Nuclear weapon is an explosive device, whose explosive force is produced by nuclear reaction (fission bomb or combination of fission and fusion reaction (hot nuclear bomb)). Both bombs release large amounts of energy from relatively small amounts of material. The first test of a fission bomb released about 20000 tons of TNT (84 TJ). The first thermonuclear ("hydrogen") bomb test released about 10 million tons of TNT (42 PJ). A thermonuclear weapon weighing just over 2400 pounds (1100 kg) can release energy equivalent to 1.2 million tons of TNT (5.0 PJ). Nuclear devices no more than conventional bombs can destroy entire cities through explosions, fires and radiation. As they are weapons of mass destruction, the proliferation of nuclear weapons is the focus of international relations policy.

Before the end of World War II, the United States used nuclear weapons against Japan twice. On August 6, 1945, the U.S. Army Air Force detonated a uranium gun type fission bomb called "little boy" in Hiroshima, Japan. Three days later, on August 9, the U.S. Army Air Force detonated a p-implosion fission bomb called "fat man" in Nagasaki, Japan. The explosions caused damage and resulted in the death of some 200000 civilians and military personnel. The ethics of these bombings and their role in Japan's surrender are the subject of debate.

Since the atomic bombings in Hiroshima and Nagasaki, nuclear weapons have been detonated more than 2000 times for testing and demonstration. Only a few countries have such weapons or are suspected of seeking them. The only countries known to have detonated and acknowledged possession of nuclear weapons are (in chronological order of the first tests) the United States, the Soviet Union (replaced by Russia as a nuclear power), the United Kingdom, France, China, India, Pakistan and North Korea. Israel is believed to have nuclear weapons, although it does not recognize them out of a deliberate policy of ambiguity. Germany, Italy, Turkey, Belgium and the Netherlands are countries sharing nuclear weapons. South Africa is the only country to develop independently and then abandon and dismantle its nuclear weapons.

The effectiveness of the Treaty on the non proliferation of nuclear weapons aimed at reducing the proliferation of nuclear weapons was questioned, and political tensions remained high in the 1970s and 1980s. Weapons modernization continues to this day.