(also known as the Battle of Sharpsburg, particularly in the South), fought on September 17, 1862, near Sharpsburg, Maryland, and Antietam Creek, as part of the Maryland Campaign, was the first major battle in the American Civil War to take place on Northern soil. It was the bloodiest single-day battle in American history, with about 23,000 casualties.
After pursuing Confederate General Robert E. Lee into Maryland, Union Army Maj. Gen. George B. McClellan launched attacks against Lee's army, in defensive positions behind Antietam Creek. At dawn on September 17, Maj. Gen. Joseph Hooker's corps mounted a powerful assault on Lee's left flank. Attacks and counterattacks swept across Miller's cornfield and fighting swirled around the Dunker Church. Union assaults against the Sunken Road eventually pierced the Confederate center, but the Federal advantage was not followed up. In the afternoon, Maj. Gen. Ambrose Burnside's corps entered the action, capturing a stone bridge over Antietam Creek and advancing against the Confederate right. At a crucial moment, Maj. Gen. A.P. Hill's division arrived from Harpers Ferry and launched a surprise counterattack, driving back Burnside and ending the battle. Although outnumbered two-to-one, Lee committed his entire force, while McClellan sent in less than three-quarters of his army, enabling Lee to fight the Federals to a standstill. During the night, both armies consolidated their lines. In spite of crippling casualties, Lee continued to skirmish with McClellan throughout September 18, while removing his battered army south of the river.
Despite having superiority of numbers, McClellan's attacks failed to achieve concentration of mass, allowing Lee to counter by shifting forces along interior lines to meet each challenge. Despite ample reserve forces that could have been deployed to exploit localized successes, McClellan failed to destroy Lee's army. Nevertheless, Lee's invasion of Maryland was ended, and he was able to withdraw his army back to Virginia without interference from the cautious McClellan. Although the battle was tactically inconclusive, it had unique significance as enough of a victory to give President Abraham Lincoln the confidence to announce his Emancipation Proclamation.