Article 2(4) of the UN Charter prohibits the use of force in international relations and separately bans any threat of force, whether explicit or implicit. However, what constitutes an implicit threat remains ill-defined, undermining the UN Charter’s system of collective security. This article addresses the central question: How can implicit threats be more reliably identified? It begins by reviewing the incomplete work of the International Law Commission on threats of force, then develops a framework to distinguish prohibited implicit threats from permissible state behaviour. Grounded in a theoretical understanding of threats of force as forms of communication rather than action, the article offers a clear definition of implicit threats. To exemplify this definition, it adapts the approach of the Definition of Aggression resolution, enabling a more precise attribution of prima facie responsibility for implicit threats. Subsequently, the article explores possible justifications for implicit threats, identifies their legal consequences and outlines modalities for international responses to such threats. Finally, the article suggests resuming the juridical effort to progressively develop international law in this area.

Which international obligations are characterised as obligations owed erga omnes or erga omnes partes is today a crucial question for the enforcement of international law, particularly through adjudication before the International Court of Justice (ICJ). The ICJ, however, has not given sufficiently clear indication as to how it understands and identifies obligations erga omnes and erga omnes partes. This lack of clarity and consistency permits varied approaches to the articulation of such obligations, ultimately leaving uncertain the enforceability, through adjudication and otherwise, of a wide variety of obligations.