A nuclear weapon derives its destructive force from nuclear reactions of fission or fusion that release vast quantities of energy from small amounts of matter. A modern thermonuclear weapon weighing over a thousand kilograms can produce an explosion comparable to over a billion kilograms of conventional explosives. They were developed during World War II under the Manhattan Project led by Robert Oppenheimer and resulted in the atomic bombings of Hiroshima and Nagasaki that killed over 150,000 people. Nuclear weapons pose dangers such as radiation exposure and their use could damage cities and countries, which is why some countries seek to limit their proliferation.