The Battle of Antietam 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 Union soil. It is the bloodiest single-day battle in American history, with a combined tally of dead, wounded, and missing at 22,717.
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, Union 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, Confederate 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 Potomac River.
Despite having superiority of numbers, McClellan's attacks failed to achieve force concentration, allowing Lee to counter by shifting forces and moving 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. McClellan had halted Lee's invasion of Maryland, but Lee was able to withdraw his army back to Virginia without interference from the cautious McClellan. Although the battle was tactically inconclusive, the Confederate troops had withdrawn first from the battlefield, making it, in military terms, a Union victory.
The cache can be found at N34 30.943 w84 0x.x18
XX= the number of McClellan's men present for duty (in thousands) minus 12