The proliferation of nuclear weapons, explosive devices which derive their destructive force from nuclear reactions (either fission or a combination of fission and fusion), is an important challenge of foreign policy.
Only a few nations possess such weapons or are suspected of seeking them. The only countries known to have detonated nuclear weapons—and that acknowledge possessing such weapons—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. In addition, Israel is also widely believed to possess nuclear weapons, though it does not acknowledge having them. One state, South Africa, fabricated nuclear weapons in the past, but has since disassembled their arsenal and submitted to international safeguards.
Only two nuclear weapons have been used in the course of warfare, both by the United States near the end of World War II. On August 6, 1945, a uranium gun-type fission bomb was detonated over the Japanese city of Hiroshima. Three days later, on August 9, a plutonium implosion-type fission bomb was exploded over Nagasaki, Japan. These two bombings resulted in the deaths of approximately 200,000 Japanese people—mostly civilians—from acute injuries sustained from the explosions.
Since the bombings of Hiroshima and Nagasaki, nuclear weapons have been detonated on over two thousand occasions for testing purposes and demonstrations. Because of the immense military power they can confer, the political control of nuclear weapons has been a key issue for as long as they have existed; in most countries the use of nuclear force can only be authorized by the head of government or head of state. In 1957, the International Atomic Energy Agency (IAEA) was established under the mandate of the United Nations to encourage development of peaceful applications for nuclear technology, provide international safeguards against its misuse, and facilitate the application of safety measures in its use.
By the 1960s, steps were being taken to limit both the proliferation of nuclear weapons to other countries and the environmental effects of nuclear testing. The Partial Test Ban Treaty (1963) restricted all nuclear testing to underground facilities, to prevent contamination from nuclear fallout, while the Nuclear Non-Proliferation Treaty (1968) attempted to place restrictions on the types of activities signatories could participate in, with the goal of allowing the transference of non-military nuclear technology to member countries without fear of proliferation. Currently, the prospect of nuclear technology falling into the hands of rogue states and terrorist organizations is considered a major threat to international security.