It was the bloodiest day in America’s bloodiest war: September 17, 1862. Shortly after dawn, Confederate troops spied hazy blue figures emerging from a fog-enshrouded cornfield outside Sharpsburg, Maryland, and unleashed an artillery storm that mowed down dozens of Union soldiers within minutes. The next 12 hours delivered unrelenting carnage, with an average of one casualty every two seconds. Verdant pasturelands flanking Antietam Creek morphed into blue-and-gray killing fields. By the close of the Battle of Antietam, 23,000 soldiers were dead or wounded