What was the most deaths and casualties in one day in a war in American history?
The bloodiest day in American history was August 6, 1945 at Hiroshima, Japan. Over 70,000 people died immediately on impact of "Little Boy", the first atom bomb. Nearly twice that number died later due to the after effects. The second bloodiest day was August 9th, 1945 at Nagasaki, Japan, where between 45,000 and 75,000 people were killed. As for American deaths, the bloodiest day was September 17, 1862 at Antietam - 3,654 deaths, over 23,000 casualties. 9/11 is second in line. However, 100,000 Japanese were killed in the firebombing of Tokyo in World War 2.