Did the so-called ‘war’ against terrorism that started after 9/11 affect the concept of international security and the principles governing the use of force in international relations? More speciﬁ cally, is the inherent right of self-defence as perceived in international law subject to different interpretations after 9/11? Could anticipatory self-defence be considered ‘less unlawful’ or ‘more acceptable’ or even legitimate in international law if foreseeable terrorist attacks against national and international security are at stake? Does the ﬁ ght against terrorism justify the resort to the use of force in any circumstance where a State evaluates that the use of armed force is the only instrument enabling the very State to defeat terrorism? Are there other means short of the use of force that can be usefully exploited to defeat this scourge?