After 9/11, many wondered why the United States had not taken military action in Afghanistan earlier to avert the deaths of more than 3,000 innocents. It was the same question many asked after 9/1 -- that would be Sept. 1, 1939, the date when Germany invaded Poland. The evil intentions of the Nazis, like those of al Qaeda, had been clear far in advance. Why had the civilized world not intervened before tragedy struck? Why had those in a position to act not listened to the anguished, urgent warnings coming from the likes of Winston Churchill and Anthony Eden in the case of the Nazis, or from Richard Clarke, Reuel Gerecht, and others in the case of the Islamists?

The answer is almost impossible to fathom in retrospect once we are aware of the consequences of inaction. Indeed, so convinced was U.S. President George W. Bush of the need to avoid making the same mistake in the future that he promulgated a doctrine of preemption that roiled traditional foreign-policy circles. Citing threats such as a terrorist attack with weapons of mass destruction, the president's 2002 National Security Strategy vowed, "To forestall or prevent such hostile acts by [its] adversaries, the United States will, if necessary, act preemptively" in exercising its inherent right of self-defense. As recently as Dec. 9, speaking at West Point, Bush reiterated that after 9/11, "We resolved that we would not wait to be attacked again. ... We understood, as I said here at West Point in 2002, 'if we wait for threats to fully materialize, we will have waited too long' -- so we made clear that hostile regimes sponsoring terror or pursuing weapons of mass destruction would be held to account."

The Iraq war was the first step toward making good on what became known as the Bush doctrine. Yet the very messiness of that intervention served as a warning of the costs of preemption. That perhaps explains why Bush, even as he continues to reaffirm that preemption is essential to U.S. national security, has failed to do more to deal with the gathering storms in Pakistan and Iran, which to future historians might stand, more than Iraq or the financial meltdown, as the greatest stains on his presidency.

In the years since Sept. 11, 2001, Pakistan has replaced Afghanistan as the leading refuge for al Qaeda and related scoundrels. Its territory has been connected to atrocities as far afield as the Mumbai attacks, which killed 170 people in November, and the London bombings, which killed 56 people in 2005. Other Pakistan-related plots have been stopped barely in the nick of time. These include Richard Reid's attempt to bring down an airliner with a shoe bomb in 2001 and plans to carry out a series of bombings in Europe by 14 would-be terrorists, who were arrested in Spain in early 2008. A few of the masterminds behind these machinations have been caught or killed. Many others, including Osama bin Laden and Ayman al-Zawahiri, remain alive and probably hiding in Pakistan. A handful of high-profile arrests notwithstanding, the Pakistani security services have made scant effort to root out jihadist networks that have long-standing links with Pakistan's own Inter-Services Intelligence agency.

In its recent report, the congressionally chartered Commission on the Prevention of Weapons of Mass Destruction Proliferation and Terrorism, chaired by former U.S. Senators Bob Graham and Jim Talent, "singled out Pakistan for special attention" because "many government officials and outside experts believe that the next terrorist attack against the United States is likely to originate from within the Federally Administered Tribal Areas (FATA) in Pakistan."

What is truly alarming is the possibility that such an attack could be carried out with weapons of mass destruction. If al Qaeda were ever to get its hands on a nuclear bomb, Pakistan would have to be considered a prime culprit. It is, after all, a state rife with Islamist extremists, and has a government unable to preserve even a modicum of order. Its capacity to safeguard its nuclear arsenal, even with the best of intentions, is in doubt. Already, the A.Q. Khan ring has been responsible for a frightening amount of nuclear proliferation. It takes a lot of credulity to imagine that Pakistan's top nuclear weapons scientist could carry out these activities without the knowledge of anyone in the Pakistani government.

Another likely source for a terrorist bomb would have to be Iran. Iran's own government admits to having more than 5,000 centrifuges in operation and plans to install many more. The International Atomic Energy Agency (IAEA) estimates that those centrifuges have already produced 630 kilograms, or 1,390 pounds, of low-enriched uranium. Once that material is purified into highly enriched uranium, it would be sufficient, or nearly sufficient, to make an atomic bomb. Intelligence estimates warn that could happen sometime in 2009. Mohamed ElBaradei, the head of the IAEA, is hardly a hard-liner, but even he says efforts to stop Iran's nuclear program have been a "failure." "We haven't really moved one inch toward addressing the issues," he recently told the Los Angeles Times.