The article examines the substance and form of 20th century positivist international law; in particular the way in which each determines the other. The political and philosophical aspects of the foundations of international law were transformed in a process which developed in the last decades of the 19th century. On the one hand, it was felt that an internationalist sense of economic interdependence had to come to the foreground in the discipline of international law. On the other, the economic Weltanschauung of several influential international lawyers inaugurated a new style of formulating theory. The article firstly describes the turn to interests in international law, which evolved slowly in scope and depth. By examining Lassa Openheim’s focus on ‘common interests’ that united states and Hans Kelsen’s focus on the ‘struggle of interests’ that constituted politics, the article studies two phenomena produced by the foundational role taken by interests during the 20th century. Firstly, this role contributed to putting an end to the moral discussion about the treatment of native populations. Secondly, it curbed debate about a common political project for a global order, thus creating conformity characterised by abuse of power – all in the name of the neutrality of positivist law. This article suggests that the work of these two leading theoreticians in the field has contributed to the shaping of the legal theory of mainstream positivist international law, and seeks to foreground discussions about the different theories on the role of law in politics. In this manner it aims to help reconceptualise law in such a way as to bring about a situation in which discussions of a common political project for the international arena are more central.