The architecture of international counterterrorism law is the subject of Chapter 2. It is a branch of public international law that has emerged from an evolving special legal regime to regulate action to prevent and punish terrorism and to tackle terrorists. International counterterrorism law is a matrix of global terrorism treaties and dedicated United Nations Security Council resolutions that require the incorporation of offences into domestic law combined with the salient rules of international humanitarian law and the law of law enforcement. These rules, which regulate when and against whom force may be used in counterterrorism, are supplemented by (though are sometimes inconsistent with) the content of regional treaties and domestic legislation. The standards and laws are subject to the constraints and oversight of international human rights law. The chapter also describes the content and adherence of regional counterterrorism treaties.

The Outlook chapter presents the achievements and challenges of what we can now call international counterterrorism law. Outside the context of armed conflict, sectoral treaties govern international terrorism involving a range of tactics and targets, notably hostage-taking, bombings, hijackings, and nuclear terrorism as well as attacks on foreign diplomats and, under a treaty approaching universality, the financing of international terrorism. But the distinction with terrorism in a situation of armed conflict in these treaties should have been drawn far more sharply. Under international humanitarian law, the definitions of terrorism in armed conflict are clear.

Chapter 5 commences by retracing how, beginning in Nuremberg, the reasonable person entered the battlefield in the form of the reasonable military commander. Subsequently the chapter explores two challenges that confront the concept of the reasonable person on the battlefield and beyond. The first challenge consists in the fact that it is easier to empathise with people who are close to us. In the theatre of war, this raises the question whether the reasonable person, when acting as the reasonable military commander, can meaningfully balance the interests of civilians on opposite sides. The second challenge relates to the fact that it is more difficult for powerful people such as military commanders (or judges) to take the perspective of others and to empathise with them.

The Conclusions reflect on the law outlined in Chapters 1−10. They recall that IHL is essentially an attempt to balance two fundamentally contradictory drivers – the need to wage war effectively, and the need to protect people and property from the excessive effects of warfare. It concludes that IHL largely succeeds in this endeavour, and that without IHL life for those caught up in armed conflict would be immeasurably worse. It notes, however, that while the fundamental principles of IHL are enduring, States can and should do more to develop new or more comprehensive laws where there is a need, such as the under-developed law of non-international armed conflict and the lack of regulation of certain weapons.

This is a general introduction to the book, explaining that the purpose of the book is to provide a concise but detailed explanation of the core rules of international humanitarian law. The contents of each chapter are summarised. It explains that the book looks at the major areas of IHL, putting them in historical context, so as to better understand how the law has evolved. This book also examines the current challenges for and pressures on the existing law, as IHL rules adopted in the time of cavalry and bayonets must adapt to deal with issues like drones, cyber warfare and autonomous weaponry. It notes that the third edition has been updated to reflect new developments in the law of armed conflict up to May 2023.

Chapter 14 examines the relationship between the law governing the use of force (jus ad bellum) and the law of armed conflict (jus in bello), including the law of neutrality. It also looks at how these branches of the law also relate to international human rights law.

This chapter formulates guidance for applying the IHL proportionality rule by looking at its objectivity; the standard of the reasonable commander and the interpretation of the term ’excessive’.

This chapter contains an analysis of the practical application of the IHL proportionality rule by looking at a number of examples of factual situations.

This chapter contains an overview of the development of the IHL rule on proportionality in treaty law and a discussion of its existence as a customary IHL rule.

In addition to the IHL proportionality rule, IHL contains a number of other rules that fulfil a similar role as a standard of moderation and with a broader scope, including effects on enemy forces.

This chapter provides insight into the practical and legal context in which the proportionality rule must be applied: the targeting process; indiscriminate attacks and precautionary measures.

The aim of this chapter is to assess the different factors which comprise the IHL proportionality rule.

Introduction to the study of proportionality rule in international humanitarian law.

IHL proportionality as a principle of IHL and the manifestations of proportionality in other legal frameworks relating to the use of force.

This chapter pleads for a refocussed application of the IHL proportionality rule, tilting the balance more towards humanity; and analyses whether it needs to be clarified or changed.

The goal of detecting future events has several implications and two of them are explored in this chapter. First, the objective of detecting future events means that whoever poses a threat has to be targeted and if the threat is posed by a number of individuals that increases over time, enmity is extended to those individuals. This is true even if they act in the name of a terrorist group that did not exist when the conflict started. From a legal perspective, this practice is facilitated by the uncertainties related to the temporal delineation of conflicts. Second, the objective of addressing future threats entails to act against individuals who are not presently perpetrating hostile acts. This practice requires that the traditional interpretation of direct participation in hostilities be subjected to a temporal change. Instead of suspending the protection of civilians solely when ‒ and only for such time as ‒ they engage in acts that reach a certain threshold of harm, targeting enemies because of the threat they pose for the future means extending direct participation in hostilities not only to preparatory acts, but also to signs revealing membership to an enemy group. This shift is facilitated by the insufficiently defined notion of “continuous combat function.”