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 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.

Thank you Chairman Dorgan, Senator Domenici and distinguished members of the Committee for giving me the opportunity to comment on the National Nuclear Security Administration's Defense Nuclear Nonproliferation programs and its 2009 budget request. I have a written statement that I would like to submit for the record.

This morning I will summarize the three main points in my statement. My opinions have been shaped by 34 years at the Los Alamos National Laboratory and nearly 20 years of practicing nonproliferation with my feet on the ground in places like Russia, China, India, North Korea and Kazakhstan.  Much of this I have done with the strong support and encouragement of Senator Domenici.

1) The proliferation of nuclear weapons and weapons capability is growing. Today, we face a nuclear threat in North Korea, nuclear ambitions in Iran, a nuclear puzzle in Syria, recently nuclear-armed states in Pakistan and India, and an improved, but not satisfactory, nuclear security situation in Russia and other states of the former Soviet Union. The danger of nuclear terrorism is real. This is not a fight the United States can win alone. We cannot simply push the dangers beyond our borders. It is imperative to forge effective global partnerships to combat the threat of nuclear terrorism and the proliferation of nuclear weapons. Meeting these challenges requires diplomatic initiative and technical cooperation. The United States must lead international diplomacy and DOE/NNSA must provide technical leadership and capabilities. The NNSA has done a commendable job in nuclear threat reduction and combating nuclear proliferation. However, funds to support these activities are not commensurate with the magnitude or the urgency of the threat.

2) CTR began with Nunn-Lugar followed by Nunn-Lugar-Domenici legislation directed at the aftermath of the breakup of the Soviet Union. We must stay engaged with Russia and the other states of the Soviet Union. Much progress has been made, but more needs to be done. We have to change the nature of the partnership to one in which Russia carries more of the burden.

We should expand the cooperative reduction programs aggressively to other countries that require technical or financial assistance. The nuclear threat exists wherever nuclear materials exist. These materials cannot be eliminated, but they can be secured and safeguarded. We should more strongly support the International Atomic Energy Agency and provide more support to countries that try to implement UNSCR 1540 to prevent nuclear terrorism, for example.

We should enlist other nations such as China, India, and for that matter, Russia, to build a strong global partnership to prevent proliferation and combat nuclear terrorism. China and India have for the most part sat on the sidelines while the U.S. has led the fight. Russia has not engaged commensurate with its nuclear status. These efforts are particularly important if nuclear energy is to experience a real renaissance.

3) The hallmark of all of these efforts must be technology, partnership and in-country presence. The DOE/NNSA has in its laboratories the principal nuclear expertise in this country. It should be applauded for sending its technical experts around the world, often in very difficult situations (I met up with the DOE team in North Korea on a bitterly cold February day). However, both for structural reasons and budgetary shortfalls, that technical talent is slowing fading away. We do not have in place the necessary personnel recruitment or the working environment in the laboratories or the pipeline of students in our universities to replenish that talent. I strongly support the NNSA's Next Generation Safeguards Initiative, which is aimed at tackling this problem.

Mr. Chairman, when I first visited Russia's secret cities in 1992 shortly after the fall of the Soviet Union, I feared that its collapse may trigger a nuclear catastrophe. The fact that nothing really terrible has happened in the intervening 16 years is in great part due to the DOE/NNSA programs that your are considering today. We must now be just as innovative and creative to deal with the changing nuclear threat today.

In my statement I also mention the implications of my recent trips to North Korea and to India. However, since I am out of time, I will need to leave those for your questions.

Thank you for your attention.

Nuclear weapons today present tremendous dangers, but also an historic opportunity. U.S. leadership will be required to take the world to the next stage -- to a solid consensus for reversing reliance on nuclear weapons globally as a vital contribution to preventing their proliferation into potentially dangerous hands, and ultimately ending them as a threat to the world.

Nuclear weapons were essential to maintaining international security during the Cold War because they were a means of deterrence. The end of the Cold War made the doctrine of mutual Soviet-American deterrence obsolete. Deterrence continues to be a relevant consideration for many states with regard to threats from other states. But reliance on nuclear weapons for this purpose is becoming increasingly hazardous and decreasingly effective.

North Korea's recent nuclear test and Iran's refusal to stop its program to enrich uranium -- potentially to weapons grade -- highlight the fact that the world is now on the precipice of a new and dangerous nuclear era. Most alarmingly, the likelihood that non-state terrorists will get their hands on nuclear weaponry is increasing. In today's war waged on world order by terrorists, nuclear weapons are the ultimate means of mass devastation. And non-state terrorist groups with nuclear weapons are conceptually outside the bounds of a deterrent strategy and present difficult new security challenges.

Apart from the terrorist threat, unless urgent new actions are taken, the U.S. soon will be compelled to enter a new nuclear era that will be more precarious, psychologically disorienting, and economically even more costly than was Cold War deterrence. It is far from certain that we can successfully replicate the old Soviet-American "mutually assured destruction" with an increasing number of potential nuclear enemies world-wide without dramatically increasing the risk that nuclear weapons will be used. New nuclear states do not have the benefit of years of step-by-step safeguards put in effect during the Cold War to prevent nuclear accidents, misjudgments or unauthorized launches. The United States and the Soviet Union learned from mistakes that were less than fatal. Both countries were diligent to ensure that no nuclear weapon was used during the Cold War by design or by accident. Will new nuclear nations and the world be as fortunate in the next 50 years as we were during the Cold War?

Leaders addressed this issue in earlier times. In his "Atoms for Peace" address to the United Nations in 1953, Dwight D. Eisenhower pledged America's "determination to help solve the fearful atomic dilemma -- to devote its entire heart and mind to find the way by which the miraculous inventiveness of man shall not be dedicated to his death, but consecrated to his life." John F. Kennedy, seeking to break the logjam on nuclear disarmament, said, "The world was not meant to be a prison in which man awaits his execution."

Rajiv Gandhi, addressing the U.N. General Assembly on June 9, 1988, appealed, "Nuclear war will not mean the death of a hundred million people. Or even a thousand million. It will mean the extinction of four thousand million: the end of life as we know it on our planet earth. We come to the United Nations to seek your support. We seek your support to put a stop to this madness."

Ronald Reagan called for the abolishment of "all nuclear weapons," which he considered to be "totally irrational, totally inhumane, good for nothing but killing, possibly destructive of life on earth and civilization." Mikhail Gorbachev shared this vision, which had also been expressed by previous American presidents.

Although Reagan and Mr. Gorbachev failed at Reykjavik to achieve the goal of an agreement to get rid of all nuclear weapons, they did succeed in turning the arms race on its head. They initiated steps leading to significant reductions in deployed long- and intermediate-range nuclear forces, including the elimination of an entire class of threatening missiles.

What will it take to rekindle the vision shared by Reagan and Mr. Gorbachev? Can a world-wide consensus be forged that defines a series of practical steps leading to major reductions in the nuclear danger? There is an urgent need to address the challenge posed by these two questions.

The Non-Proliferation Treaty (NPT) envisioned the end of all nuclear weapons. It provides (a) that states that did not possess nuclear weapons as of 1967 agree not to obtain them, and (b) that states that do possess them agree to divest themselves of these weapons over time. Every president of both parties since Richard Nixon has reaffirmed these treaty obligations, but non-nuclear weapon states have grown increasingly skeptical of the sincerity of the nuclear powers.

Strong non-proliferation efforts are under way. The Cooperative Threat Reduction program, the Global Threat Reduction Initiative, the Proliferation Security Initiative and the Additional Protocols are innovative approaches that provide powerful new tools for detecting activities that violate the NPT and endanger world security. They deserve full implementation. The negotiations on proliferation of nuclear weapons by North Korea and Iran, involving all the permanent members of the Security Council plus Germany and Japan, are crucially important. They must be energetically pursued.

But by themselves, none of these steps are adequate to the danger. Reagan and General Secretary Gorbachev aspired to accomplish more at their meeting in Reykjavik 20 years ago -- the elimination of nuclear weapons altogether. Their vision shocked experts in the doctrine of nuclear deterrence, but galvanized the hopes of people around the world. The leaders of the two countries with the largest arsenals of nuclear weapons discussed the abolition of their most powerful weapons.

* * *

What should be done? Can the promise of the NPT and the possibilities envisioned at Reykjavik be brought to fruition? We believe that a major effort should be launched by the United States to produce a positive answer through concrete stages.

First and foremost is intensive work with leaders of the countries in possession of nuclear weapons to turn the goal of a world without nuclear weapons into a joint enterprise. Such a joint enterprise, by involving changes in the disposition of the states possessing nuclear weapons, would lend additional weight to efforts already under way to avoid the emergence of a nuclear-armed North Korea and Iran.

The program on which agreements should be sought would constitute a series of agreed and urgent steps that would lay the groundwork for a world free of the nuclear threat. Steps would include:

• Changing the Cold War posture of deployed nuclear weapons to increase warning time and thereby reduce the danger of an accidental or unauthorized use of a nuclear weapon.

• Continuing to reduce substantially the size of nuclear forces in all states that possess them.

• Eliminating short-range nuclear weapons designed to be forward-deployed.

• Initiating a bipartisan process with the Senate, including understandings to increase confidence and provide for periodic review, to achieve ratification of the Comprehensive Test Ban Treaty, taking advantage of recent technical advances, and working to secure ratification by other key states.

• Getting control of the uranium enrichment process, combined with the guarantee that uranium for nuclear power reactors could be obtained at a reasonable price, first from the Nuclear Suppliers Group and then from the International Atomic Energy Agency (IAEA) or other controlled international reserves. It will also be necessary to deal with proliferation issues presented by spent fuel from reactors producing electricity.

• Halting the production of fissile material for weapons globally; phasing out the use of highly enriched uranium in civil commerce and removing weapons-usable uranium from research facilities around the world and rendering the materials safe.

• Redoubling our efforts to resolve regional confrontations and conflicts that give rise to new nuclear powers.

 
Achieving the goal of a world free of nuclear weapons will also require effective measures to impede or counter any nuclear-related conduct that is potentially threatening to the security of any state or peoples.

Reassertion of the vision of a world free of nuclear weapons and practical measures toward achieving that goal would be, and would be perceived as, a bold initiative consistent with America's moral heritage. The effort could have a profoundly positive impact on the security of future generations. Without the bold vision, the actions will not be perceived as fair or urgent. Without the actions, the vision will not be perceived as realistic or possible.

We endorse setting the goal of a world free of nuclear weapons and working energetically on the actions required to achieve that goal, beginning with the measures outlined above.

Mr. Shultz, a distinguished fellow at the Hoover Institution at Stanford, was secretary of state from 1982 to 1989. Mr. Perry was secretary of defense from 1994 to 1997. Mr. Kissinger, chairman of Kissinger Associates, was secretary of state from 1973 to 1977. Mr. Nunn is former chairman of the Senate Armed Services Committee.

A conference organized by Mr. Shultz and Sidney D. Drell was held at Hoover to reconsider the vision that Reagan and Mr. Gorbachev brought to Reykjavik. In addition to Messrs. Shultz and Drell, the following participants also endorse the view in this statement: Martin Anderson, Steve Andreasen, Michael Armacost, William Crowe, James Goodby, Thomas Graham Jr., Thomas Henriksen, David Holloway, Max Kampelman, Jack Matlock, John McLaughlin, Don Oberdorfer, Rozanne Ridgway, Henry Rowen, Roald Sagdeev and Abraham Sofaer.

The probability of a nuclear weapon one day going off in an American city cannot be calculated, but it is larger than it was five years ago. Potential sources of bombs or the fissile materials to make them have proliferated in North Korea and Iran. Russia's arsenal remains incompletely secured 15 years after the end of the Soviet Union. And Pakistans nuclear technology, already put on the market once by Abdul Qadeer Khan, could go to terrorists if the president, Gen. Pervez Musharraf, cannot control radicals in that country.

In the same period, terrorism has surged into a mass global movement and seems to gather strength daily as extremism spills out of Iraq into the rest of the Middle East, Asia, Europe and even the Americas. More nuclear materials that can be lost or stolen plus more terrorists aspiring to mass destruction equals a greater chance of nuclear terrorism.

Former Senator Sam Nunn in 2005 framed the need for Washington to do better at changing this math with a provocative question: On the day after a nuclear weapon goes off in a American city, "what would we wish we had done to prevent it?" But in view of the increased risk we now face, it is time to add a second question to Mr. Nunn's: What will we actually do on the day after? That is, what actions should our government take?

It turns out that much could be done to save lives and ensure that civilization endures in such terrible circumstances. After all, the underlying equation would remain a few terrorists acting against all the rest of us, and even nuclear weapons need not undermine our strong societies if we prepare to act together and sensibly. Sadly, it is time to consider such contingency planning.

First and foremost, the scale of disaster would quickly overwhelm even the most prepared city and state governments. To avoid repeating the Hurricane Katrina fiasco on a much larger scale, Washington must stop pretending that its role would be to support local responders. State and local governments--though their actions to save lives and avoid panic in the first hours would be essential--must abandon the pretense that they could remain in charge. The federal government, led by the Department of Homeland Security, should plan to quickly step in and take full responsibility and devote all its resources, including those of the Department of Defense, to the crisis.

Only the federal government could help the country deal rationally with the problem of radiation, which is unique to nuclear terrorism and uniquely frightening to most people. For those within a two-mile circle of a Hiroshima-sized detonation (in Washington, that diameter is the length of the Mall; in New York, three-fourths the length of Central Park; in most cities, the downtown area) or just downwind, little could be done. People in this zone who were not killed by the blast itself, perhaps hundreds of thousands of them, would get radiation sickness, and many would die.

But most of a city's residents, being further away, would have more choices. What should they do as they watch a cloud of radioactive debris rise and float downwind like the dust from the twin towers on 9/11? Those lucky enough to be upwind could remain in their homes if they knew which way the fallout plume was blowing. (The federal government has the ability to determine that and to quickly broadcast the information.) But for those downwind and more than a few miles from ground zero, the best move would be to shelter in a basement for three days or so and only then leave the area.

This is a hard truth to absorb, since we all would have a strong instinct to flee. But walking toward the suburbs or sitting in long traffic jams would directly expose people to radiation, which would be the most intense on the day after the bomb goes off. After that, the amount would drop off day by day (one third as strong after three days, one fifth as strong after five days, and so on), because of the natural decay of the radioactive components of the fallout.

More tough decisions would arise later. People downwind could leave their homes or stay, leave for a while and then come back or leave and come back briefly to retrieve valuables. The choices would be determined by the dose of radiation they were willing to absorb. Except in the hot zone around the blast and a few miles downwind, even unsheltered people would not be exposed to enough radiation to make them die or even become sick. It would be enough only to raise their statistical chance of getting cancer later in life from 20 percent (the average chance we all have) to something greater--21 percent, 22 percent, up to 30 percent at the maximum survivable exposure.

Similar choices would face first responders and troops sent to the stricken area: how close to ground zero could they go, and for how long? Few would choose to have their risk of death from cancer go up to 30 percent. But in cases of smaller probabilities--an increase to 20.1 percent, for example--a first responder might be willing to go into the radiation zone, or a resident might want to return to pick up a beloved pet. These questions could be answered only by the individuals themselves, based on information about the explosion.

Next comes the unpleasant fact that the first nuclear bomb may well not be the last. If terrorists manage to obtain a weapon, or the fissile material to make one (which fits into a small suitcase), who's to say they wouldn' have two or three more? And even if they had no more weapons, the terrorists would most likely claim that they did. So people in other cities would want to evacuate on the day after, or at least move their children to the countryside, as happened in England during World War II.

The United States government, probably convened somewhere outside Washington by the day after, would be urgently trying to trace the source of the bombs. No doubt, the trail would lead back to some government--Russia, Pakistan, North Korea or other countries with nuclear arsenals or advanced nuclear power programs--because even the most sophisticated terrorist groups cannot make plutonium or enrich their own uranium; they would need to get their weapons or fissile materials from a government.

The temptation would be to retaliate against that government. But it might not even be aware that its bombs were stolen or sold, let alone have deliberately provided them to terrorists. Retaliating against Russia or Pakistan would therefore be counterproductive. Their cooperation would be needed to find out who got the bombs and how many there were, and to put an end to the campaign of nuclear terrorism. It is important to continue to develop the ability to trace any bomb by analyzing its residues. Any government that did not cooperate in the search should of course face possible retaliation.

Finally, as buildings and lives were destroyed, so would the sense of safety and well-being of survivors, and this in turn could lead to panic. Contingency plans for the day after a nuclear blast should demonstrate to Americans that all three branches of government can work in unison and under the Constitution to respond to the crisis and prevent further destruction.

A council of, say, the president, the vice president, the speaker of the House and the majority leader of the Senate, with the chief justice of the Supreme Court present as an observer, could consider certain aspects of the government's response, like increased surveillance. Any emergency measures instituted on the day after should be temporary, to be reviewed and curtailed as soon as the crisis ends.

Forceful efforts to prevent a nuclear attack--more forceful than we have seen in recent years--may keep the day from coming. But as long as there is no way to be sure it will not, it is important to formulate contingency plans that can save thousands of lives and billions of dollars, prevent panic and promote recovery. They can also help us preserve our constitutional government, something that terrorists, even if armed with nuclear weapons, should never be allowed to take away.

William J. Perry, a professor at Stanford, and Ashton B. Carter, a professor at Harvard, were, respectively, the secretary and an assistant secretary of defense in the Clinton administration. Michael M. May, also a professor at Stanford, is a former director of the Lawrence Livermore National Laboratory.