To many people, the answer is obvious: the bomb. War had become too dangerous to contemplate, and leaders were scared straight. The balance of nuclear terror deterred them from starting a war that would escalate to a holocaust and put an end to civilization, if not human life itself. As Winston Churchill said in his last major speech to Parliament, "It may well be that we shall by a process of sublime irony have reached a stage in this story where safety will be the sturdy child of terror, and survival the twin brother of annihilation." In the same vein, the foreign policy analyst Kenneth Waltz has suggested that we "thank our nuclear blessings," and Elspeth Rostow proposed that the nuclear bomb be awarded the Nobel Peace Prize.

What went right? How is it that, in defiance of experts, doomsday clocks, and centuries of European history, World War III never happened? What allowed distinguished military historians to use giddy phrases like "a change of spectacular proportions," "the most striking discontinuity in the history of warfare," and "nothing like this in history"?

For one thing, weapons of mass destruction had never braked the march to war before. The benefactor of the Nobel Peace Prize wrote in the 1860s that his invention of dynamite would "sooner lead to peace than a thousand world conventions, [since] as soon as men will find that in one instant whole armies can be utterly destroyed, they will surely abide in golden peace." Similar predictions have been made about submarines, artillery, smokeless powder, and the machine gun. The 1930s saw a widespread fear that poison gas dropped from airplanes could bring an end to civilization and human life, yet that dread did not come close to ending war either. As Luard puts it, "There is little evidence in history that the existence of supremely destructive weapons alone is capable of deterring war. If the development of bacteriological weapons, poison gas, nerve gases, and other chemical armaments did not deter war in 1939, it is not easy to see why nuclear weapons should do so now."

Let's hope not. If the Long Peace were a nuclear peace, it would be a fool's paradise, because an accident, a miscommunication, or an air force general obsessed with precious bodily fluids could set off an apocalypse. Thankfully, a closer look suggests that the threat of nuclear annihilation deserves little credit for the Long Peace.

Also, the theory of the nuclear peace cannot explain why countries without nuclear weapons also forbore war -- why, for example, the 1995 squabble over fishing rights between Canada and Spain, or the 1997 dispute between Hungary and Slovakia over damming the Danube, never escalated into war, as crises involving European countries had so often done in the past. During the Long Peace leaders of developed countries never had to calculate which of their counterparts they could get away with attacking (yes for Germany and Italy, no for Britain and France), because they never contemplated a military attack in the first place. Nor were they deterred by nuclear godparents -- it wasn't as if the United States had to threaten Canada and Spain with a nuclear spanking if they got too obstreperous in their dispute over flatfish.

As for the superpowers themselves, Mueller points to a simpler explanation for why they avoided fighting each other: they were deterred plenty by the prospect of a conventional war. World War II showed that assembly lines could mass-produce tanks, artillery, and bombers that were capable of killing tens of millions of people and reducing cities to rubble. This was especially obvious in the Soviet Union, which had suffered the greatest losses in the war. It's unlikely that the marginal difference between the unthinkable damage that would be caused by a nuclear war and the thinkable but still staggering damage that would be caused by a conventional war was the main thing that kept the great powers from fighting.

Finally, the nuclear peace theory cannot explain why the wars that did take place often had a nonnuclear force provoking (or failing to surrender to) a nuclear one -- exactly the matchup that the nuclear threat ought to have deterred. North Korea, North Vietnam, Iran, Iraq, Panama, and Yugoslavia defied the United States; Afghan and Chechen insurgents defied the Soviet Union; Egypt defied Britain and France; Egypt and Syria defied Israel; Vietnam defied China; and Argentina defied the United Kingdom. For that matter, the Soviet Union established its stranglehold on Eastern Europe during just those years (1945-49) when the United States had nuclear weapons and it did not. The countries that goaded their nuclear superiors were not suicidal. They correctly anticipated that for anything but an existential danger, the implicit threat of a nuclear response was a bluff. The Argentinian junta ordered the invasion of the Falkland Islands in full confidence that Britain would not retaliate by reducing Buenos Aires to a radioactive crater. Nor could Israel have credibly threatened the amassed Egyptian armies in 1967 or 1973, to say nothing of Cairo.

Schelling, and the political scientist Nina Tannenwald, have each written of "a nuclear taboo"-- a shared perception that nuclear weapons fall into a uniquely dreadful category. The use of a single tactical nuclear weapon, even one comparable in damage to conventional weaponry, would be seen as a breach in history, a passage into a new world with unimaginable consequences. The obloquy has attached itself to every form of nuclear detonation. The neutron bomb, a weapon that would cause minimal blast damage but would kill soldiers with a transient burst of radiation, fell deadborn from the military lab because of universal loathing, even though, as the political scientist Stanley Hoffman pointed out, it satisfied the moral philosophers' requirements for waging a just war. The half-crazed "Atoms for Peace" schemes of the 1950s and 1960s, in which nuclear explosions would be harnessed to dig canals, excavate harbors, or propel rockets into space, are now the stuff of incredulous reminiscences of a benighted age.

To be sure, the nonuse of nuclear weapons since Nagasaki falls short of an out-and-out taboo. Nuclear bombs don't build themselves, and nations have devoted enormous thought to the design, construction, delivery, and terms of use of these weapons. But this activity has been compartmentalized into a sphere of hypotheticals that barely intersects with the planning of actual wars. And there are telltale signs that the psychology of taboo -- a mutual understanding that certain thoughts are evil to think -- has been engaged, starting with the word that is most commonly applied to the prospect of nuclear war: unthinkable. In 1964, after Barry Goldwater had mused about how tactical nuclear weapons might be used in Vietnam, Lyndon Johnson's electoral campaign aired the famous "Daisy" television ad, in which footage of a girl counting the petals of a daisy segues into a countdown to a nuclear explosion. The ad has been given some of the credit for Johnson's landslide election victory that year. Religious allusions have surrounded nuclear weapons ever since Robert Oppenheimer quoted the Bhagavad-Gita when he viewed the first atomic test in 1945: "Now I am become Death, the destroyer of worlds." More commonly the language has been biblical: Apocalypse, Armageddon, the End of Days, Judgment Day. Dean Rusk, secretary of state in the Kennedy and Johnson administrations, wrote that if the country had used a nuclear weapon, "we would have worn the mark of Cain for generations to come." The physicist Alvin Weinberg, whose research helped make the bomb possible, asked in 1985:

The nuclear taboo emerged only gradually. As we saw in chapter 1, for at least a decade after Hiroshima many Americans thought the A-bomb was adorable. By 1953 John Foster Dulles, secretary of state in the Eisenhower administration, was deploring what he called the "false distinction" and "taboo" surrounding nuclear weapons. During a 1955 crisis involving Taiwan and the People's Republic of China, Eisenhower said, "In any combat where these things can be used on strictly military targets and for strictly military purposes, I see no reason why they shouldn't be used just exactly as you would use a bullet or anything else."

Are we witnessing a gradual sanctification of Hiroshima -- that is, the elevation of Hiroshima to the status of a profoundly mystical event, an event ultimately of the same religious force as biblical events? I cannot prove it, but I am convinced that the 40th Anniversary of Hiroshima, with its vast outpouring of concern, bears resemblance to the observance of major religious holidays… This sanctification of Hiroshima is one of the most hopeful developments of the nuclear era.

Though the United States and the USSR continued to develop nuclear technology at a breakneck pace, they began, however hypocritically, to pay homage to nuclear disarmament in conferences and statements. At the same time a grassroots movement began to stigmatize the weapons. Demonstrations and petitions attracted millions of citizens, together with public figures such as Linus Pauling, Bertrand Russell, and Albert Schweitzer. The mounting pressure helped nudge the superpowers to a moratorium and then a ban on atmospheric nuclear testing, and then to a string of arms-control agreements. The Cuban Missile Crisis in 1962 was a tipping point. Lyndon Johnson capitalized on the change to demonize Goldwater in the Daisy ad and called attention to the categorical boundary in a 1964 public statement: "Make no mistake. There is no such thing as a conventional nuclear weapon. For nineteen peril-filled years no nation has loosed the atom against another. To do so now is a political decision of the highest order."

But in the following decade nuclear weapons acquired a stigma that would put such statements beyond the pale. It began to sink in that the weapons' destructive capacity was of a different order from anything in history, that they violated any conception of proportionality in the waging of war, and that plans for civil defense (like backyard fallout shelters and duck-and-cover drills) were a travesty. People became aware that lingering radiation from nuclear fallout could cause chromosome damage and cancer for decades after the actual explosions. The fallout from atmospheric tests had already contaminated rainfall all over the world with strontium 90, a radioactive isotope resembling calcium that is taken up in the bones and teeth of children (inspiring Malvina Reynolds's protest song "What Have They Done to the Rain?").

As the world's luck held out, and the two nuclear-free decades grew to three and four and five and six, the taboo fed on itself in the runaway process by which norms become common knowledge. The use of nuclear weapons was unthinkable because everyone knew it was unthinkable, and everyone knew that everyone knew it. The fact that wars both large (Vietnam) and small (Falklands) were not deterred by the increasingly ineffectual nuclear threat was a small price to pay for the indefinite postponement of Armageddon.

A norm that rests only on mutual recognition of that norm is, of course, vulnerable to a sudden unraveling. One might worry -- one should worry -- that nuclear nations outside the club of great powers, such as India, Pakistan, North Korea, and perhaps soon Iran, may not be party to the common understanding that the use of nuclear weapons is unthinkable. Worse, a terrorist organization that pilfered a stray nuclear weapon could make a point of defying the taboo, since the whole point of international terrorism is to shock the world with the most horrific spectacle imaginable. Once the precedent of a single nuclear explosion was set, one might worry, all restraints would be put aside. A pessimist might argue that even if the Long Peace has not, thus far, depended on nuclear deterrence, it is an ephemeral hiatus. It will surely end as nuclear weapons proliferate, a maniac from the developing world brings the lucky streak to an end, and the taboo comes undone among small and great powers alike.

But the only country that fulfilled his prophecy is Israel (" 'The Lord's my shepherd,' says the Psalm / But just in case -- we better get a bomb!"). Contrary to expert predictions that Japan would "unequivocally start on the process of acquiring nuclear weapons" by 1980 and that a reunified Germany "will feel insecure without nuclear weapons," neither country seems interested in developing them. And believe it or not, since 1964 as many countries have given up nuclear weapons as have acquired them. Say what? While Israel, India, Pakistan, and North Korea currently have a nuclear capability, South Africa dismantled its stash shortly before the collapse of the apartheid regime in 1989, and Kazakhstan, Ukraine, and Belarus said "no thanks" to the arsenals they inherited from the defunct Soviet Union. Also, believe it or not, the number of nonnuclear nations that are pursuing nuclear weapons has plummeted since the 1980s. Figure 5-22, based on a tally by the political scientist Scott Sagan, charts the number of nonnuclear states in each year since 1945 that had programs for developing nuclear weapons.

No judicious person can feel calm about the parlous state of nuclear safety in today's world. But even here, things are not as bad as many people think. In the next chapter, I'll examine the prospect of nuclear terrorism. For now, let's look at nuclear states.

One hopeful sign is that nuclear proliferation has not proceeded at the furious rate that everyone expected. In the 1960 presidential election debates, John F. Kennedy predicted that by 1964 there might be "ten, fifteen, twenty" countries with nuclear weapons. The concern accelerated when China conducted its first nuclear test in 1964, bringing the number of nations in the nuclear club to five in less than twenty years. Tom Lehrer captured popular fears of runaway nuclear proliferation in his song "Who's Next?" which ran through a list of countries that he expected would soon become nuclear powers ("Luxemburg is next to go / And who knows? Maybe Monaco").

How precarious is the nuclear taboo? Will a rogue state inevitably defy the taboo and thereby annul it for the rest of the world? Doesn't history show that every weapons technology will sooner or later be put to use and then become unexceptionable?

FIGURE 5-22: Nonnuclear states that started and stopped exploring nuclear weapons, 1945-2010

The downslopes in the curve show that at various times Algeria, Australia, Brazil, Egypt, Iraq, Libya, Romania, South Korea, Switzerland, Sweden, Taiwan, and Yugoslavia have pursued nuclear weapons but then thought the better of it -- occasionally through the persuasion of an Israeli air strike, but more often by choice.

Country names marked with "-" represent the year in which a nuclear program in that country was stopped. The countries labeled in gray were believed to be exploring nuclear weapons in 2010. Though Israel bombed a suspected Syrian nuclear facility in 2007, as of 2010 Syria has refused International Atomic Energy Agency inspections, so it is kept on the list of active states. Sources: Graph adapted from Sagan, 2009, with updated information in Sagan, 2010, provided by Scott Sagan and Jane Esberg.

The story of poison gas -- the quintessential horror of World War I -- is one place to look for an answer. In his book The Chemical Weapons Taboo, the political scientist Richard Price recounts how chemical weapons acquired their own stigma during the first half of the 20th century. The Hague Convention of 1899, one of a number of international agreements that aimed to regulate the conduct of war, had banned hollow-point bullets, aerial bombing (from balloons, that is, since the invention of the airplane was four years away), and projectiles that delivered poison gas. Given what was to come, the convention may seem like another candidate for history's dustbin of toothless feel-good manifestos.

But Price shows that even the combatants of World War I felt the need to pay the convention homage. When Germany introduced lethal gas to the battlefield, it claimed that it was retaliating for France's use of tear gas grenades and that anyway, it was conforming to the letter of the law because it didn't deliver the gas in artillery shells but just opened the cylinders and let the wind waft the gas toward the enemy. That these rationalizations were utterly lame shouldn't obscure the fact that Germany felt the need to justify its behavior at all. England, France, and the United States then claimed to be acting in reprisal for Germany's illegal use, and all sides agreed that the convention was no longer in force because nonsignatories (including the United States) had joined the conflict.

After the war, a revulsion against chemical weapons spread through the world. A prohibition with fewer loopholes was institutionalized in the Geneva Protocol of 1925, which declared, "Whereas the use in war of asphyxiating, poisonous or other gases, and of all analogous liquids, materials or devices, has been justly condemned by the general opinion of the civilized world… the prohibition of such use… shall be universally accepted as part of International Law, binding alike the conscience and the practice of nations." Eventually 133 countries signed it, though many of the signatories reserved the right to stockpile the weapons as a deterrent. As Winston Churchill explained, "We are, ourselves, firmly resolved not to use this odious weapon unless it is used first by the Germans. Knowing our Hun, however, we have not neglected to make preparations on a formidable scale."

Whether or not it was the piece of paper that made the difference, the taboo against the use of poison gas in interstate warfare took hold. Astonishingly, though both sides had tons of the stuff, poison gas was never used on the battlefield during World War II. Each side wanted to avoid the opprobrium of being the first to reintroduce poison gas to the battlefield, especially while the Nazis were hoping that England might accede to their conquest of continental Europe. And each side feared retaliation by the other.

The only sustained uses of poison gas in war since the 1930s were by Egypt in Yemen in 1967 and by Iraq against Iranian forces (and its own Kurdish citizens) during the war of 1980-88. Defying the taboo may have been Saddam Hussein's undoing. The revulsion against his use of poison gas muted some of the opposition to the United States-led war that deposed him in 2003, and it figured in two of the seven charges against him in the Iraqi trial that led to his execution in 2006. The world's nations formally abolished chemical weapons in 1993, and every known stockpile is in the process of being dismantled.

The restraint held even in the face of destabilizing events that might have been expected to trigger an unstoppable escalation. In at least two episodes in Europe, poison gas was accidentally released by Allied forces. Explanations were conveyed to the German commanders, who believed them and did not retaliate. A bit of cognitive compartmentalization helped too. In the 1930s Fascist Italy used poison gas in Abyssinia, and Imperial Japan used it in China. But these events were cordoned off in leaders' minds because they took place in "uncivilized" parts of the world rather than within the family of nations. Neither registered as a breach that would have nullified the taboo.

It's not immediately obvious why, out of all the weapons of war, poison gas was singled out as uniquely abominable -- as so uncivilized that even the Nazis kept it off the battlefield. (They clearly had no compunction about using it elsewhere.) It's highly unpleasant to be gassed, but then it's just as unpleasant to be perforated or shredded by pieces of metal. As far as numbers are concerned, gas is far less lethal than bullets and bombs. In World War I fewer than 1 percent of the men who were injured by poison gas died from their injuries, and these fatalities added up to less than 1 percent of the war's death toll. Though chemical warfare is militarily messy -- no battlefield commander wants to be at the mercy of which way the wind is blowing -- Germany could have used it to devastate the British forces at Dunkirk, and American forces would have found it handy in rooting out the Japanese soldiers hiding in caves in the Pacific Rim. And even if chemical weapons are difficult to deploy, that would hardly make them unique, since most new weapon technologies are ineffective when they are introduced. The first gunpowder weapons, for example, were slow to load, difficult to aim, and apt to blow up in the soldier's face. Nor were chemical weapons the first to be condemned for barbarism: in the era of longbows and pikes, gunpowder weapons were denounced as immoral, unmanly, and cowardly. Why did the taboo against chemical weapons take?

One possibility is that the human mind finds something distinctively repugnant about poison. Whatever suspension of the normal rules of decency allows warriors to do their thing, it seems to license only the sudden and directed application of force against an adversary who has the potential to do the same. Even pacifists may enjoy war movies or video games in which people get shot, stabbed, or blown up, but no one seems to get pleasure from watching a greenish cloud descend on a battlefield and slowly turn men into corpses. The poisoner has long been reviled as a uniquely foul and perfidious killer. Poison is the method of the sorcerer rather than the warrior; of the woman (with her terrifying control of kitchen and medicine chest) rather than the man. In Venomous Woman, the literary scholar Margaret Hallissy explains the archetype:

Poison can never be used as an honorable weapon in a fair duel between worthy opponents, as the sword or gun, male weapons, can. A man who uses such a secret weapon is beneath contempt. Publicly acknowledged rivalry is a kind of bonding in which each worthy opponent gives the other the opportunity to demonstrate prowess… The dueler is open, honest, and strong; the poisoner fraudulent, scheming, and weak. A man with a gun or a sword is a threat, but he declares himself to be so, and his intended victim can arm himself… The poisoner uses superior secret knowledge to compensate for physical inferiority. A weak woman planning a poison is as deadly as a man with a gun, but because she plots in secret, the victim is more disarmed.

The analogies between the chemical weapons taboo and the nuclear weapons taboo are clear enough. Today the two are lumped together as "weapons of mass destruction," though nuclear weapons are incomparably more destructive, because each taboo can draw strength from the other by association. The dread of both kinds of weapons is multiplied by the prospect of slow death by sickening and the absence of a boundary between battlefield and civilian life.

Whatever abhorrence of poisoning we might have inherited from our evolutionary or cultural past, it needed a boost from historical contingency to become entrenched as a taboo on the conduct of war. Price conjectures that the critical nonevent was that in World War I, poison gas was never deliberately used against civilians. At least in that application, no taboo-shattering precedent had been set, and the widespread horror in the 1930s about the prospect that gas-dispensing airplanes could annihilate entire cities rallied people into categorically opposing all uses of the weapons.

The world's experience with chemical weapons offers some morals that are mildly hopeful, at least by the terrifying standards of the nuclear age. Not every lethal technology becomes a permanent part of the military tool kit; some genies can be put back in their bottles; and moral sentiments can sometimes become entrenched as international norms and affect the conduct of war. Those norms, moreover, can be robust enough to withstand an isolated exception, which does not necessarily set off an uncontrollable escalation. That in particular is a hopeful discovery, though it might be good for the world if not too many people were aware of it.

If the world did away with chemical weapons, could it do the same with nuclear weapons? Recently a group of American icons proposed just that in an idealistic manifesto entitled "A World Free of Nuclear Weapons." The icons were not Peter, Paul, and Mary but George Shultz, William Perry, Henry Kissinger, and Sam Nunn. Shultz was secretary of state in the Reagan administration. Perry was secretary of defense under Clinton. Kissinger was national security advisor and secretary of state under Nixon and Ford. Nunn was a chairman of the Senate Armed Services Committee and has long been considered the American lawmaker most knowledgeable about national defense. None could be accused of starry-eyed pacifism.

As one might guess from the résumés of its supporters, Global Zero has some hardheaded realpolitik behind it. Since the end of the Cold War, the nuclear arsenal of the great powers has become an absurdity. It is no longer needed to deter an existential threat from an enemy superpower, and given the nuclear taboo, it serves no other military purpose. The threat of a retaliatory strike cannot deter stateless terrorists, because their bomb would not come with a return address, and if they were religious fanatics there would be nothing on earth that they valued enough to threaten. As praiseworthy as the various nuclear arms reduction agreements have been, they make little difference to global security as long as thousands of weapons remain in existence and the technology to make new ones is not forgotten.

Supporting them is a dream team of war-hardened statesmen from Democratic and Republican administrations going back to that of John F. Kennedy. They include five former secretaries of state, five former national security advisors, and four former secretaries of defense. In all, three-quarters of the living alumni of those positions signed on to the call for a phased, verified, binding elimination of all nuclear weapons, now sometimes called Global Zero. Barack Obama and Dmitry Medvedev have endorsed it in speeches (one of the reasons Obama was awarded the Nobel Peace Prize in 2009), and several policy think tanks have begun to work out how it might be implemented. The leading road map calls for four phases of negotiation, reduction, and verification, with the last warhead dismantled in 2030.

The psychology behind Global Zero is to extend the taboo on using nuclear weapons to a taboo on possessing them. Taboos depend on a mutual understanding that there are bright lines delineating all-or-none categories, and the line distinguishing zero from more-than-zero is the brightest of all. No country could justify acquiring a nuclear weapon to protect itself against a nuclear-armed neighbor if it had no nuclear-armed neighbors. Nor could it claim that the nuclear legacy nations were hypocritically reserving the right to keep their own weapons. A developing nation could no longer try to look like a grown-up by acquiring a nuclear arsenal if the grown-ups had eschewed the weapons as old-fashioned and repulsive. And any rogue state or terrorist group that flirted with acquiring a nuclear weapon would become a pariah in the eyes of the world -- a depraved criminal rather than a redoubtable challenger.

The problem, of course, is how to get there from here. The process of dismantling the weapons would open windows of vulnerability during which one of the remaining nuclear powers could fall under the sway of an expansionist zealot. Nations would be tempted to cheat by retaining a few nukes on the side just in case their adversaries did so. A rogue state might support nuclear terrorists once it was sure that it would never be a target of retaliation. And in a world that lacked nuclear weapons but retained the knowledge of how to build them -- and that genie certainly can't be put back in the bottle -- a crisis could set off a scramble to rearm, in which the first past the post might be tempted to strike preemptively before its adversary got the upper hand. Some experts on nuclear strategy, including Schelling, John Deutch, and Harold Brown, are skeptical that a nuclear-free world is attainable or even desirable, though others are working out timetables and safeguards designed to answer their objections.

With all these uncertainties, no one should predict that nuclear weapons will go the way of poison gas anytime soon. But it is a sign of the momentum behind the Long Peace that abolition can even be discussed as a foreseeable prospect. If it happens, it would represent the ultimate decline in violence. A nuclear-free world! What realist would have dreamed it?