Today, at the other end of the long trek down the glacier of the Cold War, the nuclear threat has seemingly calved off and fallen into the sea. In 2007, the Pew Research Center's Global Attitudes Project found that 12 countries rated the growing gap between rich and poor as the greatest danger to the world. HIV/AIDS led the list (or tied) in 16 countries, religious and ethnic hatred in another 12. Pollution was identified as the greatest menace in 19 countries, while substantial majorities in 25 countries thought global warming was a "very serious" problem. Only nine countries considered the spread of nuclear weapons to be the greatest danger to the world.

The response was very different among nuclear and national security experts when Indiana Republican Sen. Richard Lugar surveyed PDF them in 2005. This group of 85 experts judged that the possibility of a WMD attack against a city or other target somewhere in the world is real and increasing over time. The median estimate of the risk of a nuclear attack somewhere in the world by 2010 was 10 percent. The risk of an attack by 2015 doubled to 20 percent median. There was strong, though not universal, agreement that a nuclear attack is more likely to be carried out by a terrorist organization than by a government. The group was split 45 to 55 percent on whether terrorists were more likely to obtain an intact working nuclear weapon or manufacture one after obtaining weapon-grade nuclear material.

"The proliferation of weapons of mass destruction is not just a security problem," Lugar wrote in the report's introduction. "It is the economic dilemma and the moral challenge of the current age. On September 11, 2001, the world witnessed the destructive potential of international terrorism. But the September 11 attacks do not come close to approximating the destruction that would be unleashed by a nuclear weapon. Weapons of mass destruction have made it possible for a small nation, or even a sub-national group, to kill as many innocent people in a day as national armies killed in months of fighting during World War II.

"The bottom line is this," Lugar concluded: "For the foreseeable future, the United States and other nations will face an existential threat from the intersection of terrorism and weapons of mass destruction."

It's paradoxical that a diminished threat of a superpower nuclear exchange should somehow have resulted in a world where the danger of at least a single nuclear explosion in a major city has increased (and that city is as likely, or likelier, to be Moscow as it is to be Washington or New York). We tend to think that a terrorist nuclear attack would lead us to drive for the elimination of nuclear weapons. I think the opposite case is at least equally likely: A terrorist nuclear attack would almost certainly be followed by a retaliatory nuclear strike on whatever country we believed to be sheltering the perpetrators. That response would surely initiate a new round of nuclear armament and rearmament in the name of deterrence, however illogical. Think of how much 9/11 frightened us; think of how desperate our leaders were to prevent any further such attacks; think of the fact that we invaded and occupied a country, Iraq, that had nothing to do with those attacks in the name of sending a message.

Richard Butler, the former chairman of the Canberra Commission on the Elimination of Nuclear Weapons and the last chairman of UNSCOM, often makes the point that the problem with nuclear weapons is nuclear weapons. People don't always understand what he means. He means that it is the weapons themselves that are the problem, not the values of the entities that control them. U.S. nuclear weapons are just as potentially dangerous to the world as, say, North Korean nuclear weapons. More, I would say, since we have greater numbers of them and have not hesitated to brandish them--even to use them--when we thought it in our interest to do so.

That the problem with nuclear weapons is nuclear weapons may seem counterintuitive, but two centuries ago governments began to think that way about disease, with untold benefits to humanity as a result. Epidemic disease had been conceived in normative terms, as an act of God for which states bore no responsibility. The change that came when disease began to be conceived as a phenomenon of nature without a metaphysical superstructure, a public health problem, a problem for government and a measure of government's success, was revolutionary. More lives were saved, and spared, with public health measures in the twentieth century in the United States alone than were lost throughout the world in all of the twentieth century's wars.

As my Scottish friend Gil Elliot wrote in his seminal book Twentieth Century Book of the Dead, "[These lives] are not saved by accident or goodwill. Human life is daily deliberately protected from nature by accepted practices of hygiene and medical care, by the control of living conditions and the guidance of human relationships. Mortality statistics are constantly examined to see if the causes of death reveal any areas needing special attention. Because of the success of these practices, the area of public death has, in advanced societies, been taken over by man-made death--once an insignificant or 'merged' part of the spectrum, now almost the whole.

"When politicians, in tones of grave wonder, characterize our age as one of vast effort in saving human life, and enormous vigor in destroying it, they seem to feel they are indicating some mysterious paradox of the human spirit. There is no paradox and no mystery. The difference is that one area of public death has been tackled and secured by the forces of reason; the other has not. The pioneers of public health did not change nature, or men, but adjusted the active relationship of men to certain aspects of nature so that the relationship became one of watchful and healthy respect. In doing so they had to contend with and struggle against the suspicious opposition of those who believed that to interfere with nature was sinful, and even that disease and plague were the result of something sinful in the nature of man himself."

Elliot goes on to compare what he calls "public death," meaning biological death, death from disease, to man-made death: "[I do not wish] to claim mystical authority for the comparison I have made between two kinds of public death--that which results from disease and that which we call man-made. The irreducible virtue of the analogy is that the problem of man-made death, like that of disease, can be tackled only by reason. It contains the same elements as the problem of disease--the need to locate the sources of the pest, to devise preventive measures, and to maintain systematic vigilance in their execution. But it is a much wider problem, and for obvious reasons cannot be dealt with by scientific methods to the same extent as can disease."

To advance the cause of public health it was necessary to depoliticize disease, to remove it from the realm of value and install it in the realm of fact. Today we have advanced to the point where international cooperation toward the prevention, control, and even elimination of disease is possible among nations that hardly cooperate with each other in any other way. No one any longer considers disease a political issue, except to the extent that its control measures a nation's quality of life, and only modern primitives consider it a judgment of God.

In 1999, for the first time in human history, infectious diseases no longer ranked first among causes of death worldwide. Public health, a discipline which organizes science-based systems of surveillance and prevention, was primarily responsible for that millennial change in human mortality. One-half of all the increases in life expectancy in recorded history occurred within the twentieth century. Most of the worldwide increase was accomplished in the first half of the century, and it was almost entirely the result of public health measures directed to primary prevention. Better nutrition, sewage treatment, water purification, the pasteurization of milk, and the immunization of children extended human life--not surgeons cutting or doctors dispensing pills.

Public health is medicine's greatest success story and a powerful model for a parallel discipline, which I propose to call public safety.

Where nuclear weapons--the largest-scale instruments of man-made death--are concerned, the elements of that discipline of public safety have already begun to assemble themselves: materials control and accounting, cooperative threat reduction, security guarantees, agreements and treaties, surveillance and inspection, sanctions, forceful disarming if all else fails.

Reducing and finally eliminating the world's increasingly vestigial nuclear arsenals may be delayed by extremists of the right or the left, as progress was stalled during the George W. Bush administration by rigid Manichaean ideologues who imagined that there might be good nuclear powers and evil nuclear powers and sought to disarm only those they considered evil. Nuclear weapons operate beyond good and evil. They destroy without discrimination or mercy: Whether one lives or dies in their operation is entirely a question of distance from ground zero. In Elliot's eloquent words, they create nations of the dead, and collectively have the capacity to create a world of the dead. But as Niels Bohr, the great Danish physicist and philosopher, was the first to realize, the complement of that utter destructiveness must then be unity in common security, just as it was with smallpox, a fundamental transformation in relationships between nations, nondiscrimination in unity not on the dark side but by the light of day.

Violence originates in vulnerability brutalized: It is vulnerability's corruption, but also its revenge. "Perhaps everything terrible," the poet Rainer Maria Rilke once wrote, "is in its deepest being something helpless that wants help from us." As we extend our commitment to common security, as we work to master man-made death, we will need to recognize that terrible helplessness and relieve it--in others, but also in ourselves.