Third-Party Countermeasures: Making Custom Out of Ambiguous Practice?

Abstract In the wake of Russia’s aggression against Ukraine, international lawyers and policy advisors are considering the tools that are available to third States that wish to respond to the serious breach of international law and support Ukraine. Within this context, the question of third-party countermeasures is once again highly relevant. Though the topic was contentious during the drafting of the Articles on Responsibility of States for Internationally Wrongful Acts (ARSIWA), doctrine on third-party countermeasures often argues that they are permissible under customary international law, even while acknowledging that opinio juris is lacking. Whereas it has been argued that this subjective requirement can be inferred, this article maintains that, given the ambiguity surrounding unilateral sanctions practice, it is necessary to demonstrate that States believe that they are legally permitted to adopt wrongful sanctions in response to a prior breach of an obligation erga omnes (partes). It is argued that the International Law Commission was right to not include third-party countermeasures in the final ARSIWA and that, while sanctions practice has seemingly flourished over the years, there has been little progress in conclusively establishing that third-party countermeasures are accepted as custom, as illustrated by the discussion on the confiscation of Russian State assets.