Extant re s e a rch has shown that cross-national variations in the level of a country ' s democracy tend to be re l a t e d to its propensity to be involved in external conflict. The dyadic version of the theory of democratic peace contends that democracies rare l y, if ever, fight each other, and it is strongly supported by the available evidence. The monadic version, suggesting that democracies are in general more peaceful re g a rdless of the nature of the other p a rty involved in a relationship, has been less well supported. This article addresses the latter proposition, seeking to explain the variations in war involvement among the established democracies on the basis of major diff e rences in their institutions of governance. Among the various distinctions considered (such as parliamentary versus presidential forms of government, rule by a single dominant party versus a coalition government, and phases of the electoral cycle), a country ' s electoral system turns out to be the most important institutional factor that dampens war involvement. Established democracies with a pro p o rt i o n a t e -re p resentation system tend to have significantly less such involvement according to three alternative measures. We adduce insights from comparative studies of democratic politics to explain this major finding, thereby offering a more specific and cogent account of why this particular institutional arrangement matters for external belligere n c e .