North Korea’s announcement that it had successfully tested a nuclear device has renewed concerns that other countries in the region will follow suit. These are not idle concerns. Given Japan’s existing nuclear power program, and its advanced technical and industrial base, it could likely develop nuclear weapons in a matter of months.

But it is far from certain that Japan will go that route, and Prime Minister Shinzo Abe was quick to dampen speculation about an ensuing arms race. “Possession of nuclear arms is not an option at all for our country,” Mr. Abe said after the North Korean test.

If Mr. Abe – or a successor – reneges on this pledge, however, and Japan decides to develop its own nuclear deterrent, it will be only the last in a series of steps in which the Japanese have enhanced their defensive posture. This rearmament has been driven primarily by fears of North Korea. While China and South Korea worry about the ramifications of a collapse of Mr. Kim’s regime, they are even more fearful of a nuclear-armed Japan. Accordingly, the best way to forestall such an eventuality is to cooperate with Tokyo in eliminating the North Korean threat.

Since at least the early 1990s, the United States has attempted to prevent North Korea from developing offensive military capabilities, including both nuclear weapons and the ballistic missiles that could some day deliver them. The North Korean nuclear test is merely the latest in a string of embarrassing incidents revealing the utter failure of U.S. policy. Simply put, the Japanese don’t trust the United States to defend their country from North Korea.

These sentiments have been building for some time. In March of last year, an editorial in the Asahi Shimbun worried that the Bush administration “does not seem in any hurry to strike a bargain as long as Pyongyang does not export nuclear arms to a third party or pose a direct threat to the United States itself.” President Bush reiterated this position following Pyongyang’s nuclear test, but such a posture does not sit well with Mr. Abe and the Japanese: the North’s nuclear program does pose a direct threat to Japan.

What, then, is Japan likely to do? Continued strong opposition within Japan to the use of the military for offensive ends suggests that unilateral preemptive action against North Korea is highly unlikely, but not beyond the realm of possibility. Following the North Korean missile test in July, then-chief cabinet secretary Abe hinted that the constitutional restrictions on the use of force would not prevent Japan from waging preemptive attacks against North Korean missile sites.

“If we accept that there is no other option to prevent a missile attack,” he told reporters, “there is an argument that attacking the missile bases would be within the legal right of self-defense. I think we need to examine this from the perspective of defending the Japanese people and nation.”

Then, in his first speech as prime minister, Mr. Abe elaborated on this point. Japan’s responsibilities now extended beyond self-defense, he explained. Given “the rising expectations” that Japan must contribute to international security, Mr. Abe pledged to “thoroughly study individual, specific cases to identify what kind of case falls under the exercise of the right of collective self-defense which is forbidden under the Constitution.”

In other words, future military ventures, similar to the deployment of Japanese troops to Iraq, could be deemed legitimate even if the current Constitution remains unchanged.

Despite the popular conception of Japan as a “pacifist” country, the Japanese boast one of the most capable militaries on the planet. Japan’s defense expenditures trail those of the United States, China and the United Kingdom, but are nearly equivalent to France’s military budget. Japan spends more than Russia and more than twice as much as India, the country often seen as a rising power (and a prospective U.S. strategic ally) in the region.

Nor do Japanese defense expenditures pose an abnormal burden. Japan’s defense spending per capita is comparable to that of Germany and South Korea. Citizens of the United Kingdom pay more than twice as much per person, as do the French. In other words, Japan’s defense spending could be expanded if changing strategic circumstances so dictated. That time may be nigh.

Japanese military action against North Korea, even if it were found to be a legitimate exercise of the right of self-defense, would nonetheless inflame regional tension. Although U.S. policymakers should rightly be concerned about how China and South Korea would react, such concerns must be understood in the context of the current crisis, when an impoverished and increasingly desperate North Korea might be tempted to sell nuclear materials to terrorists.

For now, military action has effectively been ruled off the table. North Korea warns that economic sanctions would be regarded as a “declaration of war,” but support for such sanctions has been building in Japan for some time. Some of Japan’s most famous and respected citizens participated in a three-day “sit-in” in June 2005 in front of the prime minister’s office, demanding that then-leader Junichiro Koizumi impose economic sanctions against North Korea. Mr. Koizumi resisted these pressures, but Mr. Abe and his government seem open to the idea.

Indeed, immediately following the test, Japan announced its own set of sanctions – including a total ban on all imports from North Korea, and prohibiting North Korean ships from entering Japanese ports – that go beyond any multilateral measures that may eventually be approved by the Security Council.

We cannot be sanguine about the potential dangers represented by the North’s nuclear weapons program. The potential for nuclear proliferation in the region exists, and while the United States is eminently capable of deterring any state foolish enough to launch a direct attack against it, one dares not make any predictions about the behavior of someone as erratic and unpredictable as Kim Jong-Il. This much is clear, however: Japanese fears of North Korea have provided the catalyst for a fundamental shift in strategy, and North Korea’s nuclear testing is likely to accelerate their efforts to defend their homeland.