Fabio Rojas invokes democratic peace theory in his comment on Rachel Maddow’s new book, Drift: The Unmooring of American Military Power (via Wilkinson):
The idea is simple – for whatever reason, democracies almost never fight each other. Of course, democracies go to war against non-democracies. But for some reason, democracies just don’t fight each other.
What’s the policy implication of all this? First, the sorts of rules that Maddow proposes are useless. People will just ignore the rules when they want to when they want war. Second, you have to reduce the population of non-democracies. Thus, if the Federal government wants to protect the United States by preventing war, the best, and cheapest, way to do it is to provide support and assistance for indigenous movements for democracy and tolerance. Once people have a genuine democracy at work, they just don’t want to fight with each other. They just don’t.
Rojas’ claim depends entirely on the meaning of “genuine democracy.” Even though there are numerous examples of wars between states with universal male suffrage and elected governments (including that little dust-up known as WWI), the states in question probably don’t qualify as “genuine” democracies and so can’t be used as counter-examples. Regardless, democratic peace theory draws broad conclusions from a short period in modern history with very few cases before the 20th century. The core of democratic peace theory as I understand it is that democratic governments are more accountable to their populations, and because the people will bear the costs of the war they are going to be less willing to support a war policy. This supposedly keeps democratic states from waging wars against one another because of the built-in electoral and institutional checks on government power. One small problem with this is that it is rubbish.
Democracies in antiquity fought against one another. Political equality and voting do not abolish conflicts of interest between competing states. Democratic peace theory doesn’t account for the effects of nationalist and imperialist ideologies on the way democratic nations think about war. Democratic nations that have professional armies to do the fighting for them are often enthusiastic about overseas wars. The Conservative-Unionist government that waged the South African War (against two states with elected governments, I might add) enjoyed great popular support and won a huge majority in the “Khaki” election that followed.
As long as it goes well and doesn’t have too many costs, war can be quite popular, and even if the war is costly it may still be popular if it is fought for nationalist reasons that appeal to a majority of the public. If the public is whipped into thinking that there is an intolerable foreign threat or if they believe that their country can gain something at relatively low cost by going to war, the type of government they have really is irrelevant. Unless a democratic public believes that a military conflict will go badly for their military, they may be ready to welcome the outbreak of a war that they expect to win. Setting aside the flaws and failures of U.S.-led democracy promotion for a moment, the idea that reducing the number of non-democracies makes war less likely is just fantasy. Clashing interests between states aren’t going away, and the more democratic states there are in the world the more likely it is that two or more of them will eventually fight one another.
Selvar 
Joseph Stromberg 
tbraton 
The democratic peace theory is one of the most ridiculous theories taken seriously by smart people (right up there with “Tom Friedman is insightful”). We should all be skeptical of theories whose formal definition literally includes “for some reason we don’t understand.” I don’t think that would have cut it for Daniel Patrick Moynihan.
I think it’s definitely a statistical anomaly. Democracies go to war plenty often. Sometimes popular enthusiasm among democratic electorates is the thing that sets off what might not otherwise have happened, as in Crimea. Their citizens aren’t any more opposed on principle to it so far as I can tell. If anything, my guess would be that a lack of democracy in the Middle East has kept things as quiet as they have been, since the publics of Egypt, Saudi Arabia, and Jordan (to name a few) are almost certainly more hostile toward Israel than their rulers have been. Democracies are very willing to go to war, sometimes without any real cause. A possibly more compelling version of this theory would be the idea that trading ties make war less likely, which would explain a lot of things more compellingly. Such as, the countries toward which America has and hasn’t been hostile to over the past two decades.