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.

Developments during the last several weeks seem to suggest that Russia is reconsidering its nuclear cooperation with Iran. Just months ago, Moscow pledged with great fanfare that the Bushehr reactor would be ready for the first shipment of fuel in March and would reach criticality in September 2007. But in February, Russia backtracked, claiming it had to delay the fuel delivery because of missed payments. As for the reactor's launch, the only thing that's certain is that it will not happen in September. The situation became even more puzzling after reports that Russia warned Iran that Moscow might suspend the project if Tehran does not stop its enrichment program and that some Russian technical specialists are returning home.

Are we seeing a radical turn in Russian policy? Probably not, but the situation is more complicated.

The six-party agreement reached last week in Beijing to cap North Korea's nuclear program was a triumph for diplomacy. But contrary to much of the conventional wisdom in recent days, the fruits of the victory fall mostly to the North Koreans.

In the short term, the deal will halt the country's production of nuclear materials, limiting its ability to expand a nuclear arsenal tested in October. But for this concession, the North Koreans get to keep that arsenal intact, at least for now, and stand to make significant economic and political gains in relations with the United States, China and South Korea.

Some critics say the Beijing agreement is a lesser version of "the Agreed Framework" reached in 1994 by the Clinton administration, later cast aside by President Bush. Former Clinton-era Defense Secretary William Perry, speaking Tuesday at the Asia Society, characterized the new agreement as "thin gruel," while backing it as "a small but a very important step forward."

The ultimate judgment will await the uncertain implementation of numerous crucial, but still vaguely defined, steps down the road. The North Koreans are certain to exploit every ambiguity in the text and to drag out the phase that calls for actual dismantlement of their nuclear program and weapons.

Unfortunately, the process that led to this moment suggests that this will not go well. Contrary to the administration's version of events, Pyongyang was not dragged to this deal by pressure -- not from Washington and not from North Korea's angry patrons in Beijing.

"We don't have the North Koreans on the ropes," a former senior U.S. intelligence analyst who has watched that closeted country for decades said. "We don't have them on the run."

On the contrary, there is ample evidence that this agreement is yet another demonstration of North Korea's uniquely successful brand of negotiation via escalation: a use of brinkmanship and willingness to go up to and over the line that converts weakness into leverage.

Against that approach, the Bush administration's preference for using tools of coercion and threat, even of pre-emptive war, failed. If anything, it brought about the very opposite outcome than the United States envisioned: it encouraged North Korea to move even more rapidly to develop and test a nuclear weapon.

The pattern of brinkmanship was already clear during the Clinton years -- what Korea expert Scott Snyder famously termed "negotiating on the edge." When confronted, Snyder noted, the North Koreans typically responded by accelerating the crisis, unworried by the consequences. The fear of appearing weak has underlined all North Korean behavior.

The Bush administration came into office almost seeking a confrontation, as the president and many of his advisers were convinced the 1994 deal was fatally flawed. Ironically, the North Koreans thought they were on the verge of strategic breakthrough, after a deal to halt missile tests and preparations for President Clinton to visit Pyongyang in the final weeks of his administration. An improved relationship with the United States would balance the power of its Chinese patron, whom North Korea deeply distrusts, and give it legitimacy in an ongoing struggle with South Korea for leadership on the Korean peninsula.

Instead Bush froze the Clinton framework and sought a new, tougher approach. In January 2002, Bush delivered his famous State of the Union depiction of North Korea as a member of the "axis of evil," along with Iran and Iraq. That October, U.S. negotiators confronted Pyongyang with accusations of cheating by pursuing a clandestine uranium-enrichment program.

The 1994 agreement collapsed amid a tit-for-tat series of escalatory moves -- beginning with a U.S. cutoff of heavy fuel oil and leading to North Korea ousting international inspectors, withdrawing from the Nuclear Non-proliferation Treaty and restarting its reactor and recycling facility to produce plutonium. Bush vowed that the United States would not "be blackmailed."

Meanwhile, preparations for war in Iraq were mounting. The Bush administration was convinced the awesome display of U.S. power would successfully intimidate the other two points on the axis of evil, North Korea and Iran.

"We are hopeful," then senior State Department official John Bolton dryly said as the invasion came to a close, "that a number of regimes will draw the appropriate lesson from Iraq -- that the pursuit of weapons of mass destruction is not in their interest."

American threat

The North Korean officials drew an entirely different conclusion: they could not afford to seem weak in the face of what they perceived as an American threat to terminate their regime.

"Only tremendous military deterrent force powerful enough to decisively beat back an attack supported by ultra-modern weapons can avert a war and protect the security of the country," said an official statement issued April 6. "This is the lesson drawn from the Iraqi war."

A drawn-out process of negotiations began later that month, beginning with a three-way meeting in China and moving that summer to six-party talks that also included South Korea, Japan and Russia. The U.S. position was to deny Pyongyang what it wanted most -- direct talks with Washington -- and to demand verified dismantlement of its nuclear program, on the model of Libya, before any rewards, economic or political, were provided.

As the war in Iraq wore on, and the threat of military force became less credible, the administration looked for other coercive tools. It forged a multinational agreement to intercept suspicious cargoes and launched a crackdown on illicit North Korea trafficking in drugs and counterfeit currency and goods, which are believed to be the main source of support for the regime's elite.

The North Koreans countered with their own demands, offering a plan to freeze their nuclear program, with compensation, followed by a coordinated series of reciprocal steps leading toward eliminating the program. Their offers were accompanied by statements that they already had the bomb and were prepared to test it.

When the Bush administration started its second term in 2005, it attempted to escalate pressure -- this time with charges that North Korea was exporting nuclear materials to the Middle East and calls for China to put pressure on its difficult clients. Pyongyang moved to unload a second set of spent fuel from its reactor and reprocess it -- American experts believe North Korea created six to eight bombs worth of plutonium after 2002.

Agreement sours

A return to the bargaining table in September 2005 yielded an agreement on the principles that would underlie a denuclearization of the Korean peninsula. But that sign of progress disappeared within hours as both sides sparred over the meaning of a pledge to build nuclear power reactors for North Korea as compensation for it dismantling its nuclear weapons.

The imposition of measures to curb the flow of North Korean "illicit" money through Chinese and other banks added to the acrimony. Administration officials described this as a legal issue driven by Treasury Department efforts to curb counterfeiting. But as Bush admitted recently, it was used as leverage in the nuclear talks.

Throughout the past year, Bush administration officials expressed confidence that these measures were causing serious pain to the North Korean leadership. Some even talked boldly of "turning out the lights" in Pyongyang through such sanctions.

But Pyongyang could read the news from Iraq as well as any American voter. Instead of having its lights turned out, North Koreans put up their own light shows. On July 4, a date chosen with apparent intent, they carried out a test of a battery of ballistic missiles, in defiance of warnings, including one from China. A U.N. resolution condemning the action -- and other steps, including a South Korean suspension of food and fertilizer aid and Chinese attempts to slow trade -- followed.

In October, again in defiance of pressure from all fronts, the North Koreans tested a nuclear device. This prompted another U.N. resolution, backed by China, to impose limited economic sanctions. But although China was clearly angered, there is little evidence it moved to cut off the lifeline of trade, particularly energy supplies.

North Korea's willingness to cross what everyone believed was a "red line" changed the equation permanently. It allowed Pyongyang to return to the six-party talks, stalled for more than a year, but now from a position of strength. At the meeting in December, the North Koreans refused to discuss any other issues unless the U.S. financial sanctions were removed. North Korean officials hinted of preparations for a second test.

The United States blinked, agreeing to hold long-sought direct talks, held in Berlin in mid-January. The talks yielded the outlines of the Beijing deal but also a separate U.S. concession to lift the financial measures within 30 days of signing a broader deal.

The Beijing agreement more closely resembles North Korea's June 2004 freeze proposal than it does the U.S. insistence that dismantling nuclear weapons precede any substantial rewards. Clearly, this is a deal the Bush administration would not have made, says Scott Snyder, "if it were not tied down with so many other problems."

North Korea made its own concessions in the Beijing agreement. But "it doesn't necessarily mean Pyongyang is backing down or preparing to abandon its nuclear weapons," argues Kim Sung Han, a senior analyst at the South Korean Foreign Ministry's research institute.

N. Korea's rewards

Administration officials point out that the initial freeze of North Korea's nuclear program, to be implemented in two months, yields only minor compensation, about 50,000 tons of heavy fuel oil. But that is not what Pyongyang sees as its real reward. The lifting of financial measures will facilitate its rapidly growing trade with China and South Korea. Even more important, the South Korean government has already signaled it will now lift the ban on large-scale fertilizer and food shipments -- which are crucial to North Korea's spring planting.

Less visible, but no less vital, the North Koreans are trying to hold off a conservative comeback to power in the South Korean presidential election in December. A North-South summit meeting may take place, which would be part of an effort by the progressive South Korean government to shore up its support.

Ultimately, the Beijing agreement may yield a trade of nuclear facilities for economic and political relations, leaving the nuclear arsenal capped but still intact. For some U.S. experts, that is sufficient.

"It will limit the size of the nuclear arsenal and the amount of bomb fuel," observes former Los Alamos nuclear laboratory director and Stanford scholar Siegfried Hecker. And that, he says, should make it less likely North Korea would sell its nuclear materials or expertise to Iran.

The bargain made in Beijing flows inexorably from North Korea's skillful playing of the escalation game. But it may be the best outcome possible, given that North Korea has already crossed the nuclear threshold and that the Bush administration has squandered U.S. power in the deserts of Iraq.

Reprinted with permission from the San Jose Mercury News.