The New Peace is the quantitative decline in war, genocide, and terrorism that has proceeded in fits and starts since the end of the Cold War more than two decades ago. It has not been around for as long as the Long Peace, is not as revolutionary as the Humanitarian Revolution, and has not swept a civilization in the manner of the Civilizing Process. An obvious question is whether it will last. Though I am reasonably confident that during my lifetime France and Germany will not go to war, that cat-burning and the breaking wheel will not make a comeback, and that diners will not routinely stab each other with steak knives or cut off each other's noses, no prudent person could express a similar confidence when it comes to armed conflict in the world as a whole.

I am sometimes asked, "How do you know there won't be a war tomorrow (or a genocide, or an act of terrorism) that will refute your whole thesis?" The question misses the point of this book. The point is not that we have entered an Age of Aquarius in which every last earthling has been pacified forever. It is that substantial reductions in violence have taken place, and it is important to understand them. Declines in violence are caused by political, economic, and ideological conditions that take hold in particular cultures at particular times. If the conditions reverse, violence could go right back up.

Also, the world contains a lot of people. The statistics of power-law distributions and the events of the past two centuries agree in telling us that a small number of perpetrators can cause a great deal of damage. If somewhere among the world's six billion people there is a zealot who gets his hands on a stray nuclear bomb, he could single-handedly send the statistics through the roof. But even if he did, we would still need an explanation of why homicide rates fell a hundredfold, why slave markets and debtors' prisons have vanished, and why the Soviets and Americans did not go to war over Cuba, to say nothing of Canada and Spain over flatfish.

The goal of this book is to explain the facts of the past and present, not to augur the hypotheticals of the future. Still, you might ask, isn't it the essence of science to make falsifiable predictions? Shouldn't any claim to understanding the past be evaluated by its ability to extrapolate into the future? Oh, all right. I predict that the chance that a major episode of violence will break out in the next decade -- a conflict with 100,000 deaths in a year, or a million deaths overall -- is 9.7 percent. How did I come up with that number? Well, it's small enough to capture the intuition "probably not," but not so small that if such an event did occur I would be shown to be flat-out wrong. My point, of course, is that the concept of scientific prediction is meaningless when it comes to a single event -- in this case, the eruption of mass violence in the next decade. It would be another thing if we could watch many worlds unfold and tot up the number in which an event happened or did not, but this is the only world we've got.

Who knows? Maybe they're right. My aim in the rest of this chapter is to point out that maybe they're wrong. This isn't the first time we've been warned of certain ruin. The experts have predicted civilization-ending aerial gas attacks, global thermonuclear war, a Soviet invasion of Western Europe, a Chinese razing of half of humanity, nuclear powers by the dozen, a revanchist Germany, a rising sun in Japan, cities overrun by teenage superpredators, a world war fought over diminishing oil, nuclear war between India and Pakistan, and weekly 9/11-scale attacks. In this section I'll look at four threats toto the New Peace -- a civilizational clash with Islam, nuclear terrorism, a nuclear Iran, and climate change -- and for each one make the case for "maybe, but maybe not."

The truth is, I don't know what will happen across the entire world in the coming decades, and neither does anyone else. Not everyone, though, shares my reticence. A Web search for the text string "the coming war" returns two million hits, with completions like "with Islam," "with Iran," "with China," "with Russia," "in Pakistan," "between Iran and Israel," "between India and Pakistan," "against Saudi Arabia," "on Venezuela," "in America," "within the West," "for Earth's resources," "over climate," "for water," and "with Japan" (the last dating from 1991, which you would think would make everyone a bit more humble about this kind of thing). Books with titles like The Clash of Civilizations, World on Fire, World War IV, and (my favorite) We Are Doomed boast a similar confidence.

The impression that the Muslim world indulges kinds of violence that the West has outgrown is not a symptom of Islamophobia or Orientalism but is borne out by the numbers. Though about a fifth of the world's population is Muslim, and about a quarter of the world's countries have a Muslim majority, more than half of the armed conflicts in 2008 embroiled Muslim countries or insurgencies. Muslim countries force a greater proportion of their citizens into their armies than non-Muslim countries do, holding other factors constant. Muslim groups held two-thirds of the slots on the U. S. State Department's list of foreign terrorist organizations, and (as mentioned) in 2008 Sunni terrorists killed nearly two-thirds of the world's victims of terrorism whose perpetrators could be identified.

The Muslim world, to all appearances, is sitting out the decline of violence. More than two decades of headlines have shocked Westerners with acts of barbarity in the name of Islam. Among them are the 1989 clerical death threat against Salman Rushdie for portraying Muhammad in a novel, the 2002 sentencing of an unmarried pregnant woman in Nigeria to execution by stoning, the fatal stabbing in 2004 of Dutch filmmaker Theo van Gogh for producing Ayaan Hirsi Ali's film about the treatment of women in Islamic countries, the lethal 2005 riots after a Danish newspaper printed editorial cartoons that were disrespectful to the prophet, the jailing and threat of flogging of a British schoolteacher in Sudan who allowed her class to name a teddy bear Muhammad, and of course the 9/11 terrorist attacks, in which nineteen Muslims killed almost three thousand civilians.

The laws and practices of many Muslim countries seem to have missed out on the Humanitarian Revolution. According to Amnesty International, almost three-quarters of Muslim countries execute their criminals, compared to a third of non-Muslim countries, and many use cruel punishments such as stoning, branding, blinding, amputation of tongues or hands, and even crucifixion. Every year more than a hundred million girls in Islamic countries have their genitals mutilated, and when they grow up they may be disfigured with acid or killed outright if they displease their fathers, their brothers, or the husbands who have been forced upon them. Islamic countries were the last to abolish slavery (as recently as 1962 in Saudi Arabia and 1980 in Mauritania), and a majority of the countries in which people continue to be trafficked are Muslim. In many Muslim countries, witchcraft is not just on the books as a crime but is commonly prosecuted. In 2009, for example, Saudi Arabia convicted a man for carrying a phone booklet with characters in an alphabet from his native Eritrea, which the police interpreted as occult symbols. He was lashed three hundred times and imprisoned for more than three years.

In defiance of the rising tide of democracy, only about a quarter of Islamic countries elect their governments, and most of them are only dubiously democratic. Their leaders receive farcically high percentages of the vote, and they exercise the power to jail opponents, outlaw opposition parties, suspend parliament, and cancel elections. It's not just that Islamic countries happen to have risk factors for autocracy, such as being larger, poorer, or richer in oil. Even in a regression analysis that holds these factors constant, countries with larger proportions of Muslims have fewer political rights. Political rights are very much a matter of violence, of course, since they amount to being able to speak, write, and assemble without being dragged off to jail.

Violence is sanctioned in the Islamic world not just by religious superstition but by a hyperdeveloped culture of honor. The political scientists Khaled Fattah and K. M. Fierke have documented how a "discourse of humiliation" runs through the ideology of Islamist organizations. A sweeping litany of affronts -- the Crusades, the history of Western colonization, the existence of Israel, the presence of American troops on Arabian soil, the underperformance of Islamic countries -- are taken as insults to Islam and used to license indiscriminate vengeance against members of the civilization they hold responsible, together with Muslim leaders of insufficient ideological purity. The radical fringe of Islam harbors an ideology that is classically genocidal: history is seen as a violent struggle that will culminate in the glorious subjugation of an irredeemably evil class of people. Spokesmen for Al Qaeda, Hamas, Hezbollah, and the Iranian regime have demonized enemy groups (Zionists, infidels, crusaders, polytheists), spoken of a millennial cataclysm that would usher in a utopia, and justified the killing of entire categories of people such as Jews, Americans, and those felt to insult Islam.

The historian Bernard Lewis is not the only one who has asked, "What went wrong?" In 2002 a committee of Arab intellectuals under the auspices of the United Nations published the candid Arab Human Development Report, said to be "written by Arabs for Arabs." The authors documented that Arab nations were plagued by political repression, economic backwardness, oppression of women, widespread illiteracy, and a self-imposed isolation from the world of ideas. At the time of the report, the entire Arab world exported fewer manufactured goods than the Philippines, had poorer Internet connectivity than sub-Saharan Africa, registered 2 percent as many patents per year as South Korea, and translated about a fifth as many books into Arabic as Greece translates into Greek.

It wasn't always that way. During the Middle Ages, Islamic civilization was unquestionably more refined than Christendom. While Europeans were applying their ingenuity to the design of instruments of torture, Muslims were preserving classical Greek culture, absorbing the knowledge of the civilizations of India and China, and advancing astronomy, architecture, cartography, medicine, chemistry, physics, and mathematics. Among the symbolic legacies of this age are the "Arabic numbers" (adapted from India) and loan words such as alcohol, algebra, alchemy, alkali, azimuth, alembic, and algorithm. Just as the West had to come from behind to overtake Islam in science, so it was a laggard in human rights. Lewis notes:

In most tests of tolerance, Islam, both in theory and in practice, compares unfavorably with the Western democracies as they have developed during the last two or three centuries, but very favorably with most other Christian and post-Christian societies and regimes. There is nothing in Islamic history to compare with the emancipation, acceptance, and integration of otherbelievers and non-believers in the West; but equally, there is nothing in Islamic history to compare with the Spanish expulsion of Jews and Muslims, the Inquisition, the Auto da fé's, the wars of religion, not to speak of more recent crimes of commission and acquiescence.

Why did Islam blow its lead and fail to have an Age of Reason, an Enlightenment, and a Humanitarian Revolution? Some historians point to bellicose passages in the Koran, but compared to our own genocidal scriptures, they are nothing that some clever exegesis and evolving norms couldn't spindoctor away.

Lewis points instead to the historical lack of separation between mosque and state. Muhammad was not just a spiritual leader but a political and military one, and only recently have any Islamic states had the concept of a distinction between the secular and the sacred. With every potential intellectual contribution filtered through religious spectacles, opportunities for absorbing and combining new ideas were lost. Lewis recounts that while works in philosophy and mathematics had been translated from classical Greek into Arabic, works of poetry, drama, and history were not. And while Muslims had a richly developed history of their own civilization, they were incurious about their Asian, African, and European neighbors and about their own pagan ancestors. The Ottoman heirs to classical Islamic civilization resisted the adoption of mechanical clocks, standardized weights and measures, experimental science, modern philosophy, translations of poetry and fiction, the financial instruments of capitalism, and perhaps most importantly, the printing press. (Arabic was the language in which the Koran was written, so printing it was considered an act of desecration.) In chapter 4 I speculated that the Humanitarian Revolution in Europe was catalyzed by a literate cosmopolitanism, which expanded people's circle of empathy and set up a marketplace of ideas from which a liberal humanism could emerge. Perhaps the dead hand of religion impeded the flow of new ideas into the centers of Islamic civilization, locking it into a relatively illiberal stage of development. As if to prove the speculation correct, in 2010 the Iranian government restricted the number of university students who would be admitted to programs in the humanities, because, according to Supreme Leader Ayatollah Ali Khameini, study of the humanities "promotes skepticism and doubt in religious principles and beliefs."

Whatever the historical reasons, a large chasm appears to separate Western and Islamic cultures today. According to a famous theory from the political scientist Samuel Huntington, the chasm has brought us to a new age in the history of the world: the clash of civilizations. "In Eurasia the great historic fault lines between civilizations are once more aflame," he wrote. "This is particularly true along the boundaries of the crescent-shaped Islamic bloc of nations, from the bulge of Africa to Central Asia. Violence also occurs between Muslims, on the one hand, and Orthodox Serbs in the Balkans, Jews in Israel, Hindus in India, Buddhists in Burma and Catholics in the Philippines. Islam has bloody borders."论有什么样的历史原因,西方世界和伊斯兰世界之间横亘着巨大的鸿沟.根据政治学家塞缪尔·亨廷顿的著名理论,这一鸿沟将我们带进世界历史的新时代:文明的冲突.他写道:"欧亚文明之间巨大的历史断裂线再次燃起熊熊烈焰.""这一次冲突的分界明显是沿着从西非一直到中亚的新月形伊斯兰国家集团的边界.暴力的一方是穆斯林,另一方是巴尔干的塞尔维亚东正教徒、以色列的犹太人、印度的印度教徒、缅甸的佛教徒和菲律宾的天主教徒.伊斯兰有着与之不同的境线."

Though the dramatic notion of a clash of civilizations became popular among pundits, few scholars in international studies take it seriously. Too large a proportion of the world's bloodshed takes place within and between Islamic countries (for example, Iraq's war with Iran in the 1980s, and its invasion of Kuwait in 1990), and too large a proportion takes place within and between non-Islamic countries, for the civilizational fault line to be an accurate summary of violence in the world today. Also, as Nils Petter Gleditsch and Halvard Buhaug have pointed out, even though an increasing proportion of the world's armed conflicts have involved Islamic countries and insurgencies over the past two decades (from 20 to 38 percent), it's not because those conflicts have increased in number. As figure 6-12 shows, Islamic conflicts continued at about the same rate while the rest of the world got more peaceful, the phenomenon I have been calling the New Peace.

Part of the answer may be found in a massive Gallup poll conducted between 2001 and 2007 on the attitudes of Muslims in thirty-five countries representing 90 percent of the world's Islamic population. The results confirm that most Islamic states will not become secular liberal democracies anytime soon. Majorities of Muslims in Egypt, Pakistan, Jordan, and Bangladesh told the pollsters that Sharia, the principles behind Islamic law, should be the only source of legislation in their countries, and majorities in most of the countries said it should be at least one of the sources. On the other hand, a majority of Americans believe that the Bible should be one of the sources of legislation, and presumably they don't mean that people who work on Sunday should be stoned to death. Religion thrives on woolly allegory, emotional commitments to texts that no one reads, and other forms of benign hypocrisy. Like Americans' commitment to the Bible, most Muslims' commitment to Sharia is more a symbolic affiliation with moral attitudes they associate with the best of their culture than a literal desire to see adulteresses stoned to death. In practice, creative and expedient readings of Sharia for liberal ends have often prevailed against the oppressive fundamentalist readings. (The Nigerian woman, for example, was never executed.) Presumably that is why most Muslims see no contradiction between Sharia and democracy. Indeed, despite their professed affection for the idea of Sharia, a large majority believe that religious leaders should have no direct role in drafting their country's constitution.

Most important, the entire concept of "Islamic civilization" does a disservice to the 1.3 billion men and women who call themselves Muslims, living in countries as diverse as Mali, Nigeria, Morocco, Turkey, Saudi Arabia, Bangladesh, and Indonesia. And cutting across the divide of the Islamic world into continents and countries is another divide that is even more critical. Westerners tend to know Muslims through two dubious exemplars: the fanatics who grab headlines with their fatwas and jihads, and the oil-cursed autocrats who rule over them. The beliefs of the hitherto silent (and frequently silenced) majority make less of a contribution to our stereotypes. Can 1.3 billion Muslims really be untouched by the liberalizing tide that has swept the rest of the world in recent decades?

FIGURE 6-12: Islamic and world conflicts, 1990-2006

Source: Data from Gleditsch, 2008. "Islamic conflicts" involve Muslim countries or Islamic opposition movements or both. Data assembled by Halvard Buhaug from the UCDP⁄ PRIO conflict dataset and his own coding of Islamic conflicts.

Though most Muslims distrust the United States, it may not be out of a general animus toward the West or a hostility to democratic principles. Many Muslims feel the United States does not want to spread democracy in the Muslim world, and they have a point: the United States, after all, has supported autocratic regimes in Egypt, Jordan, Kuwait, and Saudi Arabia, rejected the election of Hamas in the Palestinian territories, and in 1953 helped overthrow the democratically elected Mossadegh in Iran. France and Germany are viewed more favorably, and between 20 and 40 percent say they admire the "fair political system, respect for human values, liberty, and equality" of Western culture. More than 90 percent would guarantee freedom of speech in their nation's constitution, and large numbers also support freedom of religion and freedom of assembly. Substantial majorities of both sexes in all the major Muslim countries say that women should be allowed to vote without influence from men, to work at any job, to enjoy the same legal rights as men, and to serve in the highest levels of government. And as we have seen, overwhelming majorities of the Muslim world reject the violence of Al Qaeda. Only 7 percent of the Gallup respondents approved the 9/11 attacks, and that was before Al Qaeda's popularity cratered in 2007.

What about mobilization for political violence? A team from the University of Maryland examined the goals of 102 grassroots Muslim organizations in North Africa and the Middle East and found that between 1985 and 2004 the proportion of organizations that endorsed violence dropped from 54 to 14 percent. The proportion committed to nonviolent protests tripled, and the proportion that engaged in electoral politics doubled. These changes helped drive down the terrorism death curve in figure 6-11 and are reflected in the headlines, which feature far less terrorist violence in Egypt and Algeria than we read about a few years ago.

Islamic insularity is also being chipped at by a battery of liberalizing forces: independent news networks such as Al-Jazeera; American university campuses in the Gulf states; the penetration of the Internet, including social networking sites; the temptations of the global economy; and the pressure for women's rights from pent-up internal demand, nongovernmental organizations, and allies in the West. Perhaps conservative ideologues will resist these forces and keep their societies in the Middle Ages forever. But perhaps they won't.

In early 2011, as this book was going to press, a swelling protest movement deposed the leaders of Tunisia and Egypt and was threatening the regimes in Jordan, Bahrain, Libya, Syria, and Yemen. The outcome is unpredictable, but the protesters have been almost entirely nonviolent and non-Islamist, and are animated by a desire for democracy, good governance, and economic vitality rather than global jihad, the restoration of the caliphate, or death to infidels. Even with all these winds of change, it is conceivable that an Islamist tyrant or radical revolutionary group could drag an unwilling populace into a cataclysmic war. But it seems more probable that "the coming war with Islam" will never come. Islamic nations are unlikely to unite and challenge the West: they are too diverse, and they have no civilization-wide animus against us. Some Muslim countries, like Turkey, Indonesia, and Malaysia, are well on the way to becoming fairly liberal democracies. Some will continue to be ruled by SOBs, but they'll be our SOBs. Some will try to muddle through the oxymoron of a Sharia democracy. None is likely to be governed by the ideology of Al Qaeda. This leaves three reasonably foreseeable dangers to the New Peace: nuclear terrorism, the regime in Iran, and climate change.

The trio suggested a simple model that is a bit like the one that Jean-Baptiste Michel and I proposed for wars, invoking nothing fancier than a combination of exponentials. As terrorists invest more time into plotting their attack, the death toll can go up exponentially: a plot that takes twice as long to plan can kill, say, four times as many people. To be concrete, an attack by a single suicide bomber, which usually kills in the single digits, can be planned in a few days or weeks. The 2004 Madrid train bombings, which killed around two hundred, took six months to plan, and 9/11, which killed three thousand, took two years. But terrorists live on borrowed time: every day that a plot drags on brings the possibility that it will be disrupted, aborted, or executed prematurely. If the probability is constant, the plot durations will be distributed exponentially. (Cronin, recall, showed that terrorist organizations drop like flies over time, falling into an exponential curve.) Combine exponentially growing damage with an exponentially shrinking chance of success, and you get a power law, with its disconcertingly thick tail. Given the presence of weapons of mass destruction in the real world, and religious fanatics willing to wreak untold damage for a higher cause, a lengthy conspiracy producing a horrendous death toll is within the realm of thinkable probabilities.

Though conventional terrorism, as John Kerry gaffed, is a nuisance to be policed rather than a threat to the fabric of life, terrorism with weapons of mass destruction would be something else entirely. The prospect of an attack that would kill millions of people is not just theoretically possible but consistent with the statistics of terrorism. The computer scientists Aaron Clauset and Maxwell Young and the political scientist Kristian Gleditsch plotted the death tolls of eleven thousand terrorist attacks on log-log paper and saw them fall into a neat straight line. Terrorist attacks obey a power-law distribution, which means they are generated by mechanisms that make extreme events unlikely, but not astronomically unlikely.

So how likely are they? What do you think the chances are that within the next five years each of the following scenarios will take place? (1) One of the heads of state of a major developed country will be assassinated. (2) A nuclear weapon will be set off in a war or act of terrorism. (3) Venezuela and Cuba will join forces and sponsor Marxist insurrection movements in one or more Latin American countries. (4) Iran will provide nuclear weapons to a terrorist group that will use one of them against Israel or the United States. (5) France will give up its nuclear arsenal.

A statistical model, of course, is not a crystal ball. Even if we could extrapolate the line of existing data points, the massive terrorist attacks in the tail are still extremely (albeit not astronomically) unlikely. More to the point, we can't extrapolate it. In practice, as you get to the tail of a power-law distribution, the data points start to misbehave, scattering around the line or warping it downward to very low probabilities. The statistical spectrum of terrorist damage reminds us not to dismiss the worst-case scenarios, but it doesn't tell us how likely they are.

I gave fifteen of these scenarios to 177 Internet users on a single Web page and asked them to estimate the probability of each. The median estimate that a nuclear bomb would be set off (scenario 2) was 0.20; the median estimate that a nuclear bomb would be set off in the United States or Israel by a terrorist group that obtained it from Iran (scenario 4) was 0.25. About half the respondents judged that the second scenario was more likely than the first. And in doing so, they committed an elementary blunder in the mathematics of probability. The probability of a conjunction of events (A and B both occurring) cannot be greater than the probability of either of them occurring alone. The probability that you will draw a red jack has to be lower than the probability that you will draw a jack, because some jacks you might draw are not red.

Yet Tversky and Kahneman have shown that most people, including statisticians and medical researchers, commonly make the error. Consider the case of Bill, a thirty-four-year-old man who is intelligent but also unimaginative, compulsive, and rather dull. In school he was strong in mathematics but undistinguished in the arts and humanities. What are the chances that Bill plays jazz saxophone? What are the chances that he is an accountant who plays jazz saxophone? Many people give higher odds to the second possibility, but the choice is nonsensical, because there are fewer saxophone-playing accountants than there are saxophone players. In judging probabilities, people rely on the vividness of their imaginations rather than thinking through the laws. Bill fits the stereotype of an accountant but not of a saxophonist, and our intuitions go with the stereotype.

You can see where I'm going. The mental movie of an Islamist terrorist group buying a bomb on the black market or obtaining it from a rogue state and then detonating it in a populated area is all too easy to play in our mind's eye. Even if it weren't, the entertainment industry has played it for us in nuclear terrorist dramas like True Lies, The Sum of All Fears, and 24. The narrative is so riveting that we are apt to give it a higher probability than we would if we thought through all the steps that would have to go right for the disaster to happen and multiplied their probabilities. That's why so many of my survey respondents judged an Iran-sponsored nuclear terrorist attack to be more probable than a nuclear attack. The point is not that nuclear terrorism is impossible or even astronomically unlikely. It is just that the probability assigned to it by anyone but a methodical risk analyst is likely to be too high.

The conjunction fallacy, as psychologists call it, infects many kinds of reasoning. Juries are more likely to believe that a man with shady business dealings killed an employee to prevent him from talking to the police than to believe that he killed the employee. (Trial lawyers thrive on this fallacy, adding conjectural details to a scenario to make it more vivid to a jury, even though every additional detail, mathematically speaking, ought to make it less probable.) Professional forecasters give higher odds to an unlikely outcome that is presented with a plausible cause (oil prices will rise, causing oil consumption to fall) than to the same outcome presented naked (oil consumption will fall). And people are willing to pay more for flight insurance against terrorism than for flight insurance against all causes.

Like television weather forecasters, the pundits, politicians, and terrorism specialists have every incentive to emphasize the worst-case scenario. It is undoubtedly wise to scare governments into taking extra measures to lock down weapons and fissile material and to monitor and infiltrate groups that might be tempted to acquire them. Overestimating the risk, then, is safer than underestimating it -- though only up to a point, as the costly invasion of Iraq in search of nonexistent weapons of mass destruction proves. The professional reputations of experts have proven to be immune to predictions of disasters that never happen, while almost no one wants to take a chance at giving the all-clear and ending up with radioactive egg on his face.

What do I mean by "too high"? "With certainty" and "more probable than not" strike me as too high. The physicist Theodore Taylor declared in 1974 that by 1990 it would be too late to prevent terrorists from carrying out a nuclear attack. In 1995 the world's foremost activist on the risks of nuclear terrorism, Graham Allison, wrote that under prevailing circumstances, a nuclear attack on American targets was likely before the decade was out. In 1998 the counterterrorism expert Richard Falkenrath wrote that "it is certain that more and more non-state actors will become capable of nuclear, biological, and chemical weapons acquisition and use." In 2003 UN ambassador John Negroponte judged that there was a "high probability" of an attack with a weapon of mass destruction within two years. And in 2007 the physicist Richard Garwin estimated that the chance of a nuclear terrorist attack was 20 percent per year, or about 50 percent by 2010 and almost 90 percent within a decade.

A few brave analysts, such as Mueller, John Parachini, and Michael Levi, have taken the chance by examining the disaster scenarios component by component. For starters, of the four so-called weapons of mass destruction, three are far less massively destructive than good old-fashioned explosives. Radiological or "dirty" bombs, which are conventional explosives wrapped in radioactive material (obtained, for example, from medical waste), would yield only minor and short-lived elevations of radiation, comparable to moving to a city at a higher altitude. Chemical weapons, unless they are released in an enclosed space like a subway (where they would still not do as much damage as conventional explosives), dissipate quickly, drift in the wind, and are broken down by sunlight. (Recall that poison gas was responsible for a tiny fraction of the casualties in World War I.) Biological weapons capable of causing epidemics would be prohibitively expensive to develop and deploy, as well as dangerous to the typically bungling amateur labs that would develop them. It's no wonder that biological and chemical weapons, though far more accessible than nuclear ones, have been used in only three terrorist attacks in thirty years. In 1984 the Rajneeshee religious cult contaminated salad in the restaurants of an Oregon town with salmonella, sickening 751 people and killing none. In 1990 the Tamil Tigers were running low on ammunition while attacking a fort and opened up some chlorine cylinders they found in a nearby paper mill, injuring 60 and killing none before the gas wafted back over them and convinced them never to try it again. The Japanese religious cult Aum Shinrikyo failed in ten attempts to use biological weapons before releasing sarin gas in the Tokyo subways, killing 12. A fourth attack, the 2001 anthrax mailings that killed 5 Americans in media and government offices, turned out to be a spree killing rather than an act of terrorism.

Each of the pathways to nuclear terrorism, when examined carefully, turns out to have gantlets of improbabilities. There may have been a window of vulnerability in the safekeeping of nuclear weapons in Russia, but today most experts agree it has been closed, and that no loose nukes are being peddled in a nuclear bazaar. Stephen Younger, the former director of nuclear weapons research at Los Alamos National Laboratory, has said, "Regardless of what is reported in the news, all nuclear nations take the security of their weapons very seriously." Russia has an intense interest in keeping its weapons out of the hands of Chechen and other ethnic separatist groups, and Pakistan is just as worried about its archenemy Al Qaeda. And contrary to rumor, security experts consider the chance that Pakistan's government and military command will fall under the control of Islamist extremists to be essentially nil. Nuclear weapons have complex interlocks designed to prevent unauthorized deployment, and most of them become "radioactive scrap metal" if they are not maintained. For these reasons, the forty-seven-nation Nuclear Security Summit convened by Barack Obama in 2010 to prevent nuclear terrorism concentrated on the security of fissile material, such as plutonium and highly enriched uranium, rather than on finished weapons.

It's really only nuclear weapons that deserve the WMD acronym. Mueller and Parachini have fact-checked the various reports that terrorists got "just this close" to obtaining a nuclear bomb and found that all were apocryphal. Reports of "interest" in procuring weapons on a black market grew into accounts of actual negotiations, generic sketches morphed into detailed blueprints, and flimsy clues (like the aluminum tubes purchased in 2001 by Iraq) were overinterpreted as signs of a development program.

The dangers of filched fissile material are real, and the measures recommended at the summit are patently wise, responsible, and overdue. Still, one shouldn't get so carried away by the image of garage nukes as to think they are inevitable or even extremely probable. The safeguards that are in place or will be soon will make fissile materials hard to steal or smuggle, and if they went missing, it would trigger an international manhunt. Fashioning a workable nuclear weapon requires precision engineering and fabrication techniques well beyond the capabilities of amateurs. The Gilmore commission, which advises the president and Congress on WMD terrorism, called the challenge "Herculean," and Allison has described the weapons as "large, cumbersome, unsafe, unreliable, unpredictable, and inefficient." Moreover, the path to getting the materials, experts, and facilities in place is mined with hazards of detection, betrayal, stings, blunders, and bad luck. In his book On Nuclear Terrorism, Levi laid out all the things that would have to go right for a terrorist nuclear attack to succeed, noting, "Murphy's Law of Nuclear Terrorism: What can go wrong might go wrong." Mueller counts twenty obstacles on the path and notes that even if a terrorist group had a fifty-fifty chance of clearing every one, the aggregate odds of its success would be one in a million. Levi brackets the range from the other end by estimating that even if the path were strewn with only ten obstacles, and the probability that each would be cleared was 80 percent, the aggregate odds of success facing a nuclear terrorist group would be one in ten. Those are not our odds of becoming victims. A terrorist group weighing its options, even with these overly optimistic guesstimates, might well conclude from the long odds that it would better off devoting its resources to projects with a higher chance of success. None of this, to repeat, means that nuclear terrorism is impossible, only that it is not, as so many people insist, imminent, inevitable, or highly probable.

All this is, to say the least, disconcerting, and many writers have concluded that Ahmadinejad is another Hitler who will soon develop nuclear weapons and use them on Israel or furnish them to Hezbollah to do so. Even in less dire scenarios, he could blackmail the Middle East into acceding to Iranian hegemony. The prospect might leave Israel or the United States no choice but to bomb its nuclear facilities preemptively, even if it invited years of war and terrorism in response. A 2009 editorial in the Washington Times spelled it out: "War with Iran is now inevitable. The only question is: Will it happen sooner or later?"

If current pundits are to be believed, then as you are reading these words the New Peace will already have been shattered by a major war, perhaps a nuclear war, with Iran. At the time of this writing, tensions have been rising over the country's nuclear energy program. Iran is currently enriching enough uranium to fashion a nuclear arsenal, and it has defied international demands that it allow inspections and comply with other provisions of the Nuclear Nonproliferation Treaty. The president of Iran, Mahmoud Ahmadinejad, has taunted Western leaders, supported terrorist groups, accused the United States of orchestrating the 9/11 attacks, denied the Holocaust, called for Israel to be "wiped off the map," and prayed for the reappearance of the Twelfth Imam, the Muslim savior who would usher in an age of peace and justice. In some interpretations of Shi'a Islam, this messiah will show up after a worldwide eruption of war and chaos.

Iran is a signatory to the Nuclear Nonproliferation Treaty, and Ahmadinejad has repeatedly declared that Iran's nuclear program is intended only for energy and medical research. In 2005 Supreme Leader Khameini (who wields more power than Ahmadinejad) issued a fatwa declaring that nuclear weapons are forbidden under Islam. If the government went ahead and developed the weapons anyway, it would not be the first time in history that national leaders have lied through their teeth. But having painted themselves into this corner, the prospect of forfeiting all credibility in the eyes of the world (including major powers on whom they depend, like Russia, China, Turkey, and Brazil) might at least give them pause.

This chilling scenario of a nuclear attack by Iranian fanatics is certainly possible. But is it inevitable, or even highly likely? One can be just as contemptuous of Ahmadinejad, and just as cynical about his motives, while imagining less dire alternatives for the world ahead. John Mueller, Thomas Schelling, and many other foreign affairs analysts have imagined them for us and have concluded that the Iranian nuclear program is not the end of the world.

Ahmadinejad: I have never said such a thing… I was talking about peace… What is being said about an apocalyptic war and -- global war, things of that nature. This is what the Zionists are claiming. Imam… will come with logic, with culture, with science. He will come so that there is no more war. No more enmity, hatred. No more conflict. He will call on everyone to enter a brotherly love. Of course, he will return with Jesus Christ. The two will come back together. And working together, they would fill this world with love. The stories that have been disseminated around the world about extensive war, apocalyptic wars, so on and so forth, these are false.

Ahmadinejad's musings about the return of the Twelfth Imam do not necessarily mean that he plans to hasten it along with a nuclear holocaust. Two of the deadlines by which writers confidently predicted that he would set off the apocalypse (2007 and 2009) have already come and gone. And for what it's worth, here is how he explained his beliefs in a 2009 television interview with NBC correspondent Ann Curry:

Curry: You've said that you believe that his arrival, the apocalypse, would happen in your own lifetime. What do you believe that you should do to hasten his arrival?

As a Jewish atheist, I can't say I find these remarks completely reassuring. But with one obvious change they are not appreciably different from those held by devout Christians; indeed, they are milder, as many Christians do believe in an apocalyptic war and have fantasized about it in bestselling novels. As for the speech containing the phrase that was translated as "wiping Israel off the map," the New York Times writer Ethan Bronner consulted Persian translators and analysts of Iranian government rhetoric on the meaning of the phrase in context, and they were unanimous that Ahmadinejad was daydreaming about regime change in the long run, not genocide in the days ahead. The perils of translating foreign bombast bring to mind Khrushchev's boast "We will bury you," which turned out to mean "outlive" rather than "entomb."

There is a parsimonious alternative explanation of Iran's behavior. In 2002 George W. Bush identified Iraq, North Korea, and Iran as the "axis of evil" and proceeded to invade Iraq and depose its leadership. North Korea's leaders saw the writing on the wall and promptly developed a nuclear capability, which (as they no doubt anticipated) has put an end to any musings about the United States invading them too. Shortly afterward Iran put its nuclear program into high gear, aiming to create enough ambiguity as to whether it possesses nuclear weapons, or could assemble them quickly, to squelch any thought of an invasion in the mind of the Great Satan.

In the case of Iran, before it decided to bomb Israel (or license Hezbollah to do so in an incriminating coincidence), with no conceivable benefit to itself, its leaders would have to anticipate a nuclear reprisal by Israeli commanders, who could match them hothead for hothead, together with an invasion by a coalition of powers enraged by the violation of the nuclear taboo. Though the regime is detestable and in many ways irrational, one wonders whether its principals are so indifferent to continuing their hold on power as to choose to annihilate themselves in pursuit of perfect justice in a radioactive Palestine or the arrival of the Twelfth Imam, with or without Jesus at his side. As Thomas Schelling asked in his 2005 Nobel Prize lecture, "What else can Iran accomplish, except possibly the destruction of its own system, with a few nuclear warheads? Nuclear weapons should be too precious to give away or to sell, too precious to waste killing people when they could, held in reserve, make the United States, or Russia, or any other nation, hesitant to consider military action."

If Iran does become a confirmed or suspected nuclear power, the history of the nuclear age suggests that the most likely outcome would be nothing. As we have seen, nuclear weapons have turned out to be useless for anything but deterrence against annihilation, which is why the nuclear powers have repeatedly been defied by their nonnuclear adversaries. The most recent episode of proliferation bears this out. In 2004 it was commonly predicted that if North Korea acquired a nuclear capability, then by the end of the decade it would share it with terrorists and set off a nuclear arms race with South Korea, Japan, and Taiwan. In fact, North Korea did acquire a nuclear capability, the end of the decade has come and gone, and nothing has happened. It's also unlikely that any nation would furnish nuclear ammunition to the loose cannons of a terrorist band, thereby giving up control over how they would be used while being on the hook for the consequences.

Though it may seem dangerous to consider alternatives to the worst-case scenario, the dangers go both ways. In the fall of 2002 George W. Bush warned the nation, "America must not ignore the threat gathering against us. Facing clear evidence of peril, we cannot wait for the final proof -- the smoking gun -- that could come in the form of a mushroom cloud." The "clear evidence" led to a war that has cost more than a hundred thousand lives and almost a trillion dollars and has left the world no safer. A cocksure certainty that Iran will use nuclear weapons, in defiance of sixty-five years of history in which authoritative predictions of inevitable catastrophes were repeatedly proven wrong, could lead to adventures with even greater costs.

These days one other gloomy scenario is on people's minds. Global temperatures are increasing, which in the decades ahead could lead to a rising sea level, desertification, droughts in some regions, and floods and hurricanes in others. Economies will be disrupted, leading to a competition for resources, and populations will migrate out of distressed regions, leading to friction with their unwelcoming hosts. A 2007 New York Times op-ed warned, "Climate stress may well represent a challenge to international security just as dangerous -- and more intractable -- than the arms race between the United States and the Soviet Union during the Cold War or the proliferation of nuclear weapons among rogue states today." That same year Al Gore and the Intergovernmental Panel on Climate Change were awarded the Nobel Peace Prize for their call to action against global warming because, according to the citation, climate change is a threat to international security. A rising fear lifts all the boats. Calling global warming "a force multiplier for instability," a group of military officers wrote that "climate change will provide the conditions that will extend the war on terror."

Once again it seems to me that the appropriate response is "maybe, but maybe not." Though climate change can cause plenty of misery and deserves to be mitigated for that reason alone, it will not necessarily lead to armed conflict. The political scientists who track war and peace, such as Halvard Buhaug, Idean Salehyan, Ole Theisen, and Nils Gleditsch, are skeptical of the popular idea that people fight wars over scarce resources. Hunger and resource shortages are tragically common in sub-Saharan countries such as Malawi, Zambia, and Tanzania, but wars involving them are not. Hurricanes, floods, droughts, and tsunamis (such as the disastrous one in the Indian Ocean in 2004) do not generally lead to armed conflict. The American dust bowl in the 1930s, to take another example, caused plenty of deprivation but no civil war. And while temperatures have been rising steadily in Africa during the past fifteen years, civil wars and war deaths have been falling. Pressures on access to land and water can certainly cause local skirmishes, but a genuine war requires that hostile forces be organized and armed, and that depends more on the influence of bad governments, closed economies, and militant ideologies than on the sheer availability of land and water. Certainly any connection to terrorism is in the imagination of the terror warriors: terrorists tend to be underemployed lower-middle-class men, not subsistence farmers. As for genocide, the Sudanese government finds it convenient to blame violence in Darfur on desertification, distracting the world from its own role in tolerating or encouraging the ethnic cleansing.

In a regression analysis on armed conflicts from 1980 to 1992, Theisen found that conflict was more likely if a country was poor, populous, politically unstable, and abundant in oil, but not if it had suffered from droughts, water shortages, or mild land degradation. (Severe land degradation did have a small effect.) Reviewing analyses that examined a large number (N) of countries rather than cherry-picking one or two, he concluded, "Those who foresee doom, because of the relationship between resource scarcity and violent internal conflict, have very little support in the large-N literature." Salehyan adds that relatively inexpensive advances in water use and agricultural practices in the developing world can yield massive increases in productivity with a constant or even shrinking amount of land, and that better governance can mitigate the human costs of environmental damage, as it does in developed democracies. Since the state of the environment is at most one ingredient in a mixture that depends far more on political and social organization, resource wars are far from inevitable, even in a climate-changed world.

No reasonable person would prophesy that the New Peace is going to be a long peace, to say nothing of a perpetual peace. There will certainly be wars and terrorist attacks in the decades to come, possibly large ones. On top of the known unknowns -- militant Islamism, nuclear terrorists, environmental degradation -- there are surely many unknown unknowns. Perhaps new leaders in China will decide to engulf Taiwan once and for all, or Russia will swallow a former Soviet republic or two, provoking an American response. Maybe an aggressive Chavismo will spill out of Venezuela and incite Marxist insurgencies and brutal counterinsurgencies throughout the developing world. Perhaps at this very moment terrorists from some liberation movement no one has heard of are plotting an attack of unprecedented destruction, or an eschatological ideology is fermenting in the mind of a cunning fanatic who will take over a major country and plunge the world back into war. As the Saturday Night Live news analyst Roseanne Roseannadanna observed, "It's always something. If it's not one thing, it's another."

But it is just as foolish to let our lurid imaginations determine our sense of the probabilities. It may always be something, but there can be fewer of those things, and the things that happen don't have to be as bad. The numbers tell us that war, genocide, and terrorism have declined over the past two decades -- not to zero, but by a lot. A mental model in which the world has a constant allotment of violence, so that every cease-fire is reincarnated somewhere else as a new war, and every interlude of peace is just a time-out in which martial tensions build up and seek release, is factually mistaken. Millions of people are alive today because of the civil wars and genocides that did not take place but that would have taken place if the world had remained as it was in the 1960s, 1970s, and 1980s. The conditions that favored this happy outcome -- democracy, prosperity, decent government, peacekeeping, open economies, and the decline of antihuman ideologies -- are not, of course, guaranteed to last forever. But nor are they likely to vanish overnight.

Of course we live in a dangerous world. As I have emphasized, a statistical appreciation of history tells us that violent catastrophes may be improbable, but they are not astronomically improbable. Yet that can also be stated in a more hopeful way. Violent catastrophes may not be astronomically improbable, but they are improbable.