Written by Valerie Plame for Newsweek

After spending years trying to thwart the nuclear black market, a former CIA spy says the only way to prevent terrorists from getting the bomb is to eliminate all of the world’s nukes.

The smoke was still drifting off the World Trade Center when the CIA discovered that Osama bin Laden had secretly met just a few days before the attack with a top Pakistani nuclear scientist, seeking help in building a nuclear bomb. Immediately, nuclear terrorism jumped to the top of the list of urgent threats to the civilized world. My clandestine work as a CIA operations officer became laser-focused on counterproliferation as we mobilized to prevent a nuclear 9/11. We knew that the horror of a nuclear bomb detonated in a major city would dwarf any catastrophe previously suffered by our country – the death toll would be in the hundreds of thousands and the economic and social devastation sudden and catastrophic.

Nine years later, who is winning this contest of wills between the civilized world and terrorist groups trying to buy, build, or steal a nuclear bomb? I would like to believe the bad guys are losing, but, in fact, time favors them as long as nuclear-bomb-grade materials and weapons exist in the world. A valiant team effort by the CIA and our many partners around the globe has prevented an attack thus far. But my experience as part of that effort tells me that the only way to end this danger is to lock down all nuclear materials and eliminate nuclear weapons in all countries.

I am now dedicated to achieving this urgent goal as a leader of the Global Zero movement to rid the world of nuclear weapons. To help deliver a wake-up call to the public and policymakers, I recently participated in a chilling documentary that’s in theaters now, Countdown to Zero, produced by Lawrence Bender and Participant Media – the team that made An Inconvenient Truth. This extraordinary film explains why living in a world with nuclear weapons and materials is simply not a viable option. Our only hope of survival is to drain the swamp as soon as we possibly can. The alternative is for nuclear weapons to spread around the world and, sooner or later, for terrorists to incinerate the heart of a major city.

According to the International Atomic Energy Agency, there have been at least 25 incidents of lost or stolen nuclear explosive material. If we estimate that the amount of recovered material represents 10 to 30 percent of the total amount that’s made it onto the black market over the years, that translates into sufficient material to build two to five nuclear weapons. The CIA is trying to ensure that none of this falls into the hands of terrorists, but it’s an uphill battle if the leakage of materials continues.

We may not precisely know the scale of the illicit trafficking in fissile materials, but we do know that rogue salesmen are peddling nuclear technology on the black market. The enterprising father of the Pakistani nuclear bomb, A. Q. Khan, hawked his wares for years before my group at the CIA caught him red-handed and put him out of business for selling a nuclear bomb to Libya in late 2003.

If terrorists get their hands on highly enriched uranium (a grapefruit-size quantity would be sufficient), they could smuggle it into a targeted city and detonate it on site. A hundred pounds of highly enriched uranium could fit in a shoebox – and 100,000 shipping containers come into the U.S. every day. Existing radiation sensors at the docks stand little chance: there are simply not enough of them, and it’s easy to hide highly enriched uranium in common materials that also give off a slight radioactive signature, like kitty litter. And building a bomb is no longer a well-guarded secret. Graduate students at U.S. scientific laboratories routinely design nuclear weapons (minus the fissile material) using off-the-shelf commercial equipment.

Countdown to Zero looks at other nuclear threats in addition to terrorism, and there are many. The spread of the bomb to more nations is especially worrisome. If Iran acquires a nuclear-weapons capability, its rivals in the region will likely follow suit in short order, and the chances of nuclear catastrophe resulting from an accident, miscalculation, or madness will rise exponentially. As the movie shows, even the well-disciplined and professional U.S. military has made very serious mistakes with nuclear weapons. The nuclear superpowers – who remain on launch-ready alert to this day – have come close to accidental nuclear war on numerous occasions. U.S. and Soviet bombers and submarines with nuclear weapons on board have crashed or sunk.

After spending years in the nuclear underworld, working to block the proliferation of nuclear weapons and material to other nations or to terrorist groups, I believe we are losing ground and that bold action is needed. The only way to avert a disaster is to put all nuclear-bomb-grade materials and nuclear weapons in all countries under ironclad control as soon as possible (as President Obama advocated during his Nuclear Security Summit earlier this year), and then to eliminate the stockpiles completely. This cannot be done overnight or unilaterally. It will require years of hard work. The United States and Russia – which together possess 95 percent of the world’s nuclear weapons – must reduce their Cold War stockpiles. Then, along with other major powers, including China, they must lead an international effort to reduce arsenals worldwide and make the elimination of nuclear weapons a global imperative, allowing no exceptions, whether Iran or Israel.

We will need to strengthen the monitoring of nuclear activities to verify compliance and root out any black market-eering. International inspectors must be able to investigate any facility in the world without restrictions. There is a strong track record to build on – since 1945 no nation has produced enough nuclear material to build a bomb without being detected by foreign intelligence agencies.

Getting to global zero will be arduous, but it can be done. Many who supported nuclear weapons as a deterrent during the Cold War now recognize that the threat today is not nuclear war with Russia or China, but proliferation and the risk of nuclear terrorism. Eliminating these present-day threats outweighs any benefits we might gain from retaining our nuclear arsenal.

The world’s nuclear stockpile has already been cut by more than half over the past 20 years – from its Cold War peak of about 70,000 warheads to today’s 23,000. And a political consensus is building in support of eliminating the rest. Presidents Obama and Medvedev have jointly declared their commitment to the goal of global zero and taken the initial step toward it by negotiating a new Strategic Arms Reduction Treaty – the first significant agreement to cut nuclear arms in decades. The United Nations Security Council has declared its unanimous support for the goal.

In the previous century, America led the world and defined the age – defeating Hitler, rebuilding Europe through the Marshall Plan, promoting civil rights for all people, sending men to the moon. Now we must again lead the world to conquer the gravest danger of this young century – nuclear terrorism.

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As a former covert CIA operations officer, Plame Wilson worked to prevent the proliferation of weapons of mass destruction. She is the author of “Fair Game: My Life as a Spy, My Betrayal by the White House,” soon to be a movie starring Naomi Watts and Sean Penn.