Bloodiest Day in American History - The Battle of Antietam

"The battle was over by 5:30 p.m. Losses for the day were heavy on both sides. The Union had 12,401 casualties with 2,108 dead. Confederate casualties were 10,318 with 1,546 dead. This represented 25% of the Federal force and 31% of the Confederate. More Americans died on September 17, 1862, than on any other day in the nation's military history, including World War II's D-Day and the terrorist attacks of September 11, 2001." --- wiki

Battle of Antietam - wiki