The article examines the extent to which Montesquieu's doux commerce thesis, which claims that commerce leads to softening of manners and therefore favours international peace, presents a challenge to democratic peace theory. It argues that Montesquieu's claim that peace may be due to commerce, and not democracy, provides a theoretical challenge to those scholars who argue that there is a Kantian virtuous triangle of democracy. The practical implication of this theoretical challenge concerns the way democratic peace theory has influenced the practice of international politics, especially American foreign policy. The article argues that Montesquieu's doux commerce thesis mediates between the contending claims of realism and liberal internationalism over the merits of democratisation as an essential means for securing peace.