The idea that democratic or republican states are more peaceful in their external relations and never (or almost never) fight each other. Modern democratic peace theory (DPT) builds on a long‐standing tradition in liberal writing on international relations and is often associated with the German philosopher Immanuel Kant (1724–1804)—hence references to ‘Kantian peace’. However, it only formed one (and not the most central) part of Kant's political thought and had already become a liberal commonplace by the end of the eighteenth century. Other precursors of modern democratic peace theory include Karl Deutsch's writing in the 1950s on security communities—groups of states (such as North America, Scandinavia, and Western Europe) in which there is real assurance that the members of that community will not fight each other physically but will settle their disputes in some other way. Overlooked or neglected by many studies of war causation, the idea of the democratic peace theory was revived in the mid‐1980s by the US political scientist, Michael Doyle. It became a major theme of both academic writing on international relations and of political and public debate on the nature of the post‐Cold War international order (as in the Clinton administration's policy of democratic enlargement or in the justifictions for EU and NATO expansion).
The democratic peace hypothesis rests on two claims:
Democratic peace theorists argue that two sets of causal factors are important in explaining the democratic peace. In the first place, the structural constraints of democratic institutions and of democratic politics make it difficult or even impossible for war‐prone leaders to drag their states into wars. They also stress the joint effect of these democratic constraints, together with the greater openness and transparency of liberal democracies. If both sides are governed by cautious, cost‐sensitive politicians that only use force defensively, then conflict is far less likely to occur. Second, democratic peace theorists highlight the importance of normative mechanisms. Liberal and democratic norms involve shared understandings of appropriate behaviour, stabilized expectations of the future, and are embedded in both institutions and political culture. Rule‐governed change is a basic principle; the use of coercive force outside the structure of rules is proscribed; and trust and reciprocity, and rule of law are at the heart of democratic politics. On this view, then, the democratic peace is produced by the way in which democracies externalize their domestic political norms of tolerance and compromise into their foreign relations, thus making war with others like them unlikely.