International law governs almost every facet of international life, from the mundane to the complex. This chapter discusses the role of international law in world politics. It begins with a discussion of the content of international law, explaining the sources of law and how we identify them. It then addresses the question of why the study of international law is foundational to the study of international relations. The chapter offers two answers to this question. First, international law is important in the study of international relations because the nature and extent of its influence are among the core debates in international relations theory. Second, the study of international law is important because law is at the heart of many international disputes today. The chapter concludes by considering two examples of ‘law in action’: the legal decision regarding Chinese activities in the South China Sea and the contemporary debate over membership of the International Criminal Court.

Chapter 7 considers how the framework proposed in Chapter 6 potentially contradicts international lawyers’ professional responsibilities. The chapter first considers different approaches to such questions in different countries and professional contexts. It then examines lawyers’ professional responsibilities when dealing with difficult jus ad bellum cases, reporting interviewees’ views of such responsibilities and of risk management techniques. By identifying and reducing risks of legal challenge to use of force, the framework potentially focuses lawyers only on their role as ‘counsellor’, helping governments do whatever they want, rather than their normative role of ‘conscience’, urging governments to change their behaviour to abide by law. Such a framework might even advise decision-makers to accept clearly unlawful force if there is little risk of an action facing legal challenge. The chapter nevertheless argues lawyers could use the framework developed in Chapter 6 while fulfilling their professional responsibilities, by seeking a form of Rawlsian ‘reflective equilibrium’ with their own personal professional judgement to address legal and factual uncertainty and extra-legal intuitions in hard cases engaging the jus ad bellum.