On September 17, 1862, the forces of Major General George B. McClellan and his Union Army of the Potomac confronted Robert E. Lee's entire Army of Northern Virginia at the Battle of Antietam in Sharpsburg, Maryland. The Union forces mounted a powerful assault on Lee's left flank in the idyllic Miller Cornfield. It was the single bloodiest day in the history of the Civil War. The elite combat units of the Union's Iron Brigade and the Confederate Texas Brigade held a dramatic showdown and suffered immense losses through vicious attacks and counterattacks sweeping through the cornstalks.