A battle in the American Civil War, fought in Maryland. After his victory at the second battle of Bull Run, General Lee invaded the North, but with only 30,000 men under his immediate command was attacked by a Union (Northern) army under General George McClellan at Sharpsburg on the Antietam Creek. Although the Confederates were badly mauled, they held their positions and made an orderly retreat on the following day. The casualties of 23,000 (divided almost equally between the two sides) were the worst of any single day of the war.