In chapter 4 we met a two-hundred-year-old theory that offers some predictions. In his essay "Perpetual Peace," Immanuel Kant reasoned that three conditions should reduce the incentives of national leaders to wage war without their having to become any kinder or gentler.

The first is democracy. Democratic government is designed to resolve conflicts among citizens by consensual rule of law, and so democracies should externalize this ethic in dealing with other states. Also, every democracy knows the way every other democracy works, since they're all constructed on the same rational foundations rather than growing out of a cult of personality, a messianic creed, or a chauvinistic mission. The resulting trust among democracies should nip in the bud the Hobbesian cycle in which the fear of a preemptive attack on each side tempts both into launching a preemptive attack. Finally, since democratic leaders are accountable to their people, they should be less likely to initiate stupid wars that enhance their glory at the expense of their citizenries' blood and treasure.

If the Long Peace is not the sturdy child of terror and the twin brother of annihilation, then whose child is it? Can we identify an exogenous variable -- some development that is not part of the peace itself -- that blossomed in the postwar years and that we have reason to believe is a generic force against war? Is there a causal story with more explanatory muscle than "Developed countries stopped warring because they got less warlike"?

The Democratic Peace, as the theory is now called, has two things going for it as an explanation for the Long Peace. The first is that the trend lines are in the right direction. In most of Europe, democracy has surprisingly shallow roots. The eastern half was dominated by communist dictatorships until 1989, and Spain, Portugal, and Greece were fascist dictatorships until the 1970s. Germany started one world war as a militaristic monarchy, joined by monarchical Austria-Hungary, and another as a Nazi dictatorship, joined by Fascist Italy. Even France needed five tries to get democracy right, interleaved with monarchies, empires, and Vichy regimes. Not so long ago many experts thought that democracy was doomed. In 1975 Daniel Patrick Moynihan lamented that "liberal democracy on the American model increasingly tends to the condition of monarchy in the 19th century: a holdover form of government, one which persists in isolated or peculiar places here and there, and may even serve well enough for special circumstances, but which has simply no relevance to the future. It was where the world was, not where it is going."

Social scientists should never predict the future; it's hard enough to predict the past. Figure 5-23 shows the worldwide fortunes of democracies, autocracies, and anocracies (countries that are neither fully democratic nor fully autocratic) in the decades since World War II. The year in which Moynihan announced the death of democracy was a turning point in the relative fortunes of the different forms of governance, and democracy turned out to be exactly where the world was going, particularly the developed world. Southern Europe became fully democratic in the 1970s, and Eastern Europe by the early 1990s. Currently the only European country classified as an autocracy is Belarus, and all but Russia are full-fledged democracies. Democracies also predominate in the Americas and in major developed countries of the Pacific, such as South Korea and Taiwan. Quite apart from any contribution that democracy might make to international peace, it is a form of government that inflicts the minimum of violence on its own citizens, so the rise of democracy itself must be counted as another milestone in the historical decline of violence.

Source: Graph adapted from Marshall & Cole, 2009. Only countries with a 2008 population greater than 500,000 are counted.

Stewart: Let me ask you a question. Argentina. Democracy?

Blair: More or less. It was when I was last there.

Stewart: Our president -- have you met him? He's a big freedom guy. He believes if everyone was a democracy, there'd be no more fighting.

Blair: Well, it's a democracy. They elect their president.

Blair: Well, as a matter of history, no two democracies have gone to war against each other.

Blair: Actually, at the time Argentina was not a democracy.

The second selling point for the Democratic Peace is a factoid that is sometimes elevated to a law of history. Here it is explained by the former U. K. prime minister Tony Blair in a 2008 interview on The Daily Show with Jon Stewart:

Stewart: Uh… didn't you guys fight?

FIGURE 5-23: Democracies, autocracies, and anocracies, 1946-2008

Stewart: England. Democracy?

• American Revolution, 1775-83: United States vs. Great Britain

If developed countries became democratic after World War II, and if democracies never go to war with one another, then we have an explanation for why developed countries stopped going to war after World War II.

• War of 1812, 1812-15: United States vs. Great Britain

• French Revolutionary Wars, 1793-99: France vs. Great Britain, Switzerland, the Netherlands

• Franco-Roman War, 1849: France vs. Roman Republic

• Spanish-American War, 1898: United States vs. Spain

• Greek Wars, 5th century BCE: Athens vs. Syracuse

As Stewart's skeptical questioning implies, the Democratic Peace theory has come under scrutiny, especially after it provided part of the rationale for Bush and Blair's invasion of Iraq in 2003. History buffs have delighted in coming up with possible counterexamples; here are a few from a collection by White:

• American Civil War, 1861-65: United States vs. Confederate States

• Punic Wars, 2nd and 3rd centuries BCE: Rome vs. Carthage

Stewart: Damn it! I thought I had him.

• Lebanese Civil War, 1978, 1982: Israel vs. Lebanon

• Kargil War, 1999: India vs. Pakistan

• First India-Pakistan War, 1947-49

• Anglo-Boer War, 1899-1901: Great Britain vs. Transvaal and the Orange Free State

• Croatian War of Independence, 1991-92: Croatia vs. Yugoslavia

• Kosovo War, 1999: NATO vs. Yugoslavia

• Israel-Lebanon War, 2006

Each counterexample has prompted scrutiny as to whether the states involved were truly democratic. Greece, Rome, and the Confederacy were slaveholding states; Britain was a monarchy with a minuscule popular franchise until 1832. The other wars involved fledgling or marginal democracies at best, such as Lebanon, Pakistan, Yugoslavia, and 19th-century France and Spain. And until the early decades of the 20th century, the franchise was withheld from women, who, as we will see, tend to be more dovish in their voting than men. Most advocates of the Democratic Peace are willing to write off the centuries before the 20th, together with new and unstable democracies, and insist that since then no two mature, stable democracies have fought each other in a war.

Critics of the Democratic Peace theory then point out that if one draws the circle of "democracy" small enough, not that many countries are left in it, so by the laws of probability it's not surprising that we find few wars with a democracy on each side. Other than the great powers, two countries tend to fight only if they share a border, so most of the theoretical matchups are ruled out by geography anyway. We don't need to bring in democracy to explain why New Zealand and Uruguay have never gone to war, or Belgium and Taiwan. If one restricts the database even further by sloughing off early pieces of the time line (restricting it, as some do, to the period after World War II), then a more cynical theory accounts for the Long Peace: since the start of the Cold War, allies of the world's dominant power, the United States, haven't fought each other. Other manifestations of the Long Peace -- such as the fact that the great powers never fought each other -- were never explained by the Democratic Peace in the first place and, according to the critics, probably came from mutual deterrence, nuclear or conventional.

A final headache for the Democratic Peace theory, at least as it applies to overall war-proneness, is that democracies often don't behave as nicely as Kant said they should. The idea that democracies externalize their law-governed assignment of power and peaceful resolution of conflicts doesn't sit comfortably with the many wars that Britain, France, the Netherlands, and Belgium fought to acquire and defend their colonial empires -- at least thirty-three between 1838 and 1920, and a few more extending into the 1950s and even 1960s (such as France in Algeria). Equally disconcerting for Democratic Peaceniks are the American interventions during the Cold War, when the CIA helped overthrow more-or-less democratic governments in Iran (1953), Guatemala (1954), and Chile (1973) which had tilted too far leftward for its liking. The advocates reply that European imperialism, though it did not vanish instantaneously, was plummeting abroad just as democracy was rising at home, and that the American interventions were covert operations hidden from the public rather than wars conducted in full view and thus were exceptions proving the rule.

Russett and Oneal untangled the knot with a statistical technique that separates the effects of confounded variables: multiple logistic regression. Say you discover that heavy smokers have more heart attacks, and you want to confirm that the greater risk was caused by the smoking rather than by the lack of exercise that tends to go with smoking. First you try to account for as much of the heart attack data as you can using the nuisance factor, exercise rates. After looking at a large sample of men's health records, you might determine that on average, every additional hour of exercise per week cuts a man's chance of having a heart attack by a certain amount. Still, the correlation is not perfect -- some couch potatoes have healthy hearts; some athletes collapse in the gym. The difference between the heart attack rate one would predict, given a certain rate of exercise, and the actual heart attack rate one measures is called a residual. The entire set of residuals gives you some numbers to play with in ascertaining the effects of the variable you're really interested in, smoking.

When a debate devolves into sliding definitions, cherry-picked examples, and ad hoc excuses, it's time to call in the statistics of deadly quarrels. Two political scientists, Bruce Russett and John Oneal, have breathed new life into the Democratic Peace theory by firming up the definitions, controlling the confounding variables, and testing a quantitative version of the theory: not that democracies never go to war (in which case every putative counterexample becomes a matter of life or death) but that they go to war less often than nondemocracies, all else being equal.

Now you capitalize on a second source of wiggle room. On average, heavy smokers exercise less, but some of them exercise a lot, while some nonsmokers hardly exercise at all. This provides a second set of residuals: the discrepancies between the men's actual rate of smoking and the rate one would predict based on their exercise rate. Finally, you see whether the residuals left over from the smoking-exercise relationship (the degree to which men smoke more or less than you'd predict from their exercise rate) correlate with the residuals left over from the exercise-heart attack relationship (the degree to which men have more or fewer heart attacks than you'd predict from their exercise rate). If the residuals correlate with the residuals, you can conclude that smoking correlates with heart attacks, above and beyond their joint correlation with exercise. And if you measured smoking at an earlier point in the men's lives, and heart attacks at a later point (to rule out the possibility that heart attacks make men smoke, rather than vice versa), you can inch toward the claim that smoking causes heart attacks. Multiple regression allows you to do this not just with two tangled predictors, but with any number of them.

A general problem with multiple regression is that the more predictors you want to untangle, the more data you need, because more and more of the variation in the data gets "used up" as each nuisance variable sucks up as much of the variation as it can and the hypothesis you're interested in has to make do with the rest. And fortunately for humanity, but unfortunately for social scientists, interstate wars don't break out all that often. The Correlates of War Project counts only 79 full-fledged interstate wars (killing at least a thousand people a year) between 1823 and 1997, and only 49 since 1900, far too few for statistics. So Russett and Oneal looked at a much larger database that lists militarized interstate disputes -- incidents in which a country put its forces on alert, fired a shot across a bow, sent its warplanes scrambling, crossed swords, rattled sabers, or otherwise flexed its military muscles. Assuming that for every war that actually breaks out there are many more disputes that stop short of war but have similar causes, the disputes should be shaped by the same causes as the wars themselves, and thus can serve as a plentiful surrogate for wars. The Correlates of War Project identified more than 2,300 militarized interstate disputes between 1816 and 2001, a number that can satisfy even a data-hungry social scientist.

Russett and Oneal first lined up their units of analysis: pairs of countries in every year from 1886 to 2001 that had at least some risk of going to war, either because they were neighbors or because one of them was a great power. The datum of interest was whether in fact the pair had had a militarized dispute that year. Then they looked at how democratic the less democratic member of the pair was the year before, on the assumption that even if a democratic state is war-averse, it still might be dragged into a war by a more belligerent (and perhaps less democratic) adversary. It hardly seems fair to penalize democratic Netherlands in 1940 for getting into a war with its German invaders, so the Netherlands-Germany pair in 1940 would be assigned the rock-bottom democracy score for Germany in 1939.

To circumvent the temptation of data snooping when deciding whether a state was democratic, especially states that call themselves "democracies" on the basis of farcical elections, Russett and Oneal got their numbers from the Polity Project, which assigns each country a democracy score from 0 to 10 based on how competitive its political process is, how openly its leader is chosen, and how many constraints are placed on the leader's power. The researchers also threw into the pot some variables that are expected to affect military disputes through sheer realpolitik: whether a pair of countries were in a formal alliance (since allies are less likely to fight); whether one of them is a great power (since great powers tend to find trouble); and if neither is a great power, whether one is considerably more powerful than the other (because states fight less often when they are mismatched and the outcome would be a foregone conclusion).

In fact, the Democratic Peace theory did even better than its advocates hoped. Not only do democracies avoid disputes with each other, but there is a suggestion that they tend to stay out of disputes across the board. And the reason they don't fight each other is not just that they are birds of a feather: there is no Autocratic Peace, a kind of honor among thieves in which autocracies also avoid disputes with each other. The Democratic Peace held not only over the entire 115 years spanned by the dataset but also in the subspans from 1900 to 1939 and from 1989 to 2001. That shows that the Democratic Peace is not a by-product of a Pax Americana during the Cold War. In fact, there were never any signs of a Pax Americana or a Pax Britannica: the years when one of these countries was the world's dominant military power were no more peaceful than the years in which it was just one power among many. Nor was there any sign that new democracies are stroppy exceptions to the Democratic Peace -- just think of the Baltic and Central European countries that embraced democracy after the Soviet empire collapsed, and the South American countries that shook off their military juntas in the 1970s and 1980s, none of which subsequently went to war. Russett and Oneal found only one restriction on the Democratic Peace: it kicked in only around 1900, as one might have expected from the plethora of 19th-century counterexamples.

So are democracies less likely to get into militarized disputes, all else held constant? The answer was a clear yes. When the less democratic member of a pair was a full autocracy, it doubled the chance that they would have a quarrel compared to an average pair of at-risk countries. When both countries were fully democratic, the chance of a dispute fell by more than half.

Even when it comes to the aversion of democracies to interstate war, it is premature to anoint democracy as the first cause. Countries with democracy are beneficiaries of the happy end of the Matthew Effect, in which them that's got shall get and them that's not shall lose. Not only are democracies free of despots, but they are richer, healthier, better educated, and more open to international trade and international organizations. To understand the Long Peace, we have to pry these influences apart.

So the Democratic Peace came out of a tough test in good shape. But that does not mean we should all be freedom guys and try to impose democratic governments on every autocracy we can invade. Democracy is not completely exogenous to a society; it is not a list of procedures for the workings of government from which every other good follows. It is woven into a fabric of civilized attitudes that includes, most prominently, a renunciation of political violence. England and the United States, recall, had prepared the ground for their democracies when their political leaders and their opponents had gotten out of the habit of murdering each other. Without this fabric, democracy brings no guarantee of internal peace. Though new and fragile democracies don't start interstate wars, in the next chapter we will see that they host more than their share of civil wars.