In the case concerning Military and Paramilitary Activities in and against Nicaragua, the International Court of Justice (ICJ) had to examine if the multilateral treaty reservation of the United States precluded the application of the Geneva Conventions. Considering certain aspects of the ICJ Judgement, this article observes the development of customary law alongside conventional law in the context of the Geneva Conventions. It looks at both the possible customary status of the Conventions at the time of their adoption as well as the subsequent passage into customary law of norms stated in the Conventions. The author notes that even though only a few international judicial decisions discuss the customary law nature of IHL instruments, these decisions nevertheless point to a tendency to ignore state practice and opinio juris. International tribunals assume that noble humanitarian principles have in fact been recognized as such by states while in reality the decisive factor is whether states observe the Geneva Conventions or not.