The Introduction outlines the nature of the study and the rationale for undertaking it. It argues that collective self-defence remains under-theorised (and collective self-defence practice under-analysed) despite the increased invocation of it by states in recent years. The Introduction also discusses the book’s methodology, particularly in relation to its focus on state practice and opinio juris as crucial ‘raw materials’ for legal standards in this context. The key facts and findings of the famous 1986 Nicaragua decision of the International Court of Justice are then summarised, because the case is referred to throughout the book. The Introduction concludes with a summary of the structure of the book.

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 how collective self-defence request needs to be issued. There are a number of unanswered questions about the necessary manner and form of such requests. First, the chapter analyses whether ‘open-ended’ requests will suffice, or whether they must be targeted at a particular state (or states). It then considers whether collective self-defence requests must take any specific form and, in particular, queries whether they can be inferred. Similarly, it examines whether a collective self-defence request must even be made publicly (or, at least, be publicised), or whether covert requests can suffice. Finally, the chapter engages with two questions concerning the timing of the request. These are whether the request must be made before the collective self-defence action begins, and whether the existence of a collective self-defence treaty arrangement between the relevant states suffices as a ‘request’ (or whether an ad hoc and specific request is still required in addition).

In the 1986 Nicaragua case, the International Court of Justice famously took the view that two additional criteria exist in customary international law for the exercise of collective self-defence, alongside the criteria that it shares with individual self-defence. These purported additional criteria have been commonly repeated in scholarship since. First, it is said that the state that has been attacked must ‘declare’ that it has been so attacked. Second, it must ‘request’ aid in its defence. This chapter sets out the manner in which the Court identified these requirements and whether it considered them to be legally determinative or merely evidentiary. It then goes on to examine state practice/opinio juris, to test whether the requirements indeed can be identified as rules of customary international law. It is argued that the first of those asserted requirements and declaration, in fact, has no legal basis. In contrast, it is argued that the issuance of a request is, as the Court indicated, a binding requirement for 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 considers the criteria for collective self-defence that are shared with individual self-defence. It is uncontentious to say that the same criteria that apply to individual self-defence – armed attack, necessity, proportionality, the reporting requirement, and the ‘until clause’ – also apply to collective self-defence. Indeed, this is an inevitable consequence of the way the concepts appear in Article 51 of the UN Charter. The nature and application of these criteria in the context of individual self-defence have been examined at great length in the existing literature. This chapter therefore does not provide in-depth analysis of all of their aspects. That said, it does provide a brief overview of these requirements to ensure that this book presents a comprehensive picture of the operation of collective self-defence today. The chapter’s main focus, though, is to examine how the operation of these criteria works specifically in the context of collective self-defence actions, which is something that has been largely overlooked in scholarship.

This chapter examines collective self-defence treaty arrangements. It engages with a diverse range of examples of the collective self-defence treaties (or treaties that contain collective self-defence aspects) that have emerged since 1945 to draw out common themes as to the nature, process, and role of such arrangements, as well as to establish notable variations. The aim is to contribute an overall picture of collective self-defence today specifically in the context of treaty relationships. The chapter argues that such relationships inevitably impose only weak obligations on their parties to defend each other and also can cause notable issues related to overlapping memberships, bureaucracy, and antagonism amongst members (amongst other difficulties). Equally, these arrangements – of which there are now hundreds – are concluded for good reason(s). They provide a range of benefits, especially in terms of their deterrent effect.

This chapter examines the relationship between collective self-defence and another legal basis for the use of force, which in scholarship is referred to as ‘military assistance on request’ or ‘intervention by invitation’. Analysing the relationship between collective self-defence and military assistance on request is crucial because these concepts are, in some respects, strikingly similar. Indeed, it has been argued that they overlap, and states often blur them in their argumentation. The chapter explores the extent to which the concepts can be differentiated at the ‘doctrinal’ or ‘conceptual’ level. It then turns to the various legal requirements (actual or, in some cases, arguable) for collective self-defence and military assistance on request, with the aim of highlighting similarities or differences, as relevant, when it comes to the operation of these two concepts.