This chapter seeks to delineate the notion of collective self-defence in international law. While the core concept can be stated relatively easily, there has been persistent controversy regarding the nature of collective self-defence. It is possible to identify no fewer than five different ‘conceptions’ of collective self-defence that have been advanced in scholarship. These conceptions are all explored in detail. The chapter also examines the question of whether collective self-defence is indeed an ‘inherent right’, as Article 51 of the United Nations Charter proclaims. The status of collective self-defence as a right (and, moreover, as a right that is inherent) has been contested. As such, its status requires theorisation based on the analysis of the views of states. Finally, the chapter considers the modality of collective self-defence: in other words, it asks what ‘qualifies’ as an act of collective self-defence. In examining this question, there is particular focus on whether the provision of weapons and logistical support in support of an attacked state amounts to the exercise of collective self-defence.

This chapter examines the history and development of collective self-defence. It is argued that – contrary to the common assertion that the concept was created in 1945 – its roots can be seen throughout history. The chapter maps that history, starting briefly with the alliances of ancient Greece and moving through to the writings of the seventeenth century, when recognisable characteristics of the modern concept truly began to emerge. It then focuses on the developments in the interwar years and during the Second World War, which saw an increase in the number of collective defence treaties. This period concluded with the emergence of a collective defence system in the Americas, which was extremely influential for the drafting of Article 51 of the UN Charter. The chapter concludes by analysing the drafting process, and the changes to collective self-defence that the adoption of the Charter brought about. It is argued that Article 51 ‘conjoined’ individual and collective self-defence in a way that had little basis in the previous historical development of collective defence arrangements under international law. This has had significant implications for how collective self-defence is understood today.

This chapter focuses specifically on the Council’s contribution to the international law on the use of force (the jus ad bellum), an area of international law that is central to the Security Council’s role in the maintenance of international peace and security and the collective security system of the United Nations. The chapter addresses, first, the general state of the rules of international law on the use of force (the jus ad bellum). It then outlines the rules themselves. This is followed by sections relating directly to the Security Council: the prohibition of the use of force; the use of force by or authorized by the Council; the Council and the right of self-defence; and the Council and ‘humanitarian intervention’ and ‘responsibility to protect’.