The United Nations is the tooth fairy of American politics: Few adults believe in it, but it's generally regarded as a harmless story to amuse the children. Since 9/11, however, the UN has ceased to be harmless, and the Democratic presidential candidates' enthusiasm for it has ceased to be amusing. The United Nations has emerged at best as irrelevant to the terrorist threat that most concerns us, and at worst as an obstacle to our winning the war on terrorism. It must be reformed. And if it cannot be reformed, the United States should give serious consideration to withdrawal.

The UN has become an obstacle to our national security because it purports to set legal limits on the United States' ability to defend itself. If these limits ever made sense at all, they do not make sense now.

Yet the UN's assertion of them forces presidents and policymakers into a horrible dilemma. If we obey the UN's rules, we compromise our national security. If we defy them, we expose ourselves to accusations of hypocrisy and lawlessness.

According to the UN Charter, nations are permitted to use military force only in two situations. Article 51 of the charter recognizes an "inherent" right to self-defense against attack. In all other cases where a nation feels threatened, it is supposed to go to the UN Security Council to seek authorization before it takes military action--even action that might forestall an attack.

The trouble is that the UN defines aggression in outdated ways. For the UN, "aggression" means invasion across national borders. Send Nazi shock troops into Poland--that's aggression. Give sanctuary to thousands of anti-American murderers, as the Taliban did in Afghanistan, that's not aggression.

In other words, if the United States had sent troops into Afghanistan to shut the camps down, we might well have been branded the aggressor. But if the U.S. had asked the Security Council for a mandate to destroy Al Qaeda's terrorist bases, could the French, Russians and Chinese have been expected to approve? Even after 9/11, there would still have been plenty of people ready to argue that however much they deplored what Al Qaeda had done, Afghanistan--a sovereign state and United Nations member--was not an Article 51 "aggressor."

In other words, under UN rules, the U.S. is obliged to let terrorists strike first before retaliating--and might even be prohibited from striking second. In an age when shadowy radical movements around the globe are seeking weapons that could kill hundreds of thousands of people, these rules are clearly out of date. We need new rules recognizing that harboring terrorists is just as much an act of aggression as an invasion and that those who are targeted by terrorists have an inherent right to defend themselves, preemptively if necessary.

Of course, it won't be easy to persuade the UN to adopt these changes. Many members--including some of our traditional allies--seem much more interested in constraining the United States than they are in defeating terrorism--at least terrorism that is aimed at us.

The UN member states know that the U.S. will in the end do whatever it has to do, regardless of what the UN says. But they also know that the United States pays a price for disregarding the UN. The French in particular benefit from pushing the United States to break the UN's rules: Under French President Jacques Chirac, they are trying to fashion the European Union as a counterweight to the United States, and the image of the U.S. as an outlaw power helps their cause.

In a little more than a decade, our world has been transformed, first by the fall of the Soviet Union and then the events of 9/11. Everything has changed--except for the UN. It remains an invention of a vanished era, designed to solve vanished problems. It must evolve or it will slide from irrelevance to oblivion. If the UN is not part of the anti-terror fight, the United States should not be part of the UN.

David Frum and Richard Perle are resident fellows at AEI.