A nuclear weapon is a weapon which derives its destructive force from the nuclear reactions of nuclear fission and/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 can be capable of destroying or seriously disabling an entire city.

In the history of warfare, nuclear weapons have been used on two occasions, 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 a plutonium implosion-type device code-named "Fat Man" was dropped on the city of Nagasaki. The use of the weapons, which resulted in the immediate deaths of at least 120,000 individuals (mostly civilians) and about twice that number over time, was and remains controversial — critics charged that they were unnecessary acts of mass killing, while others claimed that they ultimately reduced casualties on both sides by hastening the end of the war. (See Atomic bombings of Hiroshima and Nagasaki for a full discussion.)

Since that time, nuclear weapons have been detonated on over two thousand occasions, mostly for testing purposes, chiefly by the following seven countries: the United States, Soviet Union, France, United Kingdom, People's Republic of China, India and Pakistan. These countries are the declared nuclear powers (with Russia inheriting the weapons of the Soviet Union after its collapse).

Various other countries may hold nuclear weapons, but they have never publicly admitted possession, or their claims to possession have not been verified. For example, Israel has modern airbourne delivery systems and appears to have an extensive nuclear program (see Israel and weapons of mass destruction); North Korea has recently stated that it has nuclear capabilities (although it has now stated that it will abandon all of its nuclear weapons programs); Ukraine may possess an obsolete Soviet-era nuclear stockpile due to a post-Soviet administrative error; and Iran is believed to be attempting to develop nuclear capabilities (for more information see List of countries with nuclear weapons).

Nuclear weapons in modern times have been used primarily as a method of creating a strategic threat. For example, the worry that North Korea will use nuclear weapons has dominated the relations between the United States and North Korea.

Apart from their use as weapons, nuclear explosives have been proposed for various non-military uses.

From Wikipedia.