In Renaissance Europe, war and the use of force were regular phenomena and likewise subject of common rules. The theory of ‘just war’, dating back to ancient times, was further developed by legal scholars, and all belligerents claimed to have a just cause, often explained in printed pamphlets. International law consisted of theory and practice, and thus, they should be considered in a mutual context. The focus was still on the question of who had a right to wage war, i.e. on the jus ad bellum, and barely on regulations of warfare or on a containment of war. International law in Renaissance Europe rooted in its very society, in its rules and values. Legal debates and war justifications consolidated the Christian European community, which even in war times did not break apart, even if it was contradictory to the principle of sovereignty and the idea that a sovereign owes no justification to anyone. Moreover, scholars and belligerents argued with natural law and insisted in the universality of international law, although it was in fact basically European. Thus, in Renaissance Europe well-established traditions existed for how to handle war, but they were more and more challenged by the idea of sovereignty, as well as by the European expansion and by global interaction.

This chapter explores the justification and legitimisation of war and ‘imperfect’ uses of force both in legal scholarship and diplomatic practice. The Grotian synthesis of ‘just’ and ‘formal’ war entered mainstream scholarship and reached its full explanatory force in the work of Vattel. The resilience of just war in the face of its impracticality among sovereign state in scholarship can, among other, be explained by the fact that the subtle interplay of two conceptions of law in fields of legal application - , gelled well with diplomatic practice. Whereas states applied the jus in bello and jus post bellum with regards to claims to the justice of the war, the just war doctrine remained a common discourse for the justification of resort to war and force and added an instrument to the toolbox of alliances diplomacy.

This chapter examines the contours of theories and debates about the nature and function of the right to resist as a legal concept. Firstly, it identifies four main approaches to conceptualizing the nature of the right: as moral, legal, both, or other. It then considers three main conceptions of the concept’s relationship to the rule of law. Concerning its other key characteristics, the chapter considers possibilities including that it is: a fundamental ‘human right’ in the political rights cluster; an ‘unenumerated’, ‘implied’, or ‘latent’ right; an enforceable ‘claim’ right; a ‘right’ or ‘duty’ or hybrid ‘right-duty’; a primary or secondary right or both. Secondly, the chapter identifies possibilities for the legal function of the right, including as: a self-help remedy for enforcement or prevention; an exceptional immunity, justification, or temporary permission by license; a form of jus ad bellum; or a lawful exception and lex specialis rule. It concludes non-exclusively that the nature of the contemporary right to resist is a potentially enforceable human right, functioning as a lex specialis rule of exception.

This conclusion sets out a definitional framework for a prohibited ‘use of force’ under article 2(4) of the UN Charter according to the type theory developed in this monograph. It sets out the contextual elements of that provision and the elements of a ‘use of force’ that are identified and discussed in greater detail in earlier parts of the monograph. Finally, it offers some reflections on the legal nature of this framework and its potential as a tool for scholars and practitioners to assess whether forcible incidents meet the threshold of a prohibited ‘use of force’ between States under international law.

This introduction highlights the prevailing uncertainty regarding the meaning of a ‘use of force’ under article 2(4) of the UN Charter and customary international law. It sets out the key research questions that this monograph addresses regarding the meaning of ‘use of force’ under jus ad bellum, including if ‘force’ means physical/armed force only and whether kinetic means or the use of particular weapons required, if a (potential) physical effect is required and the required nature of such effects, if there is a de minimis gravity threshold, and if a coercive or hostile intent is required. It also explains why the definition of prohibited force matters and its consequences under international law, including with respect to the gap between ‘use of force’ under article 2(4) and ‘armed attack’ under article 51 as well as the rise of grey zone operations. Finally, it sets out the aims and contributions of this monograph and an outline of its structure.

This chapter examines the origin of the customary prohibition of the use of force between States and its relationship to article 2(4) of the UN Charter, focusing on the pre-UN Charter era. In doing so, it critically analyses two possibilities for the norm to have emerged prior to 1945: that it developed prior to the UN Charter and article 2(4) was declaratory of that pre-existing custom, or that article 2(4) crystallised a customary rule that was already in the process of formation. It rejects these two possibilities, arguing that article 2(4) was a significant legal development which went beyond the existing laws of the time in order to found a new international legal order in the aftermath of World War II. Any pre-existing customary limitations on the use of force were significantly broadened by article 2(4), and the drafting process was not accompanied by meaningful State practice developing in parallel with this radical change in the law. Therefore, the customary rule prohibiting recourse to force between States must have arisen after the Charter entered into force. The emergence and development of the customary rule from 1945 onwards are examined in the next chapter.

In appreciating the institutional perpetuity of war, while simultaneously acknowledging the historically informed, inherent limitations of attempts to bound its conduct by international law, this chapter introduces the three interrelated questions that serve as the organising themes of this volume: first, is there a historical continuity with legal protections in war being informed by notions of ‘civility’ and ‘barbarity’?; second, what is the relationship between the ideals and operational realities in international humanitarian law (IHL)?; and third, what are the limitations of international laws designed to restrain excess in war? Via a brief overview of the divergent evolutions of jus ad bellum and jus in bello law, this introductory chapter further explores the sub-themes present in this volume: universalism and its shortcomings; problems with punishing violations of IHL; and the degree to which modern laws of war legitimate activities that should otherwise be prohibited.

It is clear that, under customary international law, the state that has suffered an armed attack must request aid before other states can provide it with that aid in the exercise of collective self-defence. There are a range of factors that need to be considered that do (or, at least, may) have a bearing on the ‘validity’ of that request. This chapter analyses perhaps the most controversial of them: the question of who can issue a collective self-defence request. In so doing, it examines the view that only states can request aid in collective self-defence and, indeed, further asks whether the issuer of the request must be a UN member. The bulk of the chapter then examines how one identifies the de jure government of the state for the specific purpose of issuing a collective self-defence request. A traditional reference point for the recognition of governments in international law generally has been the effective control of territory. However, this is of minimal – if any – importance to the identification of the entity that can request aid in collective self-defence. Instead, other factors, such as democratic and constitutional legitimacy, are of greater importance.

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).