Atomic Bomb

The atomic bomb is a nuclear weapon that derives its explosive force from nuclear fission, the splitting of heavy atomic nuclei such as uranium-235 or plutonium-239, releasing an amount of energy per weapon orders of magnitude beyond any conventional explosive and first developed by the United States during World War II under the secret Manhattan Project. The only combat use of atomic weapons in history came in August 1945, when the United States dropped the uranium-based bomb "Little Boy" on Hiroshima and the plutonium-based "Fat Man" on Nagasaki, killing well over a hundred thousand people, most of them civilians, in the immediate blasts and firestorms, with tens of thousands more dying afterward from radiation exposure and injuries; Japan's surrender days later ended World War II. The weapon's introduction fundamentally transformed the calculus of international conflict, and its subsequent development by the Soviet Union, Britain, France, China and other states drove the Cold War nuclear arms race and the doctrine of mutually assured destruction, under which the sheer destructive scale of nuclear arsenals was argued to deter their own use between major powers for the following decades.