Fat Man and Little Boy
Fat Man and Little Boy, both weapons of unparalleled destructive power, were actually quite different. Little Boy, fueled by highly enriched uranium-235, was triggered by a simple "gun" mechanism; a small, slug-shaped piece of uranium was fired down a barrel into a larger, cup-shaped piece. This elementary design generated a destructive force of about 15 kilotons—the equivalent of 15,000 tons of TNT.
A much more complex implosion-type device triggered Fat Man. It consisted of a plutonium core surrounded by high explosives wired to explode simultaneously. The shock waves from these conventional explosions triggered the fission of the plutonium, which yielded a 22 kiloton explosion.
The Controversy
U.S. military officials believed that such a massive demonstration of U.S. military power was the only reasonable way to force an unconditional Japanese surrender. Though the islands' supply lines had been cut, the Japanese air force was a shambles, and Tokyo was nearly in ruins, it was still widely believed that no conventional military action short of an invasion could make Japan surrender. In her entire history, Japan had never been invaded or defeated. Even after the destruction of Hiroshima, she refused to capitulate.
The decision to bomb Hiroshima and Nagasaki—the first and last use of atomic weapons in combat—remains one of the most controversial in military history. Altogether, the two bombings killed an estimated 110,000 Japanese citizens and injured another 130,000. By 1950, another 230,000 Japanese had died from injuries or radiation. Though the two cities were nominally military targets, the overwhelming majority of the casualties were civilians.