The media have barely noticed, but the Bush Administration has embarked on a burst of "multilateral" cooperation. It's called the Proliferation Security Initiative, and in only a few months of existence it has already had more success than the United Nations in controlling weapons of mass destruction.

Just ask Moammar Gadhafi. As the Journal reported last week, the Libyan strongman finally agreed to open his country's weapons sites to arms inspectors only after the U.S. and its PSI allies halted the illegal shipment of uranium-enrichment equipment headed for Libya's nuclear-arms program.

It remains to be seen whether Gadhafi will actually dismantle his program, but at least it's been exposed--no thanks, by the way, to the U.N. agency charged with monitoring such things. Libya's nuclear program was news to the International Atomic Energy Agency, whose inspectors somehow missed it entirely--after they'd earlier missed secret programs in North Korea and Iran...

...But don't mistake PSI for a multilateral institution in the conventional sense. There's no headquarters, no secretary-general, no talkfests--and, perhaps most important of all, no French or Russian veto. "PSI is an activity, not an organization," a senior Administration official tells us. It's an action-oriented group that "needs to be agile and move fast."

As PSI grows, the U.S. official contemplates "dozens of other countries participating" in dozens of different ways. Call it mix-and-match multilateralism. Countries participate or not, depending on the need at hand and on their own capabilities. The one common thread is U.S. leadership...