The democratic peace theory, liberal peace theory, or simply the democratic peace is a theory and related empirical research in international relations, political science, and philosophy which holds that democracies—usually, liberal democracies—never or almost never go to war with one another.

The original theory and research on wars has been followed by many similar theories and related research on the relationship between democracy and peace, including that lesser conflicts than wars are also rare between democracies, and that systematic violence is in general less common within democracies.

Research on the democratic peace theory has to define "democracy" and "peace" (or, more often, "war").

The straightforward argument for the democratic peace is: given the number of wars over the past two centuries, if democracies fought each other as often as any other pair of states, there should have been many wars between democracies. Instead, depending on the study, there have been zero or very few. A review (Ray 1998) lists many studies finding that this peacefulness is statistically significant.

Several researchers find no wars between well-established liberal democracies. Jack Levy (1988) made an oft-quoted assertion that the theory is "as close as anything we have to an empirical law in international relations".

Others see one or two exceptions. Some wars commonly suggested as exceptions are the Spanish-American War, the Continuation War and, recently, the Kargil War. Some of those who see exceptions regard them as marginal cases.

Other authors simply describe war between democracies as "rare", "very rare", "rare or non-existent".

The question of no or few wars may be unimportant. Bremer (1992, 1993), who strongly supports the democratic peace, argues that it is impossible to prove a probability of exactly zero wars between democracies; thus is "fruitless to debate the question of whether democracies never or only very rarely fight one another". It is only possible to show a decrease in the probability of war.

However, at least one researcher (Rummel 1983) has argued that one exception will disprove the theory. Most researchers disagree (Gleditsch 1992).

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