Alleged Acquiescence of the International Community to Revisionist Claims of International Customary Law (with Special Reference to the jus contra bellum Regime)

There is a broad consensus that the protracted inaction of states faced with the conduct of other states can sometimes be analysed as evidence of acceptance as law (opinio juris) for the purpose of identifying rules of customary international law. This issue has become highly topical in the context of the alleged ‘acquiescence’ of the international community to innovative interpretations of the right of self-defence formulated by some of the states currently conducting a military intervention against the Islamic State in Syria and the Levant (‘ISIL’) on Syrian territory. While the question has been recently discussed by the International Law Commission in the context of its work on the identification of international customary law, the precise circumstances in which acceptance as law can be inferred from inaction remain largely uncertain. Analogies with the concept of acquiescence, as developed in the context of the creation, modification or extinction of subjective rights and obligations, can arguably provide useful guidance in this respect. States whose interests are directly affected by a given conduct can reasonably be expected to express their view on its legality. If they remain silent and if other contextual elements do not indicate otherwise, they can therefore be presumed to have acquiesced to the claims accompanying the concerned conduct. But the lack of reaction of third states whose interests are not directly affected should not be lightly assimilated with the manifestation of a benevolent opinio juris. Even the fact that erga omnes rights and obligations are affected by a given conduct does not create an expectation of near universal reaction, failing which the continuing validity of the underlying customary norm would be eventually undermined. Furthermore, other legal and extra-legal factors must be carefully assessed before assimilating the failure to react as the evidence of a nascent opinio juris.