The Impact of Subsequent Customary International Law on Treaties: Pushing the Boundaries of Interpretation?

Conflicts between treaty and customary norms are endemic to international law and are increasingly frequent. Yet there is nothing automatic or mechanical about interpreting and resolving such conflicts, which require a high degree of contextual sensitivity. Their identification and interpretation test the limits of the rules of treaty interpretation as codified in the Vienna Convention on the Law of Treaties, particularly where treaty modifications by subsequent customary law are concerned. This article endeavours to sketch how the latter phenomenon occurs, and the interpretative and evidentiary challenges involved—many of which remain underexplored. The analysis begins with the identification and interpretation of newly emerged customary norms, before delving into the process of determining their treaty-modifying potential. This involves the side-by-side interpretation of the pre-existing treaty and the customary norm to assess whether there is a genuine incompatibility that cannot be resolved through harmonious interpretation. The final inductive step is to ascertain the parties’ consent to displace the treaty norm in favour of the customary norm, subject to certain crucial requirements. Against the backdrop of the organic and continuous interplay between treaties and customary international law, these interpretative and evidentiary steps serve to ensure that the parties’ intention remains paramount.