Compared with the realist tradition, relatively few students of international relations explore variations within liberalism. This paper introduces a particular interpretation of Immanuel Kant's evolutionary liberalism and then compares it with Thomas Paine's revolutionary liberalism. Paine was an ebullient optimist while Kant was more guarded and cautious. These different assumptions lead to distinct liberal views on voting rights, how trade fosters peace, and defense policies. The most striking disagreement, and one that endures in contemporary liberal circles, revolves around the question of military interventions to spread democratic rule. Kant advocated nonintervention while Paine actively pursued military intervention to spread democratic rule. Differences between Kant and Paine represent some enduring tensions still residing within the liberal tradition in international relations.