Atomic bombs are nuclear weapons that generate their power from fission (in contrast to more powerful hydrogen bombs that use a combination of fission and fusion.) During the final days of World War II, the United States detonated atomic bombs on the cities of Hiroshima and Nagasaki. On August 6, 1945 an American B-29 bomber named “Enola Gay” detonated “Little Boy” over the city of Hiroshima, Japan. The United States detonated a second atomic bomb (“Fat Man”) on August 9, 1945 over the city of Nagasaki, Japan. The purpose of these bombings was to render an Allied invasion of mainland Japan unnecessary. The goal was to force the Empire of Japan to surrender with the threat of additional atomic bombings. The United States accomplished this when Japan surrendered on August 15, 1945. The casualty estimated for Nagasaki ranges from 40,000–80,000 people, while in Hiroshima there was an estimated 123,000 casualties, the majority of whom were civilians. The necessity and morality of these bombings (and of any use of nuclear weapons) remains a subject of debate today.