The Bloodiest Day in American History Came in 1862 As recently as 2009, the remains of soldiers who were interred on the field, their final resting places forgotten, have been rediscovered. Here's What You Need To Remember: Even though McClellan knew Lee's plans in advance, the Battle of Antietam was drawn-out - and horrific. Some units sustained casualties approaching three-fourths. During the September 17, 1862 Battle of Antietam, casualties piled almost too high to count. The culmination of the first invasion of the North during the American Civil War by General Robert E. Lee and the Confederate Army of Northern Virginia, it was the bloodiest single day in the history of the United States and the bloodiest day’s fighting ever in the Western Hemisphere. Due to the massive scale of the losses, the Antietam death toll has always been hard to accurately determine.