The four Geneva Conventions of 1949 for the protection of war victims open with an unusual provision: it is the undertaking of the contracting states ‘to respect and to ensure respect for [the Conventions] in all circumstances’. Why reaffirm that contracting states are bound to ‘respect’ their treaty obligations? Does ‘all circumstances’ add anything special to this fundamental rule of the law of treaties? And what about ‘ensure respect’: should that not be regarded as implicit in ‘respect’, in the sense of a positive counterpart to the negative duty not to violate the terms of the Conventions? I readily admit that common Article 1 was not the first provision of the Conventions to capture my attention: there was, after all, so much to discover in these impressive structures that Article 1 could easily be passed over as an innocuous sort of opening phrase. Two things have changed this. One was the insistence of the International Committee of the Red Cross (ICRC) that a State Party to the Conventions is not only itself bound to comply with its obligations under these instruments but is under a legal obligation to make sure that other States Parties do likewise. The more this thesis of the ICRC was forced upon us, the less likely it seemed to me that this could indeed be an international legal obligation upon contracting states.