Legal rules are not static, but are capable of evolving over time. This holds all the more true for a multilateral convention such as the UN Charter, which spells out a broad array of open-textured principles intended to regulate the relations between States for an indefinite period of time. The Charter regime indeed constitutes ‘a living, growing, and above all discursive system for applying the rules on a reasoned, principled, case-by-case basis’.

In the present chapter, we intend to shed further light on how this process of change operates in relation to the legal regime on the use of force. It may be noted at the outset that several excellent monographs attempt to identify the substance of the present-day Ius ad Bellum by analysing relevant state practice and opinio iuris, albeit without explaining at much length why or how (changing) custom influences the law on the use of force. Nonetheless, both issues are of crucial importance. The methodological approach one adopts to a large degree determines the outcome of any inquiry into the substantive content of the law on the use of force. A different approach may lead one author to acknowledge the legality of pre-emptive self-defence or humanitarian intervention, while leading another to reject it. Hence, for the sake of intellectual honesty and academic accuracy, the issue of methodology should not lightly be passed over.