Democratic peace theory

Naturally, the common people don't want war; neither in Russia nor in England nor in America, nor for that matter in Germany. That is understood. But, after all, it is the leaders of the country who determine the policy and it is always a simple matter to drag the people along, whether it is a democracy or a fascist dictatorship or a Parliament or a Communist dictatorship.

Democratic peace theory is the theory that liberal democracies don't go to war with one another. This is predicated on the theory that a large group of voters, many of whom are eligible for conscription, are less keen to go to war than one monarch or dictator who is likely to stay in the capital city ordering troops to the front, an effect that, with two democracies involved, is thought to be cumulative. Moreover, democracies have a tendency to bureaucracy, and when forms must be signed in triplicates, declaring war seems a little less heroic. Another contributing factor is that democracies tend to ally against non-democracies, and even democracies with no official ties beyond basic diplomatic relations are more likely to think of one another as friendly. Democracies also usually value equality between people of different nations and which makes wars seem less justified and necessary.

The theory is used by neoconservatives to justify war (ironically enough), as long as that war is "spreading democracy," like the 2003 Iraq War.

The theory seems to have been borne out by the fact that since they became representative democracies, the nations of Europe have refrained from war with one another, a marked contrast with the era of monarchy, in which just two nations, England and France, managed to go to war 20 times over a 900-year period, averaging once every 45 years, or about once a generation. It is possible, however, that these observations are more of a coincidence, as most the world's liberal democracies were allied during the Cold War against the Eastern Bloc, which was composed of more autocratic regimes. Since it has remained a relatively short amount of time since the Cold War ended, many of the same alliances remain; hence, the lack of conflict between the world's liberal democracies.

Things to consider
This theory does not preclude democracies from going to war with states with other systems of government. In fact, there has been some evidence that democracies are more likely to go to war with non-democracies than other non-democracies are. Which is more than just a little contradictory with the usual reasons given for the supposedly inherent peacefulness of democracies — if these mechanisms were indeed major factors in precluding offensive warfare in general, they'd work in much the same way when the opponent is an autocracy. A possible explanation for the fact that democracies are at least much less likely to wage war against another would be a sense of "kinship" and shared norms among democracies, and a higher tendency to mutually assume peaceful intentions on the other's part, which could serve to preclude security dilemmas and arms races, both of which are potential sources of conflict. Yet one more reason for democracies to be peaceful to one another is the tendency of democracies to have a healthy press and whistleblowers that expose things like secret armament that can stay under wraps for a longer period of time in a dictatorship. Take, for instance, the which, while backed by the Weimar Republic government, was ultimately exposed. Many of Nazi Germany's secret arms projects, however, only became public knowledge upon their conclusion. Democracies also tend to have less erratic foreign policies. An autocratic ruler like Erdoğan or Putin can declare another state an enemy or friend at a moment's notice, as they did to each other in the course of the Syrian civil war and the aftermath of the coup attempt in Turkey. Erdoğan also did similar things with regards to the Kurds, veering from attempts at peace to open hostility without any seeming rhyme or reason. In democracies, on the other hand, the phrase "pacta sunt servanda" still has meaning and even a diametrically opposed government won't just throw out the foreign policy of its predecessor overnight. That's why Donald Trump threatening to defund NATO is so scary: no democratic politician has done a similar thing before.

When presented with exceptions, proponents of the theory move the goalposts, changing the definitions of "democracy" or "war" so that the claim remains true (No True Scotsman). Matthew White writes:

I've witnessed this debate on Usenet several times, and it always follows the same pattern:
 * 1) Somebody casually brings up the old factoid about how no two democracies have ever gone to war with one another.
 * 2) Somebody jumps in and lists a dozen or so wars which have been fought between democracies.
 * 3) Somebody else points out that those countries weren't democratic, not really.
 * 4) Everybody gets into arguments over who was or was not democratic.
 * 5) The argument fizzles out except for two guys continuing to argue over whether the American Civil War was about slavery.

James Lee Ray points out that with a sufficiently restrictive definition of democracy, the claim becomes trivially true: define democracy as true universal suffrage, the right of all — including children — to vote, and there have been no democracies, and therefore no wars between them. On the other hand, with a generous enough definition, counterexamples become plentiful: Germany before and during World War One had universal male suffrage over the age of 25, even though the Kaiser (Emperor) retained significant power, particularly over the armed forces.

Counterexamples

 * Formal belligerency between Finland (an ally of Nazi Germany) and six Allied states during the Second World War — however, Finland only ever directly fought the Soviet Union, which was very much not a democracy
 * The 2006 Israeli-Lebanese War, although it was mainly fought between democratic Israel and Hezbollah paramilitaries (rather than the Lebanese army). Hezbollah, while being a political party in Lebanon, cannot conceivably be called democratic.
 * That Britain and France looked poised to war with each other just prior to World War I, an event which forced them to unite instead. However, they did not end up going to war against one another (instead, they declared war on the similarly semi-democratic Germany) and neither of them had full female suffrage.
 * France invaded parts of Germany in 1923, killing 130 German civilians. However, France at the time had no female suffrage, which raises the question of how democratic a country that excludes ~50% of its adult population from the vote can possibly be.
 * The ongoing conflict in Western Sahara is fought between Morocco (a constitutional monarchy) and the Polisario Front/Sahrawi Arab Democratic Republic (an unrecognised nation with a democratic structure, though led by the same guy for decades); for 30 years, the latter has been supported by Algeria, also nominally a democratic republic.
 * The 2008 Russo-Georgian War. Again, Putin is no democrat. However, South Ossetia and Abkhazia were arguably parties to this conflict and arguably democracies.
 * The Second Congo War (1998-2003) involved various highly imperfect democracies, including Namibia (in which the ruling party never polls below 60% ) on the Congolese side and Rwanda (where the current government has been in office since seizing it by force and being reelected) and Uganda supporting the rebels.
 * The Turkish state (itself cycling between flawed democracy, military coup, and authoritarianism at any given time) has had an on-again-off-again war against various Kurdish forces, some of them arguably with democratic internal structures.
 * The military coup against the perpetrators of the Rwandan genocide was supported by the British, but opposed to by the French, causing a proxy war between both despite both being full democracies at the time.
 * and several more...

In her 1999 book Ballots and Bullets, Joanne Gowa finds strong empirical support for the democratic peace only in the period after World War II, which presented a bipolar rather than multipolar world system. The claim could be made by a supporter of the democratic peace theory that while it isn't perfect in preventing wars between democracies, it does seem to drastically reduce the frequency of wars between democratic states. This can be seen as something of a weak form of the theory.