This article focuses on Hans Kelsen's theory of international law and pacifism. Following an analytical reconstruction of Kelsen's theses, the author makes a number of critical observations. In particular, the article examines Kelsen's ideas on the primacy of international law, the necessary demise of the concept of sovereignty and the assumption of the doctrine of justum bellum as the basis for the juridical character of international law. Special attention is given to Kelsen's idea of a 'Permanent League for the Maintenance of Peace', inspired by a kind of 'judicial cosmopolitanism', and developed in his "Peace through Law". It is the author's opinion that Kelsen's internationalism and pacifism brought about an important turning-point in the theory of international law and anticipated by 50 years many of the issues that the international community is today discussing: in particular, individuals as subjects of international law (and not only states) and the use of international criminal tribunals for the punishment of those responsible for war crimes and crimes against humanity. Finally, the author argues that it is uncertain that Kelsen's theoretical and political goals inspired by the Kantian idea of the moral unity of humanity and by a normativist conception of law, may be fulfilled or even desirable.