Chapter 1 provides an exposition of the general breadth and scope of the contemporary prohibition of the threat or use of force. After setting the prohibition in its historical context, it then examines the sources of the prohibition and its status as a ‘peremptory’ or ‘jus cogens’ norm of international law and what this means in terms of the prospects for its adaptation and modification in the future. While much focus has been traditionally placed upon the use of force, by comparison relatively little attention has been given to the threat of force, despite both coming under the contemporary rule against force. As such, the chapter sets out and provides an examination of the concept of a threat of force. While the prohibition is of undoubted importance, the decentralised nature of the international legal system means that accountability and responsibility for its violation are uncertain. The chapter seeks to examine this issue in detail, while also providing attention to the ‘human element’ to the prohibition, as well as assessing the continued relevance and effectiveness of the norm.

Chapter 8 seeks to untangle various issues in addressing when and how a state might be able to resort to the use of force in self-defence against non-state actors. It firstly raises some important general considerations in relation to self-defence against non-state actors, before moving on to examine the situation of self-defence measures which target both the non-state actors and the host state, as was the case with the United States’s response to the terrorist attacks of 11 September 2001. A distinction needs to be made between self-defence taken against both the non-state actor perpetrators of the attack or those posing a threat and the state within which they are located, and those actions that are more limited in only specifically targeting the non-state actors. The chapter finally addresses the particular phenomenon of so-called targeted killing, which engages not only the jus ad bellum, but also the legal framework of international human rights law and, potentially, international humanitarian law.

The purpose of this chapter is to examine the use of force under the auspices of the UN, in particular the Security Council. Chapter 3 first sets out and examines the relevant provisions of the UN Charter and the division of competence between the various organs of the UN – the Security Council, the General Assembly and the Secretariat – to provide an understading as to how force was originally envisaged as being employed under the auspices of the UN. Given the Security Council’s primacy both within the Charter and in practice, the chapter goes on to examine the specific powers of the Council in relation to the use of forcible measures within the Charter and the limitations imposed upon these. Following this it then examines how these powers and responsibilities have been implemented in practice in both the Cold War and post-Cold War era. In particular, it has developed the practice of ‘authorising’ states, coalitions of states and regional organisations to use force to implement its mandate. The chapter finally addresses the relationship between the UN and regional – and, by extension, collective defence – organisations in the context of the use of force.

Chapter 6 provides an overview of the general aspects of the right of self-defence. It first examines the concept of an ‘armed attack’ as found in Article 51, with the aim of shedding some light on the difficult issues and questions raised by this prerequisite for the invocation of the right of self-defence. It then goes on to provide an examination of the twin customary principles of necessity and proportionality, including a specific look at the controversial concept of armed reprisals, before moving on to examining military action for the protection of nationals who are located abroad. The right of self-defence exists in both individual and collective forms, and the chapter takes a specific look at the right of collective self-defence. Finally, it examines the role of the UN Security Council in the invocation and implementation of the right of self-defence, an aspect of the right which is prominent throughout Article 51.

Chapter 4 addresses some of the issues that have arisen in connection with the ‘authorisation method’ that was set out in Chapter 3. It first examines some of the initial ways the Council has attempted to obtain a balance between multilaterialsm and unilateralism in the design and implementation of its resolutions which have authorised ‘all necessary means’. One issue that has not been resolved, and is apparent in many sections of this chapter, is the authority to interpret resolutions of the Security Council. In this respect, the chapter addresses the possibility of ‘reviving’ past authorisations, something that has thus far been restricted to the use of force in Iraq betweem 1991 and 2003. The chapter then examines the issue of determining the breadth and scope of an authorisation and the issues of ‘implied’ authorisation, in particular in attempts to enforce what might be perceived as the ‘collective will’ of the Council, as well as the Council providing what might be seen as retrospective or ex post facto authorisation or approval for military action. Finally, it addresses the issue of the forcible implementation of the ‘Responsibility to Protect’ concept by the Council.

It is a sad and depressing reality that, for any international lawyer interested and working in the field of international law governing the use of military force by and between states (the ‘jus ad bellum’), business tends to be bad during moments of relative peace and stability. Arguably even more depressing, however, is the fact that this is not often the case.

Chapter 5 has as its focus not the institution of peacekeeping per se, but more specifically the use of force within, and as a now an integral part of, United Nations peacekeeping. It begins by attempting to define UN peacekeeping, given its absence from the UN Charter, and goes on to address the legal basis for peacekeeping operations and the basic principles of peacekeeping. The chapter then traces the development of peacekeeping, from the early UN Charter-era operations, which saw the establishment of the basic principles, through to the challenges to these principles, which did not take long to manifest themselves, in particular through the use of forcible measures by peacekeeping forces. Finally, the chapter examines the evolution of the use of force within peacekeeping missions, from simple self-defence to the implementation and enforcement of robust mandates, and assesses not only whether the various forms of forcible peacekeeping can be reconciled with the fundamental principles of peacekeeping, but also whether peacekeeping has now taken on a war-fighting role.

Chapter 10 focuses on the somewhat controversial doctrine of humanitarian intervention. It assesses the issue of whether the doctrine can be reconciled with the UN Charter, before examining state practice in connection with the doctrine, including taking a look at Cold War and post-Cold War practice in assessing the position of the doctrine during the UN era. Given that the forcible entry by the North Atlantic Treaty Organization in the Kosovo crisis in 1999 proved something of a landmark in terms of events that have shaped the path of the modern doctrine, the status of the doctrine in the immediate aftermath of this intervention is given particular attention, as well as the impact that the war in Ukraine has had and whether there is an obligation to intervene in the context of genocides. The chapter then looks at the Syrian civil war and the relevance of the doctrine of humanitarian intervention to that particularly tragic conflict, which continues at the time of writing. A conception of the doctrine that has come to dominate contemporary debates is that of the ‘Responsibility to Protect’. Given its relatively recent rise to prominence, its impact upon the doctrine of humanitarian intervention is assessed.

Chapter 2’s purpose is to provide some focused discussion on the meaning of the ‘force’ prohibited in Article 2(4) and customary international law. In seeking to expose the uncertainties regarding this particular term, various factors or common elements of ‘force’ are distilled. After looking at the prohibition of force in the context of the principle of non-intervention, the chapter moves on to look at the type of force that the prohibition is concerned with and, concluding that it is ‘armed force’, then moves on to attempt to distil the key elements of such force, including whether it is the means used or the effects created which is of importance, and whether force can be used indirectly. The chapter then addresses the ‘gravity’ or severity aspects of a use of force, in particular by distinguishing it from an armed attack or act of aggression, but also by examining whether there is a level of force – or de minimis threshold – below which an action falls out of the remit of the prohibition. Finally, and having distilled the key practical components of a prohibited use of force, the chapter focuses upon the mens rea component.

Chapter 9 will examine the elements of a ‘valid’ consent to the use of force. The chapter sets out the ways in which those with authority to consent to intervention might be identified. While this has traditionally been reserved for governments of states, clarity in this respect is often lacking meaning that issues such as effective control, recognition and the principle of self-determination all have relevance in providing a complete picture of this issue. The chapter then examines the issue of consent to intervention, specifically within the context of a civil war, and examines the various issues to which this gives rise, including that of whether a right of counter-intervention exists should a prior intervention have occurred. The chapter then addresses the form that consent might take while, finally, it takes a look at the express and implied limits of the consent to intervention.

One theme that it is hoped emerges from this book is that the law governing the use of force is not purely epiphenomenal. It is possible to suggest that as states on the whole refrain from resorting to the use of armed force the law clearly has a restraining influence upon their actions. However, such assumptions must give way to the reality that the law actually has a far more subtle influence over the actions of states. There can be no escaping the fact that states, and some more often than others, resort to force in their international affairs. Yet, at the same time, states, in general, often engage with the law in justifying their actions, as well as in responding to the actions of other states, and in attempting to shape the contours of the law through interpretation and practice. While there are occasions when states use force without offering a legal justification for doing so, as was the case with the US strikes against Syria in April 2017, it is more often the case that states engage in justificatory discourse of a legal nature which is indicative that they consider the law to be at least of some relevance, even if violations of its strictures are not supported by clear and reliable enforcement mechanisms. It arguably follows, then, that if they feel compelled to offer at least a justification, they feel similarly compelled to adjust their actions to make them at least plausible within the substantive rules and procedures that exist. Furthermore, the law in this area, if nothing else – as is arguably the case with much of international law – provides a language and framework through which discursive intercourse between states and other actors can occur.

The purpose of Chapter 7 is to examine the various forms of preventative self-defence, that is whether the right of self-defence can be invoked before an armed attack has been launched, or at least before the physical manifestations of one have begun to occur. Given that there have been no attempts at formal reform of Article 51, the question arises as to how, if at all, might preventative self-defence be reconciled with this key requirement of an armed attack. This chapter therefore examines the three main forms of preventative self-defence: interceptive, anticipatory and pre-emptive. All three have been of central interest in the light of contemporary threats and challenges. In this respect, while there has been a greater acceptance of the possibility to invoke self-defence in the face of the threat of an ‘imminent’ attack, there has been, as explored in this chapter, disagreement as to exactly whether this is restricted to being interpreted in its ordinary temporal meaning or whether it is today necessary to take it into other contextual factors.