ONE month after the terrorist assault on the World TYade Center and the Pentagon, on October 11,2001, / President George W. Bush faced a more terrifying prospect At that morning s presidential daily intelligence briefing, George Tenet, the director of central intelligence, informed the president that a CIA agent codenamed "Dragonfire" had reported that Al Qaeda terrorists possessed a 10-kiloton nuclear bomb, evidently stolen from the Russian arsenal. According to Dragonfire, this nuclear weapon was in New York City.

The government dispatched a top-secret nuclear emergency support team to the city. Under a cloak of secrecy that excluded even Mayor Rudolph Giuliani, these nuclear ninjas searched for the bomb. On a normal workday, half a million people crowd the area within a half-mile radius of Times Square.

A noon detonation in midtown Manhattan would kill them all instantly. Hundreds of thousands of others would die from collapsing buildings, fire and fallout in the

hours thereafter. The electromagnetic pulse generated by the blast would fry cell phones and other electronic communication. The wounded would overwhelm hospitals and emergency services. Firemen would fight an uncontrolled ring of fires for days afterward.

In the hours that followed, Condoleezza Rice, then national security adviser, analysed what strategists call the "problem from hell." Unlike the Cold War, when the US and the Soviet Union knew that an attack against the other would elicit a retaliatory strike or greater measure, Al Qaeda — with no return address -- had no such fear of reprisal. Even if the president were prepared to negotiate, Al Qaeda has no phone number to call.

Concerned that Al Qaeda could have smuggled a nuclear weapon intoWashington as well, the president ordered Vice-President Dick Cheney to leave the capital for an "undisclosed location," where he would remain for weeks to follow -standard procedure to ensure "continuity of government" in case of a decapitation strike against US political leadership.

Several hundred federal employees from more than a dozen government agencies joined the

vice-president at this secret site, the core of an alternative government that would seek to cope in the aftermath of a nuclear explosion that destroyedWashington.

Six months earlier, the CIA's Counterterrorism Center had picked up chatter in Al Qaeda channels about an "American Hiroshima." The CIA knew that Osama bin Laden's fascination with nuclear weapons went back at least to 1992, when he attempted to buy highly enriched uranium from South Africa. Al Qaeda operatives were alleged to have negotiated with Chechen separatists in Russia to buy a nuclear warhead, which the Chechen warlord Shamii Basayev claimed to have acquired from Russian arsenals.

The CIA's special task force on Al Qaeda had noted the terrorist group's emphasis on thorough planning, intensive training and repetition of successful tactics. The task force highlighted Al Qaeda's preference for symbolic targets and spectacular attacks.

As CIA analysts examined Dragonfire's report and compared it with other bits of information, they noted that the September attack on the World TYade Center had set the bar higher for future terrorist attacks. Psychologically, a nuclear

attach would stagger the world's imagination. New York was, in the jargon of national-security experts, "target rich."

As it turned out, Dragonfire's report proved to be a false alarm. But the central takeaway from the case is this: The US government had no grounds in science or logic to dismiss this possibility, nor could it do so today.

There's no established methodology for assessing the probability of an unprecedented event that could have such catastrophic consequences. Nonetheless, in "Nuclear Terrorism," I state my considered judgment that if the US and other governments just keep doing what they are doing today, a nuclear terrorist attack in a major city is more likely than not by 2014.

Richard Garwin, a designer of the hydrogen bomb, whom Enrico Fermi once called "the only true genius I had ever met," told Congress in March 2007 that he estimated a "20 percent per year probability of a nuclear explosion with American cities and European cities included."

My Harvard colleague Matthew Bunn has created a model that estimates the probability of a nuclear terrorist attack over a 10-year period to be 29% -- identical to the average estimate from a poll of security experts commissioned by Senator Richard Lugar in 2005.

Former secretary of defense William Perry has expressed his own view that my work may underestimate the risk. Warren Buffet, the world's most successful investor and legendary odds-maker in pricing insurance policies for unlikely but catastrophic events, concluded that nuclear terrorism is "inevitable." As he has stated: "I don't see anywaythatitwon'thappen."

The good news is that nuclear terrorism is preventable by a feasible, affordable agenda of actions that, if taken, would shrink the risk of nuclear terrorism to nearly zero. A global strategy to prevent this ultimate catastrophe can be organised under a Doctrine ofThree No's: No loose nukes, no new nascent nukes, no new nuclear weapons. The first requires securing all nuclear weapons and weapons-usable material, on the fastest possible timetable, to a new "gold standard." The second does not allow for any new national capabilities to enrich uranium or reprocess plutonium. The third draws a line under the current eight and a half nuclear powers - the five members of the Security Council and India, Israel, Pakistan and North Korea -- and says unambiguously: "Stop. No More."

The US cannot unilaterally sustain a successful strategy to

prevent nuclear terrorism. Nor can the necessary actions simply be commanded, compelled or coerced. Instead, they require deep and steady international cooperation rooted in the recognition that nations share a common threat that requires a common strategy.

A Global Alliance Against Nuclear Terrorism is, therefore, in order. The mission of this alliance should be to minimise the risk of nuclear terrorism by taking every action physically, technically and diplomatically possible to prevent nuclear weapons or materials from falling into the hands of terrorists.

Constructing such an alliance will require the US and other nuclear-weapons states to confront the question of a "fourth no" — no nuclear weapons. While US or Russian possession of nuclear arsenals is not a major driver of Iran’s nuclear ambitions, and while Osama bin Laden would not be less interested in acquiring a nuclear weapon if the US eliminated its current arsenals, the proposition that nuclear weapons are necessary for the security of US and Russia but intolerably dangerous if acquired by Iran or South Africa is difficult to sell to nuclear have-nots.

The question of a categorical "fourth no” has come to the fore with the January 2007 opinion piece in the Wall Street Journal by George P Shultz, William J. Perry, Henry A Kissinger and Sam Nunn, calling upon the US and other states to act to realise their Non-Proliferation TYeaty commitment and President Reagan's vision of "a world free of nuclear weapons." Towards that goal, the immediate agenda should be to devalue nuclear weapons and minimise their role in international affairs.

This should begin with nuclear-weapons states pledging to the following principles: no new national enrichment, no nuclear tests, no first use of a nuclear bomb and no newnuclearweapons.

Faced with the possibility of an American Hiroshima, many are paralysed by a combination of denial and fatalism. This is unwarranted. Through a combination of imagination, a clear agenda for action and fierce determination to pursue it, the countdown to a nuclcar9/l 1 can bestopped.

Graham Allison is tho director of the Bolter Center for Sdcnce and International Affairs at tho Harvard Kennedy School. Ho served as assistant secretary of defense for policy and plans, and is tho author of Nuclear Terrorism: Tho Ultimoto Preventable Catastrophe in which tho argument stated here Is developed at book length

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