Four potential scenarios—all bad.

Kim Jong-il has now done what the Iranian mullahs are still a few years from accomplishing and what Saddam Hussein never came close to pulling off. He has apparently exploded an atom bomb. He probably can't yet pack a nuke into the nose cone of a missile or drop one from a plane. But as the term is generally (and aptly) defined, North Korea is now a nuclear-armed power. What's the rest of the world going to do about it?

The "international community" has a chance to behave as if the term were more than a polite or ironic euphemism. If there's a single national leader in the world who likes this new development, he hasn't said so. The U.N. Security Council quickly voted 13-0 to condemn the nuclear test. Several nonmembers have joined in the criticism. Now all we need is a next step—action.

This is nothing to shrug off. The combination of Kim Jong-il and a nuclear arsenal is a nightmare. It doesn't mean he's going to fire A-bombs at the United States or, for that matter, at South Korea or Japan. Kim may be a monster, but he's not suicidal; his top priority is the survival of his regime, and he must know that a nuclear attack would be followed by obliterating retaliation.

But what nuclear weapons do provide is cover for lesser sorts of aggression. The "club" of nuclear nations is a sort of mafia. The bomb provides protection, and thus a certain swagger, whether the other club members like it or not.

It doesn't take more than a handful of nukes to become a "made man" in this club. If Saddam Hussein had possessed some nukes in 1990, before he invaded Kuwait, it is doubtful that the U.S.-led coalition (and that really was a coalition) would have mobilized armed forces to push his troops back. If Mao Zedong had not possessed an atomic arsenal in 1969, during intense border clashes with the Soviet Union, it is likely that Leonid Brezhnev would have mounted an invasion. More to the point, without the nukes, Mao wouldn't have had the nerve to trigger the border clashes to begin with.

Kim Jong-il—like his father, Kim Il-Sung, before him—has kept his tiny, impoverished country afloat all these decades precisely by stirring up trouble and provoking confrontation (to justify his totalitarian rule), then playing his bigger neighbors off one another (to keep the tensions from spinning out of control and into his borders). His quest for nukes was propelled by a desire for the ultimate protection, mainly against an American attack. But now that he has them, he can be expected to play his games of chicken more feistily—and with still more opportunities for miscalculation.

Sunday's nuclear test has four other potential, dreadful consequences.

First, Kim Jong-il could churn out more bombs and sell at least some of them to the highest bidders. North Korea is dreadfully short of resources; his scheme to counterfeit American money has run into roadblocks; nukes might be his new cash cow. During the run-up to the invasion of Iraq, President Bush rallied domestic support by invoking the image of Saddam Hussein selling A-bombs to al-Qaida. It was a highly improbable scenario; even if Saddam had been building A-bombs, he would almost certainly have kept them under tight control. Kim, on the other hand, is a guerrilla-anarchist; he maintains his power not by trying to shape, or seek greater influence in, the international system but rather by throwing the system into a shambles. He's much less likely to have qualms about trading bombs for hard currency, regardless of the customer.

The second possible consequence of a nuclear North Korea is the unleashing of a serious regional arms race. The Japanese have long had the technical know-how and the stash of plutonium to build atomic (or possibly even hydrogen) bombs. They've foresworn that route because of moral qualms stemming from their own militarism in World War II. They also cite their security arrangement with the United States. But it's an open question how long these 60-year-old qualms would endure in the face of a clear and present danger. Just last month, a Japanese think tank run by former Prime Minister Yasuhiro Nakasone published a study calling on the nation to "consider the nuclear option." North Korea's nuclear test can only fuel these temptations.

If Japan goes nuclear, the Chinese might decide that it's in their security interests to resume nuclear testing. China's moves could incite India to accelerate its nuclear program, which would almost certainly compel Pakistan to match that effort. The South Koreans, meanwhile, might feel they need their own bomb to deter any crazy ideas from their northern neighbor, which could push the cycle into still higher gear.