In the wake of World War II, leading thinkers were desperate to figure out what had gone wrong and tossed around a number of schemes for preventing a repeat performance. Mueller explains the most popular one:

Some Western scientists, apparently consumed with guilt over having participated in the development of a weapon that could kill with new efficiency,… took time out from their laboratories and studies to consider human affairs. They quickly came to conclusions expressed with an evangelical certainty they would never have used in discussing the physical world. Although he had done his greatest work in physics while a citizen of the sovereign nation of Switzerland, Einstein proved as immune to the Swiss example as everyone else. "As long as there are sovereign nations possessing great power," he declared, "war is inevitable."… Fortunately, he and other scientists had managed to discover the one device that could solve the problem. "Only the creation of a world government can prevent the impending self-destruction of mankind."

Today the campaign for world government lives on mainly among kooks and science fiction fans. One problem is that a functioning government relies on a degree of mutual trust and shared values among the people it governs which is unlikely to exist across the entire globe. Another is that a world government would have no alternatives from which it could learn better governance, or to which its disgruntled citizens could emigrate, and hence it would have no natural checks against stagnation and arrogance. And the United Nations is unlikely to morph into a government that anyone would want to be governed by. The Security Council is hamstrung by the veto power that the great powers insisted on before ceding it any authority, and the General Assembly is more of a soapbox for despots than a parliament of the world's people.

World government seems like a straightforward extension of the logic of the Leviathan. If a national government with a monopoly on the use of force is the solution to the problem of homicide among individuals and of private and civil wars among factions, isn't a world government with a monopoly on the legitimate use of military force the solution to the problem of wars among nations? Most intellectuals did not go as far as Bertrand Russell, who in 1948 proposed that the Soviet Union should be given an ultimatum that unless it immediately submitted to world government, the United States would attack it with nuclear weapons. But world government was endorsed by, among others, Einstein, Wendell Willkie, Hubert Humphrey, Norman Cousins, Robert Maynard Hutchins, and William O. Douglas. Many people thought world government would gradually emerge out of the United Nations.

In "Perpetual Peace," Kant envisioned a "federation of free states" that would fall well short of an international Leviathan. It would be a gradually expanding club of liberal republics rather than a global megagovernment, and it would rely on the soft power of moral legitimacy rather than on a monopoly on the use of force. The modern equivalent is the intergovernmental organization or IGO -- a bureaucracy with a limited mandate to coordinate the policies of participating nations in some area in which they have a common interest. The international entity with the best track record for implementing world peace is probably not the United Nations, but the European Coal and Steel Community, an IGO founded in 1950 by France, West Germany, Belgium, the Netherlands, and Italy to oversee a common market and regulate the production of the two most important strategic commodities. The organization was specifically designed as a mechanism for submerging historic rivalries and ambitions -- especially West Germany's -- in a shared commercial enterprise. The Coal and Steel Community set the stage for the European Economic Community, which in turn begot the European Union.

Russett and Oneal propose that membership in intergovernmental organizations is the third vertex of a triangle of pacifying forces which they attribute to Kant, the other two being democracy and trade. (Though Kant did not single out trade in "Perpetual Peace," he extolled it elsewhere, so Russett and Oneal felt they could take some license in drawing their triangle.) The international organizations needn't have utopian or even idealistic missions. They can coordinate defense, currency, postal service, tariffs, canal traffic, fishing rights, pollution, tourism, war crimes, weights and measures, road signs, anything -- as long as they are voluntary associations of governments. Figure 5-25 shows how membership in these organizations steadily increased during the 20th century, with a bump after World War II.

Many historians believe that these organizations helped keep war out of the collective consciousness of Western Europe. By making national borders porous to people, money, goods, and ideas, they weakened the temptation of nations to fall into militant rivalries, just as the existence of the United States weakens any temptation of, say, Minnesota and Wisconsin to fall into a militant rivalry. By throwing nations into a club whose leaders had to socialize and work together, they enforced certain norms of cooperation. By serving as an impartial judge, they could mediate disputes among member nations. And by holding out the carrot of a vast market, they could entice applicants to give up their empires (in the case of Portugal) or to commit themselves to liberal democracy (in the case of former Soviet satellites and, perhaps soon, Turkey).

Might Kant have been right in an even grander sense? Russett and Oneal defended the Kantian triangle with sophisticated correlations. But a causal story derived from correlational data is always vulnerable to the possibility that some hidden entity is the real cause of both the effect one is trying to explain and the variables one is using to explain it. In the case of the Kantian triangle, each putative pacifying agent may depend on a deeper and even more Kantian cause: a willingness to resolve conflicts by means that are acceptable to all the affected parties, rather than by the stronger party imposing its will on the weaker one. Nations become stable democracies only when their political factions tire of murder as the means of assigning power. They engage in commerce only when they put a greater value on mutual prosperity than on unilateral glory. And they join intergovernmental organizations only when they are willing to cede a bit of sovereignty for a bit of mutual benefit. In other words, by signing on to the Kantian variables, nations and their leaders are increasingly acting in such a way that the principle behind their actions can be made universal. Could the Long Peace represent the ascendancy in the international arena of the Categorical Imperative?

To verify whether IGO membership made an independent contribution to peace, or just went along for the ride with democracy and trade, Russett and Oneal counted the number of IGOs that every pair of nations jointly belonged to, and they threw it into the regression analysis together with the democracy and trade scores and the realpolitik variables. The researchers concluded that Kant got it right three out of three times: democracy favors peace, trade favors peace, and membership in intergovernmental organizations favors peace. A pair of countries that are in the top tenth of the scale on all three variables are 83 percent less likely than an average pair of countries to have a militarized dispute in a given year, which means the likelihood is very close to zero.

Source: Graph from Russett, 2008.

FIGURE 5-25: Average number of IGO memberships shared by a pair of countries, 1885-2000

Many scholars in international relations would snort at the very idea. According to an influential theory tendentiously called "realism," the absence of a world government consigns nations to a permanent state of Hobbesian anarchy. That means that leaders must act like psychopaths and consider only the national self-interest, unsoftened by sentimental (and suicidal) thoughts of morality.

Realism is sometimes defended as a consequence of the existence of human nature, where the underlying theory of human nature is that people are selfinterested rational animals. But as we shall see in chapters 8 and 9, humans are also moral animals: not in the sense that their behavior is moral in the light of disinterested ethical analysis, but in the sense that it is guided by moral intuitions supported by emotions, norms, and taboos. Humans are also cognitive animals, who spin out beliefs and use them to guide their actions. None of these endowments pushes our species toward peace by default. But it is neither sentimental nor unscientific to imagine that particular historical moments can engage the moral and cognitive faculties of leaders and their coalitions in a combination that inclines them toward peaceful coexistence. Perhaps the Long Peace is one of them.

Can we identify exogenous causes of the new humanitarian aversion to war among developed countries? In chapter 4 I conjectured that the Humanitarian Revolution was accelerated by publishing, literacy, travel, science, and other cosmopolitan forces that broaden people's intellectual and moral horizons. The second half of the 20th century has obvious parallels. It saw the dawn of television, computers, satellites, telecommunications, and jet travel, and an unprecedented expansion of science and higher education. The communications guru Marshall McLuhan called the postwar world a "global village." In a village, the fortunes of other people are immediately felt. If the village is the natural size of our circle of sympathy, then perhaps when the village goes global, the villagers will experience greater concern for their fellow humans than when it embraced just the clan or tribe. A world in which a person can open the morning paper and meet the eyes of a naked, terrified little girl running toward him from a napalm attack nine thousand miles away is not a world in which a writer can opine that war is "the foundation of all the high virtues and faculties of man" or that it "enlarges the mind of a people and raises their character."

In addition to the three proximate Kantian causes, then, the Long Peace may depend on an ultimate Kantian cause. Norms among the influential constituencies in developed countries may have evolved to incorporate the conviction that war is inherently immoral because of its costs to human well-being, and that it can be justified only on the rare occasions when it is certain to prevent even greater costs to human well-being. If so, interstate war among developed countries would be going the way of customs such as slavery, serfdom, breaking on the wheel, disemboweling, bearbaiting, cat-burning, heretic-burning, witch-drowning, thief-hanging, public executions, the display of rotting corpses on gibbets, dueling, debtors' prisons, flogging, keelhauling, and other practices that passed from unexceptionable to controversial to immoral to unthinkable to not-thought-about during the Humanitarian Revolution.

The end of the Cold War and the peaceful dissolution of the Soviet empire have also been linked to the easier movement of people and ideas at the end of the 20th century. By the 1970s and 1980s the Soviet Union's attempt to retain its power by totalitarian control of media and travel was becoming a significant handicap. Not only was it becoming ludicrous for a modern economy to do without photocopiers, fax machines, and personal computers (to say nothing of the nascent Internet), but it was impossible for the country's rulers to keep scientists and policy wonks from learning about the ideas in the increasingly prosperous West, or to keep the postwar generation from learning about rock music, blue jeans, and other perquisites of personal freedom. Mikhail Gorbachev was a man of cosmopolitan tastes, and he installed in his administration many analysts who had traveled and studied in the West. The Soviet leadership made a verbal commitment to human rights in the 1975 Helsinki Accords, and a cross-border network of human rights activists were trying to get the populace to hold them to it. Gorbachev's policy of glasnost (openness) allowed Aleksandr Solzhenitzyn's The Gulag Archipelago to be serialized in 1989, and it allowed debates in the Congress of People's Deputies to be televised, exposing millions of Russians to the brutality of the past Soviet leadership and the ineptitude of the current one. Silicon chips, jet airplanes, and the electromagnetic spectrum were loosing ideas that helped to corrode the Iron Curtain. Though today's authoritarian China may seem to be straining the hypothesis that technology and travel are liberalizing forces, its leadership is incomparably less murderous than Mao's insular regime, as the numbers in the next chapter will show.

There may be another reason why antiwar sentiments finally took. The trajectory of violent deaths in Europe that we saw in figure 5-18 is a craggy landscape in which three pinnacles -- the Wars of Religion, the French Revolutionary and Napoleonic Wars, and the two world wars -- are followed by extended basins, each at a lower altitude than the preceding one. After each hemoclysm, world leaders tried, with some success, to make a recurrence less likely. Of course their treaties and concerts did not last forever, and an innumerate reading of history may invite the conclusion that the days of the Long Peace are running out and that an even bigger war is waiting to be born. But the Poisson pitter-patter of war shows no periodicity, no cycle of buildup and release. Nothing prevents the world from learning from its mistakes and driving the probability lower each time.

Lars-Erik Cederman went back to Kant's essays and discovered a twist in his prescription for perpetual peace. Kant was under no illusion that national leaders were sagacious enough to deduce the conditions of peace from first principles; he realized they would need to learn them from bitter historical experience. In an essay called "Idea for a Universal History with a Cosmopolitan Purpose," he wrote:

Wars, tense and unremitting preparations, and the resultant distress which every state must eventually feel within itself, even in the midst of peace -- these are the means by which nature drives nations to make initially imperfect attempts, but finally, after many devastations, upheavals and even complete inner exhaustion of their powers, to take the step which reason could have suggested to them even without so many sad experiences -- that of abandoning their lawless state of savagery.

Cederman plotted the historical trajectory of militarized disputes from 1837 to 1992 within pairs of democracies and other pairs of countries (figure 5-26). The inclined sawtooth for democracies shows that they started out warlike and thereafter underwent periodic shocks that sent their rate of disputes skyward. But after each peak their dispute rate quickly fell back to earth. Cederman also found that the learning curve was steeper for mature democracies than for newer ones. Autocracies too returned to more peaceable levels after the sudden shocks of major wars, but they did so more slowly and erratically. The fuzzy idea that after the 20th-century Hemoclysm an increasingly democratic world "got tired of war" and "learned from its mistakes" may have some truth to it.

Cederman suggests that Kant's theory of peace-through-learning should be combined with his theory of peace-through-democracy. Though all states, including democracies, start off warlike (since many democracies began as great powers), and all states can be blindsided by sudden terrible wars, democracies may be better equipped to learn from their catastrophes, because of their openness to information and the accountability of their leaders.

Source: Graph from Cederman, 2001. The curves plot 20-year moving averages for at-risk pairs of countries.

FIGURE 5-26: Probability of militarized disputes between pairs of democracies and other pairs of countries, 1825-1992

A popular theme in the antiwar ballads of the 1960s was that evidence of the folly of war had always been available but that people stubbornly refused to see it. "How many deaths will it take till they learn that too many people have died? The answer, my friend, is blowin' in the wind." "Where have all the soldiers gone? Gone to graveyards, every one. When will they ever learn?" After half a millennium of wars of dynasties, wars of religion, wars of sovereignty, wars of nationalism, and wars of ideology, of the many small wars in the spine of the distribution and a few horrendous ones in the tail, the data suggest that perhaps, at last, we're learning.