This was mentioned as fact in my International Politics class and it brought up a bunch of questions for me.
First of all, is it even true? If so, is it meaningful about the nature of democracies? It occurs to me that democracies have only been common for a very short amount of time in very recent history, namely post world war. I suspect that there is more to be said here about the nature of post-nuclear international history than the peacefulness of democracies. Secondly, just because no war has been declared doesn't necessarily mean that warfare hadn't been waged. (I remember reading once that the US has had military operations in every nation in the Western Hemisphere for some reason at some point in time, though I don't have a source for this, can someone verify/debunk this for me?) Third, not all democracies were created equal, and I suspect that in many conflicts one party would attempt to argue that the other wasn't "really" democratic. Whether they were or not, in the modern world democracy is seen as virtuous and I can see states in conflict using this as a way of discrediting their enemies. Am I correct in this assumption? Besides that, are there any other things I haven't considered?
This is absolutely false and I'm still not sure why people still say it. One of the earliest wars involving a democracy was waged against another democracy. The Sicilian Expedition during the Peloponnesian War, in which the democrats at Athens warred against the Syracusans, was fought between two democracies. Thucydides notes that the Syracusans and Athenians would have been better employed fighting together against the Lacedaemonians, and that it was a fleet tragedy that they were so shortsighted. And this is without any real consideration of what constitutes democracy since both cities had pretty much identical constitutions (the democratic constitution at Syracuse was modeled on the Athenian). If we have to consider what constitutes democracy then we've got all kinds of considerations to think about. Did the Carthaginian constitution represent a limited form of democracy (most scholars would say no, but it's still a question)? Did the assemblies of many of the Gallic tribes constitute forms of democracy? And what about the wars between the Romans, which were oligarchical with democratic elements, and the Achaean League, which although composed of separate political entities still maintained political parity of all members within the League, allotting them the same votes?
Much of this, of course, depends on how you define "democracy" and "war."
For one example I believe meets both criteria, the United States government overthrew the freely elected government of Guatemala in 1954 using a U.S.-armed and funded proxy army, as well as CIA agents on the ground and Air Force crewmen in sanitized aircraft bombing and strafing the capital.
What you're referring to is called the democratic peace theory, which posits that democracies never go to war with each other (or they are extremely hesitant to).
But it really isn't true. If you take a look at this list (I apologize for wikipedia but it's adequate for making a general point), you can see that there have been many conflicts between entities that can be called democracies.
Of course, you can agree that XYZ was not a true democracy, but then we get to the near impossible task of creating definitions of "war" and "democracy" that everyone will accept.
Saying "Democracies never go to war with each other" really can't be taken as accurate and is little more than a political statement meant to glorify democracy as the one true political system.
Mansfield and Snyder's 1995 FA article does a thorough debunking of the democratic peace theory, by showing that it really only applies to countries with established liberal democratic civil institutions (e.g. free press, independent judiciary, etc.) and that democaticizing/newly democratic states that lack those prerequisites tend to be more prone to war than almost any other country.
I've answered this question before, here. The general point was that the answer depended on your specific definitions, but according to my interpretation, "democratic" governments have certainly committed acts of war against each other. I'll reproduce the answer below.
It depends upon how you define "war". If you define perpetrating or supporting a coup d'état as an act of war, then the United States has gone to war with a number of democracies.
The U.S. and the U.K. overthrew the parliamentary government of Iran, under Prime Minister Mohammad Mosaddegh, in favor of absolute rule by the Shah in 1953.
In 1954, the United States, through the CIA and Department of State and under the influence of the United Fruit Company (now Chiquita), waged a campaign of terror and violence against the government of Guatemala, deposing democratically elected President Jacobo Árbenz in favor of a series of military regimes, including those that would perpetrate the Mayan genocide with U.S. support during the 1980s.
The United States supported the 1964 overthrow of João Goulart in Brazil and, in addition to various multinational corporate interests, played a major role in the 1973 Chilean coup d'état, which left democratically elected socialist President Salvador Allende dead and right-wing military dictator Augusto Pinochet in power.
Through training, funding, and support for the Contras in Nicaragua, even after the internationally recognized 1984 democratic election, the United States effectively waged war on the Sandinista government of Nicaragua, a conflict that arguably influenced the 1990 election through the intimidation of the peasants by the pro-opposition Contras. In the 1986 ICJ case Nicaragua v. United States, the U.S. was found guilty of violations of international law and Nicaraguan sovereignty during that conflict.
A failed 2002 coup attempt in Venezuela against the democratic government of Hugo Chávez is alleged to have had CIA support, and the successful 2004 Haitian coup against the government of Jean-Bertrand Aristide was in part orchestrated by the United States government.
I suppose it also depends on how you define "democracy", which is a slippery term. I've written about differences in conception between American democracy and Cuban democracy. If you were to accept both forms of government as valid interpretations of democracy, then Cuba could be added to the list, since U.S. bombings and a U.S.-backed invasion certainly constitute acts of war. Also, both Fatah and Hamas have been democratically elected in Palestine at different times, and if one accepts Israel as a democracy (depending upon how one classifies the status of Palestinians under Israeli occupation), the conflict between Israel and Palestine could be a war between democracies.
If one applies a very traditional view of war, requiring formal declarations of war from the involved governments, then perhaps war between democracies is very rare indeed. But, I think that the list of coups I gave constitute acts of war, so I would say that the theory in question is weak.
You can find more of my answers to related questions on my user profile page, including answers on U.S.-backed coup attempts in general, U.S. involvement in Central America, and U.S. involvement in the Chilean coup.
The United States and Great Britain fought each other during the War of 1812. They weren't as expansive democracies as they are today, but both of them could be called such without too much quibbling.
France, Germany, and the UK in 1914 were all representative democracies to varying degrees, although in Germany the Monarch held more power than in the UK.
Even if you limit this question to the most strict definition of a democracy, then I can still think of one example: The Cod Wars between the UK and Iceland in the 1950s and 1970s. Admittedly we're not exactly talking about total war here, but they were still bona fide military confrontations and they did lead to fatalities on both sides.