The Battle of Antietam, also known as the Battle of Sharpsburg, particularly in the Southern United States, is a battle of the American Civil War, fought on September 17, 1862, between Confederate General Robert E. Lee's Army of Northern Virginia and Union General George B. McClellan's Army of the Potomac, near Sharpsburg, Maryland and Antietam Creek.

Part of the Maryland Campaign, it is the first field army–level engagement in the Eastern Theater of the American Civil War to take place on Union soil.

It is the bloodiest day in United States history, with a combined tally of twenty-two thousand seven hundred and seventeen dead, wounded, or missing.

After pursuing the Confederate general Robert E. Lee into Maryland, Maj. Gen. George B. McClellan of the Union Army launches attacks against Lee's army, in defensive positions behind Antietam Creek.

At dawn on September 17, Maj. Gen. Joseph Hooker's corps mount a powerful assault on Lee's left flank.

Attacks and counterattacks sweepacross Miller's Cornfield, and fighting swirls around the Dunker Church.

Union assaults against the Sunken Road eventually pierce the Confederate center, but the Federal advantage is not followed up.

In the afternoon, Union Maj. Gen. Ambrose Burnside's corps enter the action, capturing a stone bridge over Antietam Creek and advancing against the Confederate right.

At a crucial moment, Confederate Maj. Gen. A. P. Hill's division arrives from Harpers Ferry and launches a surprise counterattack, driving back Burnside and ending the battle.

Although outnumbered two-to-one, Lee commits his entire force, while McClellan sends in less than three-quarters of his army, enabling Lee to fight the Federals to a standstill.

During the night, both armies consolidate their lines

In spite of crippling casualties, Lee continues to skirmish with McClellan throughout September 18, while removing his battered army south of the Potomac River.

Despite having superiority of numbers, McClellan's attacks fail to achieve force concentration, which allows Lee to counter by shifting forces and moving along interior lines to meet each challenge.

Therefore, despite ample reserve forces that could have been deployed to exploit localized successes, McClellan fails to destroy Lee's army.

McClellan's persistent but erroneous belief that he is outnumbered contributes to his cautiousness throughout the campaign.

McClellan has halted Lee's invasion of Maryland, but Lee is able to withdraw his army back to Virginia without interference from the cautious McClellan.

McClellan's refusal to pursue Lee's army leads to his removal from command by President Abraham Lincoln in November.

Although the battle is tactically inconclusive, the Confederate troops had withdrawn first from the battlefield, and abandoned their invasion, making it a Union strategic victory.

It is a sufficiently significant victory to give Lincoln the confidence to announce his Emancipation Proclamation, which discouragesthe British and French governments from pursuing any potential plans to recognize the Confederacy.