The official US attitude towards the prosecution of crimes against humanity and war crimes changed dramatically from the universality of Nuremberg to the exceptionalism of the Rome Treaty negotiation. This article argues that the history of US legal thinking indicates that both stances are the result of a conceptual battle between legal realism and legal idealism – strains of international legal thought that pose a battle of opposites which is never fully resolved into a coherent approach. Although Nuremberg would seem to illustrate the idealist extreme and the abstention from Rome the realist one, in fact both stances were the culmination of intense negotiation and argumentation between the two strains of thought.