Throughout its history the Soviet Union attempted to spread its political and economic system regionally and globally. Holding together the disparate Caucasus, Baltic, and Central Asian Soviet Socialist Republics, defending Eastern European states, propping up client states like Cuba, and intervening in third-world insurgencies, led to its exhaustion and collapse. The United States, too, has attempted to spread its political and economic system, and spreading democracy has become a common justification for war in the United States, sometimes in the name of humanitarian intervention. Is spreading democracy the path to security or the path to exhaustion?
Today's Liberals and Conservatives tout the democratic peace theory and advocate the spread of liberal democracy as the path to U.S. and international security. The use of military force to spread democracy through multilateral institutions like Woodrow Wilson's League of Nations and Franklin Roosevelt's United Nations had long been the preferred method of the Democratic Party. More recently, using force to spread democracy has been adopted by the new Republican Party through unilaterally initiated action with whatever contributions might be made by an ad hoc coalition of the willing. Presidents Clinton and Bush increased the operational tempo of military force tenfold over Cold War levels. The belief in democratic peace--that democracies tend not to war, or that democracies tend not to war against each other--was used to justify the use of force around the world to spread democracy.
The issue has been subjected to rigorous empirical studies by scholars with impeccable credentials, and the issue is far from resolved. Political scientists have agreed to a definition of war but are in less in agreement on what constitutes a democratic government. Regular citizens might legitimately disagree with the technical definition of war, but those who study war have settled and most studies rely on the consensus definition. Many uses of force fail to meet the criteria, including U.S. covert operations against left-leaning democracies during the Cold War. It's easy enough to define democracy in such a way that no democracies have waged wars against each other. It's also easy enough to define democracy so that there are plenty of examples
As for the war proneness of democratic regimes, the overwhelming evidence is that stable democracies and stable autocracies are equally prone to war, but they are much less prone to war than are states transitioning between the two forms of government. Stable governments are less prone to war than unstable governments in transition. Given this evidence, one should easily have anticipated a period of widespread hostility after communist regimes and regimes artificially propped up by the Soviet Union collapsed. The authors warned against the "naïve enthusiasm for spreading peace by promoting democratization." Spreading democracy by forced regime change followed by nation building, or by supporting popular revolutions, will create war prone democracies in the near term. The hoped for peace dividend will come only after the fledgling democracies become stable. Evidence from Iraq and Afghanistan suggests that will take many years.
As for democracies not warring against each other, advocates for spreading democracy by force often cite an important 1976 study spanning 1816 to 1965. The authors observed that democracies during much of the period were few and dispersed. Simple geography--states wage wars against neighbors, and democracies weren't neighbors--may be a better explanation than form of government. The study's authors were skeptical about the claim that democracy explained the correlation. It's ironic that this study is so often cited as the basis of the democratic peace theory.
Democratic peace theory is only one theory explaining past and predicting future international behavior. During the Cold War, many democracies entered into alliance against communist regimes and given the common threat were unlikely to war against each other. More broadly, there are other explanations lumped under the idealist and realist schools of international relations theories.
The idealist school offers several complementary explanations. Sociological liberalism argues for the dampening effect of transnational social bonds like shared language or religion. Interdependence liberalism focuses on economic interdependence as a dampening effect. Institutional liberalism places high value on the cooperative opportunities provided by international institutions; a pattern of cooperation reduces the likelihood of war. Republican liberalism elevates the republican form of government--the democratic peace theory.
The realist school focuses on interests and power. States are acquisitive and competitive. Conflict is inevitable. Survival of the state is vital, and only power assures survival. Realists differ on how much power is enough. Some favor only defensive power and others argue for offensive power. In between are those who think in terms of balancing power, that is, power that can be aggregated through a coalition based on shared interests that is sufficient to deter or defeat a threat to those interests.
Still, the democratic peace theory is used to justify intervention and regime change to spread democracy and thereby assure peace. As the late Gore Vidal sardonically called it, "Perpetual War for Perpetual Peace." Realists since Thucydides have asserted "that states expand in the absence of countervailing power; unbalanced power will act without moderation, and states not subject to external restraint tend to observe few limits on their behavior." There are justifications for war, but justifying the use of military force to spread democracy should be viewed with some skepticism.