A nuclear weapon is a weapon which derives its destructive force from nuclear reactions of fission or fusion. As a result, even a nuclear weapon with a small yield is significantly more powerful than the largest conventional explosives, and a single weapon is capable of destroying an entire city.
In the history of warfare, nuclear weapons have been used twice, both during the closing days of World War II. The first event occurred on the morning of 6 August 1945, when the United States dropped a uranium gun-type device code-named "Little Boy" on the Japanese city of Hiroshima. The second event occurred three days later when, again, the United States dropped a plutonium implosion-type device code-named "Fat Man" on the city of Nagasaki. The use of these weapons, which resulted in the immediate deaths of around 120,000 people and even more over time, was and remains controversial. (See Atomic bombings of Hiroshima and Nagasaki for a full discussion).
Since the Hiroshima and Nagasaki bombings, nuclear weapons have been detonated on over two thousand occasions for testing and demonstration purposes. The only countries known to have detonated such weapons are (chronologically) the United States, The former Soviet Union, the United Kingdom, France, The People's Republic of China, India, Pakistan, and North Korea.
Various other countries may hold nuclear weapons but have never publicly admitted possession, or their claims to possession have not been verified. For example, Israel has modern airborne delivery systems and appears to have an extensive nuclear program with hundreds of warheads (see Israel and weapons of mass destruction), though it officially maintains a policy of "ambiguity" with respect to its actual possession of nuclear weapons. According to some estimates, it possesses as many as 200 nuclear warheads. Iran currently stands accused by the United Nations of attempting to develop nuclear capabilities, though its government claims that its acknowledged nuclear activities, such as uranium enrichment, are for peaceful purposes. South Africa also secretly developed a small nuclear arsenal, but disassembled it in the early 1990s (For more information see List of states with nuclear weapons).
Apart from their use as weapons, nuclear explosives have been tested and used for various non-military uses. Synthetic elements, such as Einsteinium and Fermium, created by nuclear fission, were discovered in the aftermath of the first hydrogen bomb test.