Can a few primarily Western States expand the right to self-defence against non-State actors, incorporating the unwilling or unable standard? Even on a traditional reading of customary law formation, the answer is no because proponents have failed to attract consistent and widespread support. What is more, using our interactional international law approach, we show that efforts to date have not been successful because they have failed to address fundamental rule of law concerns. The current state of world politics has perhaps caught proponents of the unwilling or unable standard in a difficult bind. We suggest how proponents might carefully develop the law on self-defence against non-State actors.