What nuclear threats do we face today? America went to war because its leadership believed Iraq had nuclear and other weapons of mass destruction. We are reminded daily of the potential dangers of Iran turning its quest for nuclear energy into a weapons capability. We are locked in a deep struggle to get North Korea to give up its nuclear status demonstrated in last fall’s test. Concerns about Russia’s nuclear arsenal are resurfacing. And, we are constantly reminded that we must wage America’s “war on terror” to avoid the nexus of international terrorism and nuclear weapons.

All nuclear threats are not alike. How do these and other nuclear threats compare in terms of severity or likelihood? And how can we effectively address them? It is useful to think of today’s nuclear threats at three levels. First is an all-out exchange of nuclear warheads—hundreds of them—that would destroy civilization as we know it. Next is a limited, but still disastrous exchange—tens of warheads —that would create levels of destruction not seen since World War II. The third level is the use of one or several nuclear bombs, which would threaten our way of life. Reframing the nuclear threat in this way allows us to gauge our level of concern and formulate meaningful preventive strategies.

An all-out nuclear exchange could occur today only between the United States and Russia, which still maintain many thousands of warheads in their nuclear inventories. A nuclear war between these two countries represents the only existential threat to the United States.

The end of the Cold War rendered this threat highly improbable but not impossible. An accidental or unauthorized launch followed by a response is still possible. To eliminate this threat, the United States and Russia should follow through on detargeting and commit to de-alerting their nuclear forces—to remove them from high alert status that allows a launch within minutes to pre-identified targets.

The two nations should commit to making major reductions in their nuclear stockpiles and eventually eliminating them. In the midst of the Cold War, President Ronald Reagan and Soviet General Secretary Mikhail Gorbachev reduced their stockpiles and even came close to an agreement to lead the world in abolishing nuclear weapons. Last January in a Wall Street Journal op-ed, George Shultz, William J. Perry, Henry Kissinger, and Sam Nunn called for a renewal of that vision by outlining steps to be taken now.

To move more rapidly toward much smaller numbers, I would add that leaders in both nations should undertake a zero-base nuclear assessment that would answer this question: If you were creating a stockpile from scratch today, how many weapons would you need to meet the current threat? Such a calculus would yield much lower numbers than trying to decide how many weapons you can live without. U.S. and Russian nuclear postures toward China should also carefully avoid provoking a Chinese nuclear buildup.

An exchange of tens of nuclear warheads is somewhat less improbable than nuclear war between Russia and the United States. But at this level, potential confrontations include nuclear exchanges between India and Pakistan, or between the United States and China—over Taiwan, for example, or on Russia’s southern border, or in the Middle East, between Israel and possibly Iran in the future. To limit the possibilities, it is crucial to stop more countries from acquiring nuclear weapons. The fewer fingers on the nuclear trigger, the better.

The United States should play a leading role in reinforcing the nuclear nonproliferation regime, centered on the 37-year-old Non-Proliferation Treaty (NPT), which allows a country to come within a whisker of building a bomb. A global expansion of nuclear power will pose additional challenges to the system. We need new rules of engagement for expanding nuclear power, including viable international controls on uranium enrichment and plutonium reprocessing.

To encourage non-nuclear weapon states to keep their end of the NPT bargain and refrain from acquiring the bomb, the five nuclear weapon states must show a greater commitment to working in good faith toward eventual elimination of their arsenals, as pledged under Article VI of the treaty.

Security guarantees from the United States and other nuclear weapon states can help curb some countries’ nuclear ambitions by alleviating fears of invasion by major world powers or by regional foes. India and Pakistan—two nuclear weapon states that aren’t parties to the NPT—should continue to pursue confidence-building measures to avoid miscalculation and potential nuclear war. We should help realize the nuclear-free zones that states are calling for in the Middle East, on the Korean peninsula, in Central Asia and in as many other regions as possible.

The United States and other states with nuclear weapons can also lower the risk of limited war by declaring a no-first-use policy, reserving nuclear weapons only as weapons of last resort.

The use of one or several nuclear bombs today is more likely than it was during the Cold War. If detonated in a big city, the damage would be catastrophic. Humankind would survive such a catastrophe, but it could gravely threaten our way of life. A country or a terrorist group in possession of a rudimentary nuclear bomb could deliver such a weapon in a van, boat, or plane. North Korea could do so, in desperation; Israel could do so in response to an existential threat; and under current doctrine, the United States or Russia could do so in response to a chemical, biological, or radiological attack. More likely, and hence of greater concern, is that terrorists would use a nuclear bomb, if they could get one.

The most likely route for terrorists to acquire a bomb is to devise one from stolen or diverted fissile materials. Theft or diversion of a ready-made weapon is far less likely.Building a rudimentary bomb is not easy but is judged to be within the capabilities of some sophisticated terrorist groups if they are able to obtain fissile materials.

Although it is widely recognized that keeping bomb materials out of terrorists’ hands is essential, the difficulty of doing so, especially from a technical standpoint, is not well understood. Only a few tens of kilograms of plutonium or highly enriched uranium are required for a bomb, yet almost 2 million kilograms of each exist in the world today, and some of it is not adequately secured. Securing these materials requires greater commitment to nuclear materials safeguards by all countries that possess them. It calls for greater urgency to protect and eventually eliminate highly enriched uranium in research reactors and facilities around the world. Bilateral or multilateral sting operations to intercept nuclear black market trade may help locate material already outside of state control. International cooperation in building databases and detection systems will improve nuclear forensics and attribution.

Each level of nuclear threat implies a different strategy of prevention. But three common aspects emerge as priorities for national and international policymaking:

• The fewer nuclear weapons, the better.
• The fewer fingers on a nuclear trigger, the better.
• Keeping fissile materials out of terrorists’ hands is essential.

Finally, this is not a problem for the United States alone to solve. It can only be solved through international cooperation.

The diplomatic initiative launched by President Bush in the wake of North Korea's nuclear weapon test in October 2006 has made substantial progress in rolling back the nation's drive to become a nuclear power.

That success, however, will be for naught if the administration fails to follow through on promises it made to encourage the Democratic People's Republic of Korea to destroy its nuclear weapons programs. The United States must honor its commitments in order to begin normalizing relations with North Korea.

Unfortunately, a recent barrage of criticism against the administration's policy aims to derail this process. Even as Pyongyang has taken more than 80 percent of the required steps to disable its Yongbyon nuclear weapons facilities, the fulfillment of US obligations has stalled.

The critics who want to stymie all forward movement, are, for the most part, the same specialists who can take credit for jettisoning in 2003 the agreement with North Korea, known as the 1994 Agreed Framework, which had stopped its plutonium production for almost a decade. Only after the collapse of the Agreed Framework did the North Koreans process the fissile material needed to build and test nuclear weapons. Three years later, in 2006, the president adopted a more realistic policy that is now under attack.

Many of the policy's critics denounce a declaration of the North's nuclear programs that has not yet been finished and argue that we must have clarity about North Korea's role in the construction of a Syrian nuclear facility and its uranium enrichment path to nuclear weapons.

Whatever the role of Korea in the Syrian reactor project, that facility no longer exists. Israel destroyed it last September.

Last year, the United States downgraded from medium to low its confidence level that North Korea continues to pursue a uranium enrichment program. In October, Pyongyang allowed US inspectors into a missile factory, where it said that aluminum tubes suspected of being used in that program were being remade into missile parts. North Korea handed over aluminum samples that later showed traces of enriched uranium, but analysis was inconclusive.

The United States apparently has secured Pyongyang's agreement to pursue these types of "clarifying" activities. Moreover, China has agreed on the importance of a verification regime aimed at assuring a "complete and correct" declaration, and a key goal of the next round of talks would be to fashion that regime.

Recent developments are even more impressive. On May 8, the North Koreans passed to a US State Department official a trove of 18,822 pages of operating records for the Yongbyon 5MWe reactor and reprocessing plant, which date back to 1986. That is 18,822 pages more than we ever had before, and begins a verification process previously impossible.

Also, the International Atomic Energy Agency and US nuclear experts have overseen the shutdown and continuing disablement of all key plutonium production facilities at Yongbyon. Discussions have been held to ship out the monitored unused reactor fuel rods.

If diplomacy is to succeed, Washington needs to begin delivering on some of the promises it has made as part of the Six-Party agreements. It must move toward normalizing relations with Pyongyang: That means beginning "the process of removing the designation of the DPRK as a state-sponsor of terrorism" and starting to "advance the process of terminating the application of the Trading with the Enemy Act with respect to the DPRK."

We know that North Korea seeks better relations with the United States to create the environment essential to facilitate economic recovery, give it more diplomatic space, and smooth the way for an upcoming political succession. Since the New York Philharmonic concert in Pyongyang last February, it has begun portraying the United States in a more positive light to its own people, laying the groundwork for a major breakthrough in relations with the United States.

This breakthrough is needed if the United States is to achieve its ultimate objective: to cap, roll back, and completely eliminate the North's nuclear weapons program.

Critics in Washington, like those in Pyongyang, are afraid of exploring the future and only want to cling to the past. That isn't the way out.

John W. Lewis, professor emeritus at Stanford University, is coauthor of "Negotiating with North Korea: 1992-2007."

The Bush administration's North Korea strategy is being criticized from the right and the left for letting Pyongyang off the hook. Some advocate scuttling the six-party talks. Others suggest slowing our own compliance with the agreement to get North Korea to make a full declaration of its nuclear program first. We disagree with both positions. Our mantra should be: It's the plutonium, stupid.

North Korea does have the bomb -- but a limited nuclear arsenal and supply of plutonium to fuel its weapons. The Yongbyon plutonium production facilities are closed and partially disabled.

In separate visits to North Korea in February, we concluded that the disablement was extensive and thorough. We also learned that Pyongyang is prepared to move to the next crucial step of dismantling Yongbyon, eliminating plutonium production. This would mean no more bombs, no better bombs and less likelihood of export. After this success, we can concentrate on getting full declarations and on rolling back Pyongyang's supply of weapons and plutonium.

We must not miss this opportunity, because we have the chance to contain the risk posed by North Korea's arsenal while we work to eliminate it. As dismantlement proceeds, negotiations should focus concurrently on the plutonium declaration, the extent of the uranium enrichment effort and Pyongyang's nuclear exports.

Pyongyang's declaration of 30 kilograms of plutonium (sufficient for roughly four to five bombs) falls short of the estimate of 40 to 50 kilograms, based on our past visits. We believe that North Korea is prepared to produce operating records and permit access to facilities, equipment and waste sites for verification. Obtaining and verifying its declaration of plutonium production and inventories is imperative. Let's proceed.

Pyongyang continues to claim that it has made no efforts to enrich uranium, despite strong evidence to the contrary. Although it appears unlikely that these efforts reached a scale that constitutes a weapons threat, a complete accounting is required. Dismantlement of the Yongbyon facilities should not, however, be postponed to resolve this issue. In October 2002, the Bush administration accused North Korea of covert uranium enrichment, only to have Pyongyang withdraw from the nuclear Non-Proliferation Treaty and produce plutonium to fuel the arsenal that we are now attempting to eliminate.

Nuclear exports are of greater concern. As recently revealed evidence demonstrates, North Korea sold nuclear technology to Syria, much as it sold missile technology. North Korea must cooperate if we are to get to the bottom of the Syrian incident and ensure that it is not repeated elsewhere. Israel eliminated the Syrian threat, for now, by bombing the reactor at Al Kibar. But it is imperative that Pyongyang reveal the nature and extent of its export operations and, most important, whether it has similar deals underway with Iran.

We do not advocate letting Pyongyang off the hook, but a "confession" regarding Syria is not the critical issue. We have good knowledge of what the North Koreans supplied to Syria. What we really need is information from North Korea that will help us deal with potential threats. For example, was North Korea acting alone, or was it part of a more sophisticated proliferation ring involving Pyongyang's trading partners and suppliers? North Korea's leadership must resolve all three declaration issues fully, and these will take time to verify.

To ultimately succeed in the peaceful elimination of nuclear weapons, we must understand why North Korea devoted its limited resources to going nuclear. The September 2005 six-party joint statement addresses many of these concerns, promising mutual respect for national sovereignty, peaceful coexistence, and a commitment to stability and lasting peace in Northeast Asia, as well as normalization of relations. Given the acrimonious history of our relations, such steps require a transformation in the relationship between North Korea and the United States, a change that will first require building trust -- step by step.

The six-party negotiations have put us on that path, and there is much evidence of winds of change blowing in North Korea that will make navigating that path easier (the recent New York Philharmonic concert in Pyongyang is one such symbol of change; the joint industrial facility at Kaesong is another). But North Korea's reluctance to provide full declarations and the Syria revelations have moved us in the wrong direction.

Nevertheless, walking away from the talks or slowing them at this point would be counterproductive. Instead, in its remaining months, the Bush administration should focus on limiting North Korea's nuclear capabilities by concluding the elimination of plutonium production. If it can also get answers on the Syrian operation and resolve the question of uranium enrichment, it will put the next administration in a stronger position to finally end the nuclear threat from North Korea.

Siegfried S. Hecker and William J. Perry are with the Center for International Security and Cooperation at Stanford University. Hecker was director of the Los Alamos National Laboratory from 1986 through 1997. Perry was secretary of defense from 1994 through 1997.