The purpose of this dissertation is to discuss the theme of fundamental international institutions by studying the role that just war plays in modernity. My argument is that just war can be understood as a constitutive institution of the modern international. The development of my argument is divided into three parts. First, I analyze the thought of Francisco de Vitoria and discuss how the Amerindians were placed at a lower level of development in comparison to the Europeans. That characterization permitted to justify wars of colonization under a civilizing moral purpose. Based on Christian Reus-Smit s proposal of a normative complex to explain the development of fundamental institutions, I interpret just war, in sixteenth-century Spain, as an institution designed to legitimize the practices of the conquest and colonial expansion. Second, I present an historical development of natural law and a discussion of how it participated in the normative structure of the colonial and imperial systems. I intend to show that despite the abandonment of jusnaturalism from the eighteen century on, natural law still remained in the expression of a supposed superiority of the European peoples. Finally, by understanding that just war represents a way through which the European states express and affirm their civilization before the non-European peoples, I finish my characterization of just war as a constitutive institution of the modern international. Besides attempting to establish a constructive and complementary dialogic relationship with Political Theory, the present dissertation constitutes a critical effort that calls for a greater consideration of historical contingencies and of the moral dimension in the theoretical study of international relations.