Roberto Ago, the International Law Commission’s second Special Rapporteur on the topic of state responsibility for internationally wrongful acts, defined self-defence as a faculté of a state to use force in response to an act of another state through which a breach of the principal obligation under Article 2(4) Charter is committed. On this basis, he then inserted a provision in Chapter V to Part One of the Draft Articles on State Responsibility expressing self-defence as a specific factual circumstance precluding the wrongfulness of the use of force which constitutes a response to state aggression. This conception of self-defence, although misunderstood from the onset, remained in the backdrop of the study of the law of state responsibility for a considerable period. It was only dismantled during the reign of the last Special Rapporteur on the topic of state responsibility, James Crawford. The last Rapporteur, at the onset, submitted that it is not the function of the Draft Articles to specify the content of the primary rules, including that referred to in Article 51 Charter. He then redefined the function of the circumstance of self-defence as that of precluding the wrongfulness of non-performance of certain obligations other than the general prohibition insofar as such non-performance is connected with the exercise of the right under Article 51 Charter. This contribution first scrutinises this paradigmatic shift and finds it to be symptomatic of the conviction on the part of Crawford that the notion of self-defence could also encompass the use of force against speculative threats of state origin as well as actual threats that emanate from individuals or groups which are disconnected from the organisation of any state. It then uses this finding as a springboard towards the examination of the controversy surrounding the notion of self-defence under international law. In that connection, it first outlines the findings of the World Court on the requirements of armed attack and attribution and describes the points of criticism which those findings have elicited. Next, it argues that state practice is unable to settle the controversy concerning the scope of the concept of self-defence and that the question whether or not valid invocation of Article 51 Charter must be preceded by a state’s breach of the general prohibition can be conclusively answered only as part of efforts geared to devising an explanatory account of self-defence which accurately translates its normative origins and which preserves its logical relations with the other, more ordinary but fundamental legal concepts. The study’s conclusion is that respect for such criteria demands that the notion of self-defence under international law be defined in line with Ago’s proposition, i.e. in terms of a de facto situation, recognised by a secondary rule, whereby the principal obligation under the primary rule contained in Article 2(4) Charter is pushed into abeyance to the detriment of a state which conducts itself in breach of that very same obligation in respect of another state; or, correlatively, in terms of a factual circumstance precluding the wrongfulness of the use of force which constitutes a riposte to an unlawful armed attack of another state.