The United States dominates the international scene like no other state. Indeed, the modern state system has never seen a comparable global power. Despite its enormous economic, political, and military strength, however, the United States cannot defend its vital interests. It cannot even defend its homeland.

For more than 50 years, the United States has depended on deterrence for defense against its principal adversaries. Though deterrence has never been as fulfilling as denial—that is, preventing an enemy’s access to the homeland—deterrence has worked or, more precisely, not failed to work. But deterrence can be trusted no longer. Today’s adversary values his life less than our death. This adversary is not a candidate for deterrence. Moreover, while he lacks a ballistic missile delivery system, he has such a variety of other means to deliver a nuclear weapon, from commercial airliners to commercial trucks to container ships, that the United States cannot have any confidence in its ability to mount a sustained defense by denial.

Can this adversary plausibly acquire a nuclear weapon to attack the United States? Unless many changes are made, I think it is more likely than not that Al Qaeda or one of its affiliates will detonate a nuclear weapon in a US city within the next five to ten years. The loss of life will be measured not in the thousands, not in the tens of thousands, but in the hundreds of thousands. In this sense, the United States is at once extraordinarily powerful and tragically vulnerable.

Consider the more likely scenarios under which the United States could be struck. An Al Qaeda cell, operating out of Central or Southeast Asia or perhaps Africa, drawing on substantial financial resources and excellent contacts among ideological sympathizers, purchases 50 or so kilograms of highly enriched uranium. Today, the sellers might be in Pakistan or Russia; tomorrow, they might be in North Korea or Iran. This fissile material could be shipped with little or no shielding and would be very difficult to detect even if it passed through radiation monitors. The weapon’s design would be of the simplest “gun type:” a substantial metal tube with slugs of highly enriched uranium at either end, with an explosive charge behind one end to drive it down the tube into the other end so as to create a supercritical mass. So simple is this type of bomb that the United States successfully dropped one on Hiroshima without ever testing it. Finally, the Al Qaeda cell would include some experts in physics, nuclear engineering, metallurgy, and conventional explosives. It would not require anyone who had ever worked on or even seen a nuclear weapon, although a nuclear weapons designer would be helpful. And because there are thousands unemployed in Russia alone, one or two might be had for a reasonable price.

This scenario, however grim, depends on highly enriched uranium “leaking” out of a country without the assistance or knowledge of the government. A nuclear facility might lose uranium to theft by criminals, terrorists, or even insiders cooperating for ideological or financial reasons. Either way, fissile material today is inadequately secured. After more than a decade of efforts to improve physical security in the former Soviet Union, much is still to be done. The situation in Pakistan is less clear, but clear enough to be a cause for concern.

Another scenario, equally worrying, is the transfer of fissile material by a government or with one’s acquiescence, which could become plausible in the not-too-distant future should North Korea or Iran continue on their current course. A third scenario involves the acquisition, by theft or transfer, of a completed nuclear weapon. Some are designed to prevent unauthorized use, but many are not. Russian stockpiles of tactical nuclear weapons, including so-called suitcase bombs, are large, and their security remains a concern. While the numbers of weapons in North Korea and Iran are orders of magnitude smaller, the possibility of loss or transfer exists and is presumably increasing as stockpiles grow.

All this addresses the question of capability. The matter of intention, the other element in the classic equation of threat, is not in doubt. Ample evidence shows that Al Qaeda seeks nuclear weapons and all the components necessary to make them. It is a fair deduction from past activity and attitude that Al Qaeda would not hesitate to detonate a nuclear weapon in a US city.

None of this is news to the US government, either to the current presidential administration or to its predecessor. Dedicated officials have been working for years on programs designed to secure nuclear weapons and fissile material in the military, energy, and research sectors of the former Soviet Union and elsewhere. The United States and others have also pursued policies to prevent nuclear weapons proliferation. Additionally, the US intelligence community has integrated counterterrorism efforts with non-proliferation activity. All this is good, but not good enough. There is no greater threat to the security of the nation than that of terrorists armed with nuclear weapons. Yet, against this threat, the United States has failed to mobilize political, military, and intelligence assets at anything near the proper level. If a US city is leveled in 2005, how will the government explain the paltry percentage of resources from the defense and intelligence budget devoted to counter this threat?

What to do? The first thing is to educate the policymakers, the press, and the public. The quickest way is to make Harvard Kennedy School of Government professor Graham Allison’s book, Nuclear Terrorism: The Ultimate Preventable Catastrophe, required reading. Not only is Allison’s description of the problem compelling and comprehensive, but his ten-step prescription is on point and essential to an appropriate policy response. He advocates creating a global alliance to deal with nuclear terrorism, eliminating inadequately secured fissile material, raising the standard by which fissile material is stored, blocking fissile material production in Iran and North Korea, shutting down Abdul Qadeer Khan-like nuclear black markets, and strengthening both non-proliferation and counterterrorist policies.

While all of this is important, and none of it easily accomplished, the question remains whether it will be enough. The United States should at least consider two additional policies: preventive war to deal with rogue suppliers and expanded deterrence to obtain the cooperation needed to stop leakage of fissile material.