A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either nuclear fission or a combination of fission and nuclear fusion reactions, producing a nuclear explosion. Both bomb types release large quantities of energy from relatively small amounts of matter. Nuclear warfare is widely considered a global catastrophic risk. Nine sovereign states are known or believed to possess nuclear weapons as of 2026: the United States, Russia, the United Kingdom, France, China, India, Pakistan, North Korea and Israel.