IL is ubiquitous in modern life. Its significance for international relations is beyond doubt. Accordingly, IR scholarship has come a long way since dismissing IL as irrelevant; as long a way as legal theory has come since rejecting it as not law. Yet our theoretical grasp of the phenomenon is neither comprehensive nor secure. There is a lacuna where there should be an account of, or at least a debate about, the effectiveness of IL. Specifically the question of how IL relates to interests and non-legal norms remains largely unanswered. How can law make a difference in international relations, where states create legal rules that accord with their interests and normative beliefs, while no central authority enforces those legal rules that do not?

This first chapter does not specifically discuss the regulation of war. Rather it systematises scholarship about IL in IR and in legal theory. I ask three questions that scholars of IR, faced with the phenomenon of IL, have posed: why do states create IL, why do states comply with IL and what is distinctive about the norms of IL? In addition, I discuss a question mostly addressed by legal scholars: what does IL mean? Or rather, to what extent does the meaning of IL arise during its interpretation? The aim is to show that the answers to these four questions provided by the major theories of IR and by legal theory prejudicially pre-judge the notion that IL can make a counterfactual difference for behaviour. I conclude that existing scholarship, in law as well as IR, casts considerable doubt on IL’s ability to be what I call ‘behaviourally relevant’.