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 A1 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. …