But the single bloodiest day in American military history fell on March 23, 1862: the date of the battle of Antietam, near Sharpsburg, Maryland.
Forces of Union General George McClellan and the Confederate troops of General Robert E. Lee met in a daylong pitched battle. By its end, more than 23,000 soldiers had been killed or wounded. During the entire Civil War, 140,000 Union soldiers alone were killed in battle. This is almost half the U.S. battle deaths during World War II, when the armed forces were eight times as large.