THE INTERCEPTION of a North Korean ship carrying 15 Scud missiles was starting to look like a practical example of the Bush administration's muscular new policy of preemptive action -- until the ship and its cargo were handed over to Yemen. Now the episode stands as an illustration of why that much-discussed doctrine is looking more formidable on paper -- and in the scary accounts of critics -- than in the real world. A paper issued by the White House this week described "effective interdiction" as "a critical part of the U.S. strategy to combat weapons of mass destruction and their delivery means." That certainly would seem to apply to an unflagged North Korean vessel carrying missiles capable of delivering chemical and biological warheads to a nation that is one of al Qaeda's principal bases. As the Yemeni government recently promised the Bush administration that it would no longer import such weapons -- and as North Korea's export of them is one of the most important challenges of proliferation -- the logic of preemption would mandate the confiscation and destruction of the missiles. Instead, faced with protests from the nominally friendly Yemeni government, which said the Scuds had been legally purchased, the administration backed off. "There is no clear authority to seize the ship," was the unmuscular explanation of White House spokesman Ari Fleischer.

Behind this response is a difficult balance of interests. On the one hand, it is dangerous to allow the import of Scud missiles anywhere in the Middle East at this moment. Though the Yemeni government has declared itself an ally of the United States in the war on terrorism -- and has allowed U.S. forces to operate against al Qaeda on its territory -- it controls neither its own lawless countryside nor, in the past, its own weapons. According to a report in this week's issue of Newsweek, the SA-7 missiles fired at an Israeli airliner in Kenya last month probably were stolen from a Yemeni government arsenal. Yemen's claim that it needs the North Korean missiles for defense is absurd; the Scud is an offensive weapon whose most common use has been to murder civilians in cities.

For the administration, these considerations, and its own preemption principle, seem to have been outweighed by the perceived advantages of continuing a partnership, however attenuated, with the Yemeni regime. If so, that would be the second time in months the White House has ignored its doctrine to satisfy real-world demands: Pakistani President Pervez Musharraf, another dodgy ally in the fight with al Qaeda, has paid no penalty for his swap of nuclear bomb technology for North Korean Scuds.

There is also the problem of international law: As construed by many of its advocates, particularly in Europe, this would bar the United States from ever boarding ships or crossing borders, even if to stop a rogue regime from supplying weapons to al Qaeda. Some of the more breathless critiques of the Bush administration's security doctrine have claimed that it will lead to unilateral U.S. military action against enemies and perceived enemies around the world; the Yemen episode demonstrates that that is most unlikely. In practice, the Bush team's reaction to such challenges has been measured and prudent. Yet the administration is right to be rethinking questions of international sovereignty and security. To defend itself and its allies in this era, the United States will need to stop rogue states from producing and distributing weapons that could be used to murder its citizens en masse without warning. It's not enough to issue papers about this principle; sooner or later, it will be necessary to act on it.