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ON SEPTEMBER 17, 1862, General Robert E. Lee’s Army of Northern Virginia and Maj. Gen. George B. McClellan’s Army of the Potomac fought a bitter, daylong battle near the town of Sharpsburg, Md. The fight, called the Battle of Antietam by the Federals and the Battle of Sharpsburg by the Confederate forces, resulted in an appalling 23,000 casualties, The engagement remains the bloodiest day in American history.

McClellan’s roughly 80,000-man army outnumbered Lee’s 38,000 troops, but the Confederates managed to hold off the Union attacks that began at dawn and continued to sunset. A New York soldier remembered the “air was full of the hiss of bullets and the hurtle of grape-shot.”

Lee’s army held its ground at the end of the day, but his battered force had to retreat into Virginia on September 18, ending his first campaign north of the Potomac River. The Lincoln administration seized the opportunity to cast the battle as a victory for the Union cause.

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