1945: In the heat of World War II the United States succeeds in exploding the first atomic bomb in the desert of New Mexico.

On August 6, a similar bomb is dropped on the Japanese city of Hiroshima, razing the city. Three days later the United States drops a second bomb on another Japanese city, Nagasaki.

The attacks mark the only occasion to date on which nuclear weapons have been used in anger; the total death toll is estimated at between 150,000 and 200,000.

1949: The Soviet Union, by then engaged in the Cold War with the West, explodes its first bomb.

1952: The US military explodes the first thermonuclear, or hydrogen, bomb in a test on the Pacific island of Eniwetok. Atmospheric nuclear tests become common, to be replaced by underground ones after they are banned in 1963.

1950s and 60s: The United States and the Soviet Union, plus Britain, China and France, build up massive nuclear arsenals, with enough power to destroy the world several times over.

However nuclear arms are paradoxically seen as an insurance against their own use, since they are so potent that any power foolish enough to use them is certain to be destroyed in its turn. This is aptly summed up in the acronym MAD: "mutual assured destruction".

In October 1962 the world seems close to a nuclear conflict when it is revealed that the Soviet Union has stationed nuclear missiles on the United States' doorstep, in Cuba. After a US blockade of the island, the Soviets back down.

1968: The United Nations adopts the Nuclear Non-Proliferation Treaty, which comes into force in 1970. It allows signatories to peacefully develop nuclear power, but bans new countries from obtaining nuclear arms and commits those which already have them to disarming.

However several countries which are either known or believed to have such weapons -- notably India, Israel and Pakistan -- later refuse to sign it.

1990s: The collapse of the Soviet Union brings the Cold War to an end, although Russia inherits the former superpower's weapons, and still holds them to this day. In 1996 the main powers agree to ban all nuclear weapons tests.

2000-2006: New nuclear-armed powers emerge, in the form of Pakistan and India. North Korea claims to possess its own weapon, and Western nations accuse Iran of seeking to do the same.

Meanwhile the same Western powers, and notably the United States, state their intention to develop new types of nuclear weapons, which some claim could be used for limited tactical purposes rather than "mutual assured destruction".

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