the midday phase, Union forces under General William Sumner attacked the Confederate center through a sunken road that the Confederates were using as a natural trench. After hours of slaughter, Union troops finally flanked the road and broke the Confederate line. Approximately 5,500 men fell in the Sunken Road — afterward called "Bloody Lane" — in a few hours of fighting.
In the afternoon, Union forces under General Ambrose Burnside spent hours trying to cross a stone bridge over Antietam Creek that could have been forded. Once across, Burnside's men drove the Confederate right back toward Sharpsburg — and then were counterattacked by A.P. Hill's division, arriving from Harpers Ferry in a forced march that took them 17 miles in eight hours. The Union advance stopped.
The Aftermath and the Emancipation Proclamation
Antietam was tactically a draw. Lee's army retreated back to Virginia, which McClellan called a Union victory. McClellan, despite holding a numerical advantage of roughly 2-to-1 throughout the battle, failed to commit his reserves, refused to pursue Lee's retreating army, and was relieved of command six weeks later.
But the strategic consequences of Antietam were decisive. Lincoln had been waiting for a Union military success to issue the Emancipation Proclamation without it appearing to be an act of desperation. Antietam gave him that pretext. On September 22, 1862, Lincoln issued the preliminary Emancipation Proclamation, declaring that enslaved people in Confederate states would be "forever free" as of January 1, 1863. The war's character changed permanently.
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Frequently Asked Questions
Why is Antietam called America's bloodiest day?
Antietam produced approximately 22,717 casualties — killed, wounded, captured, or missing — in a single day of fighting on September 17, 1862. This remains the highest single-day casualty total in American military history.
Who won the Battle of Antietam?
Tactically, Antietam was a draw — Lee's army was not destroyed, and McClellan held ground but did not pursue. Strategically, it was a Union victory because it ended Lee's invasion of the North and gave Lincoln the military success he needed to issue the Emancipation Proclamation.
What was the significance of the Emancipation Proclamation at Antietam?
Lincoln issued the preliminary Emancipation Proclamation on September 22, 1862, five days after Antietam. By doing so after a Union military success (however ambiguous), he reframed the war as a fight against slavery, which discouraged European intervention on the Confederate side and gave Union soldiers a more explicit moral purpose.
