Identifying obligations erga omnes is only one of the problems to which the Court's jurisprudence gives rise. As important is the question how States can respond to erga omnes breaches – often described as ‘the acid test of the erga omnes concept’. Indeed, if violations of obligations erga omnes did not trigger any special rights of response, the concept, at least for the purposes of law enforcement, would be of rhetorical value only. For the reasons given in the Introduction, only two means of enforcement are addressed in the present study: ICJ proceedings and countermeasures. It is convenient to begin by discussing the former of these, as it has historically provided the starting-point for the debate about erga omnes effects in the field of standing.

Since 1970, it has been disputed whether all States should be entitled to bring ICJ proceedings in response to breaches of obligations erga omnes. The different points of view are defined relatively clearly. According to most commentators, the question hardly needs to be addressed; the Court is said to have conclusively settled it in Barcelona Traction, when recognising the legal interest of all States in seeing obligations erga omnes observed. Byers, to take but one example, laconically states: ‘Generality of standing … is the essence of erga omnes rules.’ Following this interpretation (which will be referred to as the broad approach), obligations erga omnes are another category of obligations allowing for a general right to respond against breaches.