Established democracies have rarely, if ever, gone to war with each other

Immanuel Kant argued in 1795 that republics would be slow to fight, since citizens bear the costs of war. In the 1980s political scientist Michael Doyle and others found that established liberal democracies have very rarely fought each other, though they fight non-democracies often. Scholars still argue over why: shared values, trade, alliances, or simply how 'democracy' and 'war' are defined.

— Michael Doyle, Kant, Liberal Legacies, and Foreign Affairs — Philosophy & Public Affairs, 1983