At the Battle of Antietam Creek near Sharpsburg in western Maryland, General Lee's first invasion of the North was halted by Union troops under General McClellan. This was the bloodiest one day battle of the civil War. Each side lost over 2000 killed and 9000 wounded.
Carruth, Gorton. "The Encyclopedia of American Facts and Dates". 10th Ed. New York: Harper Collins Publishers. ©1997.