A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either nuclear fission (fission or atomic bomb) or a combination of fission and nuclear fusion reactions (thermonuclear weapon), 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. The majority of nuclear weapons have energy yields between 100 and 1,000 kilotons of TNT. Yields in the low kilotons can destroy cities. The effects of nuclear weapons include extreme blast damage, heat, ionizing radiation, and firestorms, followed by radioactive nuclear fallout, an electromagnetic pulse, radar blackout, and acute or chronic radiation syndrome in humans and animals. Hundreds of nuclear explosions would likely cause nuclear winter and nuclear famine. The first nuclear weapons were developed by the United States in collaboration with the United Kingdom and Canada during World War II in the Manhattan Project. Production requires a large scientific-industrial complex, primarily for fissile material, either nuclear reactors and reprocessing plants producing plutonium, or uranium enrichment facilities. Nuclear weapons have been used twice in war, in the 1945 atomic bombings of Hiroshima and Nagasaki that killed between 150,000 and 246,000 people. Nuclear deterrence, including mutually assured destruction, aims to prevent nuclear warfare via the threat of unacceptable damage and the danger of escalation to nuclear holocaust. A nuclear arms race for weapons and their delivery systems was a defining component of the Cold War. Nuclear weapons have had yields between 50 megatons for the Tsar Bomba, and 10 tons for the W54. Strategic nuclear weapons are targeted against civilian, industrial, and military infrastructure, while tactical nuclear weapons are intended for battlefield use. Strategic weapons led to the development of dedicated intercontinental ballistic missiles, submarine-launched ballistic missile, and nuclear strategic bombers, collectively known as the nuclear triad. Tactical weapons options included shorter-range ground-, air-, and sea-launched missiles, artillery, demolition munitions, torpedoes, and depth charges, but they have become less salient since the end of the Cold War. Two nuclear-armed states have nuclear sharing arrangements, stationing their weapons in six other countries: US weapons are deployed in Belgium, Germany, Italy, Netherlands, and Turkey; Russian weapons are deployed in Belarus. Roughly 40 countries have defense policies based on nuclear deterrence, including more than 30 non-nuclear-weapon states under extended deterrence (also known as nuclear umbrella) agreements such as the North Atlantic Treaty Organization. Nuclear weapons are weapons of mass destruction, and their control is a focus of international security through measures to prevent nuclear proliferation, arms control, or nuclear disarmament. The total global stockpile peaked at more than 64,000 weapons in 1986, and is around 9,600 as of 2025. Key international agreements and organizations include the Treaty on the Non-Proliferation of Nuclear Weapons, the Comprehensive Nuclear-Test-Ban Treaty and Organization, the International Atomic Energy Agency, nuclear-weapon-free zones, and the Treaty on the Prohibition of Nuclear Weapons.
Testing and deployment Nuclear weapons have only twice been used in warfare, both times by the United States against Japan at the end of World War II. On 6 August 1945, the United States Army Air Forces (USAAF) detonated a uranium gun-type fission bomb nicknamed "Little Boy" over the Japanese city of Hiroshima; three days later, on 9 August, the USAAF detonated a plutonium implosion-type fission bomb nicknamed "Fat Man" over the Japanese city of Nagasaki. These bombings caused injuries that resulted in the deaths of approximately 200,000 civilians and military personnel. The ethics of these bombings and their role in Japan's surrender are to this day, still subjects of debate. Since the atomic bombings of Hiroshima and Nagasaki, nuclear weapons have been detonated more than 2,000 times for testing and demonstration. Only a few nations possess such weapons or are suspected of seeking them. The only countries known to have detonated nuclear weapons—and acknowledge possessing them—are (chronologically by date of first test) the United States, the Soviet Union (succeeded as a nuclear power by Russia), the United Kingdom, France, China, India, Pakistan and North Korea. Israel is believed to possess nuclear weapons, though, in a policy of deliberate ambiguity, it does not acknowledge having them. Germany, Italy, Turkey, Netherlands, Belgium, and Belarus are nuclear weapons sharing states. South Africa is the only country to have independently developed and then renounced and dismantled its nuclear weapons. On the 30 October 2025, US President Donald Trump called for renewed nuclear weapons testing in order to keep pace with other nuclear armed states such as Russia and China, but did not make clear whether he referred to nuclear explosive testing or testing of delivery systems for nuclear warheads.
Types
There are two basic types of nuclear weapons: those that derive the majority of their energy from nuclear fission reactions alone, and those that use fission reactions to begin nuclear fusion reactions that produce a large amount of the total energy output.
Fission weapons
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