Japan's ruling party suffered a historic defeat Sunday. For the first time since the conservative Liberal Democratic Party (LDP) was formed in 1955, an opposition party has become the largest party in the upper house.

The powerful message delivered by Japanese voters has significant implications not only for Japan but also for the rest of the world, not least for its close ally, the United States.

The election result revives momentum in Japan toward creation of a viable two-party system, potentially ending the conservative postwar monopoly on power. Japanese voters expressed deep anxiety about the impact of economic change upon their treasured social order. They embraced the campaign of the Democratic Party (the main opposition) against growing income inequality and the failure of the state to take care of an aging population.

Equally important, the vote was a humiliating defeat for Prime Minister Shinzo Abe's agenda of giving priority to revising Japan's antiwar Constitution and allowing its military to take on a global role in support of the US. Democratic Party leader Ichiro Ozawa effectively portrayed Mr. Abe as a man out of touch with the concerns of ordinary Japanese. But he also articulated an alternative vision of Japan's international role, calling for closer ties to its Asian neighbors and sending troops overseas only under the auspices of United Nations peacekeeping missions.

Since 9/11, Japan has been among the most loyal, if not unquestioning, of US allies. It sent troops to Iraq, provided logistical support to the war in Afghanistan, and outdid the US in putting pressure on North Korea. Most recently, Abe echoed the rhetoric of the Bush administration, calling for formation of a "values-based" alliance of democracies along with India and Australia, implicitly aimed at containing a rising China. The election results will certainly slow, if not reverse, this tight synchronization.

For the business community, the vote will raise concerns that needed economic policy actions such as fiscal reforms will get stalled in a gridlocked parliament. The vote reminds politicians that the economic recovery has left an awful lot of Japanese behind, with real wages falling, youth unemployment high, and the elderly drawing down their savings to survive. Abe's feel-good rhetoric and focus on security just angered those Japanese.

There remains strong support for gradual change. Most Japanese want the country to take on a more "normal" security role, but one that will stop far short of overdrawn fears of a remilitarized Japan. And many Japanese, particularly in the younger generation, back economic reform, though not at the expense of social stability.

The most intriguing question is the future of Japan's democracy. Abe is resisting calls for his resignation, attributing the vote to a series of scandals in his Cabinet and most of all to the revelation that the government's national pension system had lost the records of some 50 million people. The election result was bad luck, Abe claimed, not a repudiation of his administration's overall policies -- a view shared by Washington policymakers.

Exit polls do confirm that voters were strongly motivated by these issues. But they also express little faith in the personal leadership of Abe, who tried to cover up the pension debacle. He suffered from an unfavorable comparison to his predecessor, Junichiro Koizumi, one of Japan's most popular postwar leaders.

But the election suggests that Mr. Koizumi's personal charisma only temporarily reversed a longer trend of drift away from the ruling conservatives, particularly by unaffiliated swing voters in Japan's cities and suburbs. Mr. Ozawa, one of Japan's most brilliant politicians, managed to both regain those voters and steal away traditional conservative backers in rural areas among farmers and pensioners worried about their future.

Ozawa, whom I have known for more than two decades, is a man of uncommon political vision. He is a former LDP stalwart who has relentlessly pursued the goal of creating a clearly defined two-party system that can create real competition. He was the architect of a split in the LDP that briefly brought the opposition to power in the early 1990s.

Over dinner last fall, Ozawa laid out to me what seemed then like an incredibly audacious plan to regain power. First to win a series of local elections, leading up to a defeat of the LDP in the upper house election, forcing in turn the dissolution of the lower house and new elections. He clearly hopes to split the LDP again and pry away its coalition partner, the New Komeito Party, as part of his strategy of realignment.

The Democratic Party has yet to demonstrate its own ability to rule, but it would be unwise to underestimate Ozawa. And it would be foolish to dismiss the desire for change delivered by Japanese voters on Sunday.

Reprinted with permission by the Christian Science Monitor.

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.