Yesterday's claim by North Korea to have successfully detonated an underground nuclear explosion has escalated fears of a destructive new arms race in east Asia, and dealt a potentially devastating blow to hopes of preventing a new round of nuclear proliferation around the globe.
The country regarded as the world's most secretive and unpredictable communist dictatorship called the bluff of the international community by carrying out the test less than a week after it had announced its plan, and in the face of virtually unanimous condemnation.
Although international scientists have yet to confirm that it was a genuine nuclear test - and some scientists suggested yesterday that the explosion had fizzled out - governments from Washington to Tokyo, Beijing and Moscow are assuming the worst.
At stake is the survival of the arms control system that has restrained the number of nuclear-armed states for almost 40 years, but which has looked increasingly fragile for the past decade. The International Atomic Energy Agency, the guardian of the nuclear non-proliferation treaty, said the test amounted to "the breaking of a de facto global moratorium on nuclear explosive testing that has been in place for nearly a decade".
North Korea's decision to demonstrate its own nuclear capacity represents a deliberate flouting not only of the US administration, which has threatened dire consequences, but also of China and Russia, its two nuclear neighbours. It also amounts to a calculated challenge to non-nuclear Japan, the country that sees itself as the most obvious target of any North Korean attack. Shinzo Abe, the new Japanese prime minister, landed in neighbouring South Korea for an official visit at the precise moment that Pyongyang announced its test.
The decision amounts to the second and most dramatic setback to international efforts to contain proliferation in recent days, after Iran's refusal to suspend its own nuclear fuel enrichment plans to negotiate a diplomatic solution with the permanent five members of the United Nations Security Council. Yet the North Korean move was both predicted and predictable.
"Everything has changed, and nothing has changed," says Gareth Evans, chief executive of the International Crisis Group, and former Australian foreign minister. "We have to confront overtly the weaponisation of North Korea. But this has been a reality for several years."
Japan and the US are the most hawkish members of the international community. Japan's response may determine whether the North Korean action precipitates a regional arms race. China, on the other hand, as Pyongyang's closest partner, holds the key to persuasion by more peaceful means.
"The US and Japan will go back to the UN Security Council to press for follow-on resolutions authorising political and economic sanctions," according to Bruce Klingner, of the Eurasia group. He says Tokyo would certainly participate in any multilateral sanctions or other non-military response.
"The critical policy now is to ensure that things don't get any worse," says Mr Evans. "Japan's reaction is the key."
Last month, Yasuhiro Nakasone, Japan's former prime minister, argued his country should consider developing nuclear weapons. "There are countries with nuclear weapons in Japan's vicinity," he said. "We are currently dependent on US nuclear weapons [as a deterrent], but it is not necessarily known whether the US attitude will continue."
Mr Nakasone set out his argument, thought to be shared by some officials in South Korea - and possibly in Taiwan as well - as a last resort, only to be enacted if the world's non-proliferation regime fell apart.
"Today's events certainly heighten tension and spur on those in Japan who believe that Japan should consider its own nuclear weapons option, but that is still a minority view and will remain so as long as Japan feels that it is protected by the US umbrella," says Mark Fitzpatrick, a non-proliferation expert at the International Institute for Strategic Studies in London.
"The bigger impact will be on non-proliferation worldwide, on countries that want to have the prestige or deterrent effect they believe nuclear weapons create . . . This will increase the number of countries that want to consider those options."
China and Russia, as well as South Korea, are all expected to be cautious about imposing draconian sanctions, not least because of their fears of precipitating a chaotic collapse of the Pyongyang regime and a humanitarian disaster.
