If North Korea really has tested a hydrogen bomb — a claim that experts dispute — it would become just one of a small number of nations to have successfully tested the powerful nuclear weapon. And, perhaps more importantly, it would be the only nation known to have tested the weapon in almost 20 years.

The United States, Britain, France, Russia (as the Soviet Union) and China are known to have conducted hydrogen weapon tests. All these nations are signatories to the Non-Proliferation Treaty (NPT), an agreement that seeks to limit the spread of nuclear weapons.

Israel, which has not joined the NPT, is widely thought to have a covert nuclear weapons program. Some suspect that Israel conducted a nuclear test in 1979 off the coast of South Africa, though there has never been any official acknowledgement and many experts remain skeptical. The Union of Concerned Scientists has said that it believes Israel possesses only fission bombs, rather than the more powerful hydrogen bombs.

In 1996, negotiations on the Comprehensive Test Ban Treaty were concluded. Although the agreement, which seeks to prohibit nuclear weapon tests, has not been ratified by many nations and has not come into effect, most countries have not conducted nuclear tests since. The exceptions are India, Pakistan and North Korea.

India conducted five nuclear tests in 1998. Although the tests were said to include a hydrogen bomb, a former coordinator of India's nuclear program said in 2009 that the hydrogen bomb had been a dud and "completely failed to ignite." Pakistan performed nuclear tests in 1998 after India's, but the scale of the tests has been disputed and Pakistan has said that the weapons were fission devices, rather than hydrogen bombs.

Which leaves North Korea. The country had already conducted three nuclear tests, but it claims that Wednesday's blast was its first involving a hydrogen bomb. Whether it has really joined the small club of countries confirmed to have conducted hydrogen bomb tests remains to be seen.

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