Then there are the two sympathetic Pakistani nuclear scientists who met with top Al-Qaeda leaders in Afghanistan in August 2001. Pakistani intelligence officers say the scientists found Bin Laden to be "intensely interested" in chemical, biological and nuclear weapons, but insist that the talks were wide-ranging and "academic", likely rendering little critical help on bomb design.

In what would seem to be other frightening news, a hand-written 25-page document entitled "Superbomb" was found in the home of an Al-Qaeda leader in Afghanistan. But according to physicist David Albright, some sections are sophisticated while others are "remarkably inaccurate and naïve." Many critical steps for making a nuclear weapon are missing; the bomb design figures, "not credible." In short, the entire program seems "relatively primitive."

When in full-on fantasyland, we even worry about decade-old reports of Al-Qaeda's purchase of twenty nuclear warheads from Chechen mobsters for $30 million and two tons of opium. And then there's the supposed acquisition of nuclear suitcase bombs in Russia, asserted by Al-Qaeda's second in command, Ayman al-Zawahiri, on the eve of Al-Qaeda's collapse in Afghanistan. Given the circumstances, this seems a desperate bluff, and it has been much disputed by Moscow officials and experts on the Russian program. Even if they still exist, these Soviet-era bombs have a short shelf life and today are nothing more than "radioactive scrap metal."

Of course, absence of evidence, we need hardly be reminded, is not evidence of absence. Thus, Allison reports that, when no abandoned nuclear-weapons material was found in Afghanistan, some intelligence analysts responded: "We haven't found most of Al-Qaeda's leadership either, and we know that they exist." Since we know Mount Rushmore exists, maybe the tooth fairy does as well.

Even if there is some desire for the bomb, fulfilling that desire is another matter. Though Allison assures us that it would be "easy" for terrorists to assemble a crude bomb if they could get enough fissile material, we see how difficult it is for states to acquire these capabilities (it took Pakistan 27 years)-let alone the Lone Ranger. Al-Qaeda would need people with great technical skills, a bevy of corrupted but utterly reliable co-conspirators and an implausible amount of luck to go undetected for months, if not years while developing and delivering their capabilities.

Perhaps aware of these monumental difficulties, terrorists around the world seem in effect to be heeding the advice found in a memo on an Al-Qaeda laptop seized in Pakistan in 2004: "Make use of that which is available . . . rather than waste valuable time becoming despondent over that which is not within your reach." That is, "Keep it simple, stupid."

Threats and the Incentives

CONSIDERING ALL the false scares and attendant unnecessary civilian casualties they've engendered, it may be time to think a bit about the strategic consequences of treating nuclear proliferation as the "supreme priority" of foreign policy.

As Langewiesche points out, the nuclear genie is out of the bottle, and just about any state can eventually obtain nuclear weapons if it really wants to make the effort-although in many cases that might involve, as a former president of Pakistan once colorfully put it, "eating grass" to pay for it.

Despite the predictions of generations of alarmists, nuclear proliferation has proceeded at a remarkably slow pace. In 1958 the National Planning Association predicted "a rapid rise in the number of atomic powers . . . by the mid-1960s", and a couple of years later, John Kennedy observed that there might be "ten, fifteen, twenty" countries with a nuclear capacity by 1964. But over the decades a huge number of countries capable of developing nuclear weapons has not done so-Canada, Sweden and Italy, for example-and several others-Brazil, Argentina, South Africa, South Korea and Taiwan-have backed away from or reversed nuclear-weapons programs.

There is, then, no imperative for countries to obtain nuclear weapons once they have achieved the appropriate technical and economic capacity to do so. Insofar as states that considered acquiring the weapons, they came to appreciate several defects: The weapons are dangerous, distasteful, costly and likely to rile the neighbors. If one values economic growth and prosperity above all, the sensible thing is to avoid the weapons unless they seem vital for security.

It has often been assumed that nuclear weapons would prove to be important status symbols. However, as Columbia's Robert Jervis has observed, "India, China, and Israel may have decreased the chance of direct attack by developing nuclear weapons, but it is hard to argue that they have increased their general prestige or influence." How much more status would Japan have if it possessed nuclear weapons? Would anybody pay a great deal more attention to Britain or France if their arsenals held 5,000 nuclear weapons, or would anybody pay much less if they had none? Did China need nuclear weapons to impress the world with its economic growth? Perhaps the only such benefit the weapons have conferred is upon contemporary Russia: With an economy the size of the Netherlands, it seems unlikely that the country would be invited to participate in the G-8 economic club if it didn't have an atomic arsenal.

It is also difficult to see how nuclear weapons benefited their owners in specific military ventures. Israel's nuclear weapons did not restrain the Arabs from attacking in 1973, nor did Britain's prevent Argentina's seizure of the Falklands in 1982. Similarly, the tens of thousands of nuclear weapons in the arsenals of the enveloping allied forces did not cause Saddam Hussein to order his occupying forces out of Kuwait in 1990. Nor did the bomb benefit America in Korea or Vietnam, France in Algeria or the Soviet Union in Afghanistan.

The handful of countries that have pursued nuclear-weapons programs seem to have done so as an ego trip (think, again, of France) or else (or additionally) as an effort to deter a potential attack on themselves: China, Israel, India, Pakistan and now North Korea. Although there were doubtless various elements in their motivations, one way to reduce the likelihood such countries would go nuclear is a simple one: Stop threatening them. From this perspective, Bush's 2002 declaration grouping Iraq, Iran and North Korea into an "axis of evil" was, to put it mildly, foolish. However, many of his supporters, particularly in the neoconservative camp, went quite a bit further. In an article in this journal in the fall of 2004 proposing what he calls "democratic realism", Charles Krauthammer urged taking "the risky but imperative course of trying to reorder the Arab world", with a "targeted, focused" effort on "that Islamic crescent stretching from North Africa to Afghanistan." And in a speech in late 2006, he continued to champion what he calls "the only plausible answer", an amazingly ambitious undertaking that involves "changing the culture of that area, no matter how slow and how difficult the process. It starts in Iraq and Lebanon, and must be allowed to proceed." Any other policy, he has divined, "would ultimately bring ruin not only on the U.S. but on the very idea of freedom."

In their 2003 book, The War Over Iraq, Lawrence Kaplan and William Kristol stress that, "The mission begins in Baghdad, but does not end there. . . . War in Iraq represents but the first installment. . . .Duly armed, the United States can act to secure its safety and to advance the cause of liberty-in Baghdad and beyond." At a speech given at the Army War College as Baghdad was falling in 2003, Richard Perle triumphantly issued an extensive litany of targets, adding for good measure, and possibly in jest, France and the State Department.

Most interesting is a call issued in Commentary by neoconservatism's champion guru, Norman Podhoretz, in the run-up to the war. He strongly advocated expanding Bush's "axis of evil" beyond Iraq, Iran and North Korea "at a minimum" to "Syria and Lebanon and Libya, as well as ‘friends' of America like the Saudi royal family and Egypt's Hosni Mubarak, along with the Palestinian Authority." More realistic about democracy than other neoconservatives, Podhoretz pointedly added, "the alternative to these regimes could easily turn out to be worse, even (or especially) if it comes into power through democratic elections." Accordingly, he emphasized, "it will be necessary for the United States to impose a new political culture on the defeated parties."

These men, with their extravagant fantasies, do not, of course, directly run the Bush Administration. However, given the important role such people have played in the administration's intellectual development and military deployments, the designated target regimes would be foolish in the extreme not to take such existential threats very seriously indeed.

It is certainly preferable that none of these regimes (and quite a few others) ever obtain nuclear weapons. But if they do so they are by far most likely to put them to use the same way other nuclear countries have: to deter.

Nonetheless, even threatened states may not develop nuclear weapons. In the wake of the Iraq disaster, an invasion by the ever-threatening Americans can probably now be credibly deterred simply by maintaining a trained and well-armed cadre of a few thousand troops dedicated to, and capable of, inflicting endless irregular warfare on the hapless and increasingly desperate and ridiculous invaders. The Iranians do not yet seem to have grasped this new reality, but perhaps others on the Bush Administration's implicit hit list will.