Introduction The legal regime governing recourse to force in international law, or what is called the ius ad bellum, needs little introduction. Its basic tenets, as established at the end of the Second World War, are well known. Above all, it comprises a general prohibition on the threat or use of armed force by states, as per Article 2(4) of the UN Charter; this general prohibition admits of one exception - the use of force by states in individual or collective self-defence (Article 51 of the UN Charter) - and it does not apply to collective enforcement measures authorised by the UN Security Council (Article 42 of the UN Charter). The precise scope of these rules is highly contested, and has been so since their inception. Notwithstanding this uncertainty, the legal regime regulating recourse to force is generally considered to be of crucial importance for international relations: The relevant UN Charter rules are referred to as the ‘cornerstone’ of the contemporary international legal order, and are considered to form part of the various categories of ‘higher law’ developed over the last decades – from ius cogens to erga omnes. Perhaps as a consequence, compliance with the rules governing the use of force has always been seen – for better or worse – as a test case for the success or failure of international law more generally: Once the prohibition is allegedly violated, inevitably the soul-searching begins, and scholars engage in speculation about the death (or properly legal character) of international law.