For years now, the political science literature has been exploring the phenomenon of the "democratic peace," according to which, to state it in its bluntest form, democracies do not go to war with one another. It's not that democracies are pacifist by nature. Democratic countries, acting alone or in concert, do go to war with nondemocratic countries from time to time, for example the United States and others against Saddam Hussein's Iraq and NATO against former Yugoslavia over the attempted ethnic cleansing of Kosovo.

Moreover, the record of peace among democracies is not without its asterisks. As the neorealist scholar Kenneth N. Waltz has noted, Germany on the eve of the First World War was, by the standards of the day, "democratic." German "militarism" in the late 19th century was not an authoritarian imposition on the German people but something they and their elected representatives supported--as indeed going to war in 1914 was popular in Britain as well. Reclassifying a country as "nondemocratic" because it has chosen to make war on a democracy is a temptation to which the "democratic peace" thesis may give rise.

Young democracies also pose problems; for example, Ecuador (less than three years democratic) and Peru (less than one year) went to war in 1981. The Kargil war between India and Pakistan in 1999 exemplifies another category of exception: India's democratic bona fides are beyond challenge, but Pakistan, though it had had a democratically elected president and a civilian government since 1988, was a distinctly illiberal democracy. Indeed,

The Bush administration, of course, has invested heavily in the idea of the beneficial effects of democracy. As the president put it in his second inaugural address, "It is the policy of the United States to seek and support the growth of democratic movements and institutions in every nation and culture, with the ultimate goal of ending tyranny in our world." Bush pointed to two reasons for this policy, one metaphysical ("no one is fit to be a master, and no one deserves to be a slave") and one practical: "as long as whole regions of the world simmer in resentment and tyranny--prone to ideologies that feed hatred and excuse murder--violence will gather, and multiply in destructive power, and cross the most defended borders, and raise a mortal threat." Now, one can agree with the metaphysical claim, and even with the long-term prognosis--as well as the inescapable fact that there is no way to get to the long term except via the short term--without relinquishing reasonable qualms about who might be empowered by free elections in the short run.

But for all the imperfections of a thesis claiming that democracies do not go to war with each other, we are left with one big and inescapable fact: A large number of mature, liberal democracies have no intention whatsoever of going to war with each other, not now, not ever. Such disputes as arise among them, they will settle by peaceful means. And one of the reasons they will be able to settle disputes peacefully is that they have learned they have no "vital interests" that conflict. That's no small thing historically.