For decades, there was just one Asian nuclear power: China. But in the past several years, three other Asian countries either have developed nuclear bombs or began trying to, posing new challenges for efforts to stop nuclear proliferation.
In Asia, China had long been the only nuclear power. During the height of Cold War animosities with the United States, it began building a nuclear armory of about 450 missiles and bombs.
But in 1998, Beijing's neighbors, India and Pakistan, joined the elite club of nuclear-armed nations, when both tested nuclear devices. They now are thought to have between 30 and 80 nuclear bombs or missiles each. The two South Asian nations have been foes since independence from Britain in 1947, and their new weapons raise the specter of a nuclear holocaust on the subcontinent.
However, Uday Bhaskar, director of India's Institute for Defense Studies and Analysis, a government-funded agency, says it would be wrong to think the countries went nuclear only to threaten each other. He says nuclear weapons give a nation a new degree of diplomatic and scientific clout, and a broader overall defense policy.
"Neither [for] India, or that matter, Pakistan, ?the suggestion that nuclear capability is either targeted or aimed at only one person may sound very dramatic, but I think it would be misleading in terms of strategic reality," said Mr. Bhaskar.
In the past year, as relations between New Delhi and Islamabad have improved, the fear of a nuclear war between the two has receded. Another fear, however, remains: that their nuclear secrets might be shared with other countries either as official policy, or through the work of rogue scientists.
Indeed, earlier this year, Pakistan dismissed the head of its nuclear program, Abdul Qadeer Khan, for selling technology to other countries, and he now lives essentially under house arrest.
That case created a problem for the United States, which is trying to push North Korea to give up its efforts to build nuclear weapons. While the United States condemned Mr. Khan's activities, it did not impose new sanctions on Pakistan.
Alexander Lennon is an expert on weapons proliferation at the Center for Strategic and International Studies, an independent research organization in Washington. He says that in the Khan case, the Bush administration had to avoid putting too much pressure on Pakistan President Pervez Musharraf, who has been a close ally in the war on terror.
Still, Mr. Lennon says, Washington must be even-handed. That is particularly important, he says, regarding recent revelations that South Korea conducted secret nuclear tests as recently as 2000.
"If it doesn't take the South Korean investigation seriously, then there are concerns that are raised about the U.S. approaches to North Korea as well as Iran and the Middle East, about whether the United States is unfairly discriminating against these countries or whether it is actually treating all potential proliferators equally," said Mr. Lennon.
South Korea says the tests were scientific, and insists it is not building weapons. The United States has criticized Seoul for conducting the tests, but praised it for allowing the International Atomic Energy Agency to investigate.
While the United States has been restrained in its response to the South Korean revelations, nearby Japan has been concerned. Japan, the only nation to have suffered an atomic bomb attack, is fiercely anti-nuclear.
Choi Jin Wook is a senior researcher at the government-run Korean Institute of National Unification in Seoul. He talks about Tokyo's response to the South Korean nuclear experiments.
"They clearly oppose to the South Korean nuclear program, and they probably would try to pressure the United States to take tougher action, even though the United States does not take it very serious," he said.
One Japanese politician has said it would be understandable if South Korea built nuclear weapons to fend off North Korea. But experts in Japan and other countries say a nuclear-armed Korean Peninsula would cause concerns. Katsuya Kodama heads Japan's private International Peace Research Association.
"In that case, Japan will also have such a project," he added. "That will be possible. So this means the entire area may be nuclearized."
South Korea's nuclear tests could be the latest obstacle to pressuring North Korea to give up its nuclear ambitions.
There have been three big concerns about North Korea's efforts to build nuclear weapons. The first is that the isolated Stalinist state might use them. The second is that Pyongyang could sell the technology to other countries or terrorist groups. And finally, a nuclear North Korea would set off an arms race in North Asia.
The United States and other governments trying to limit the spread of nuclear weapons are still coming to grips with the reality of a nuclear Asia. While many experts say there is little likelihood that either Seoul or Tokyo will go nuclear in the next several years, they warn that possibility complicates non-proliferation efforts.