Introduction

Chapter 1 introduces the topic and explains the book’s context, purpose and importance. The chapter elaborates on how the International Court of Justice, scholars and states have approached necessity and proportionality and situates the book in relation to the current understanding of each requirement. The chapter proceeds to examine the provenance of necessity and proportionality (including consideration of the much-celebrated Caroline incident), as well as explaining the approach that this author takes to explore their contemporary meaning. Chapter 1 concludes by offering some initial observations on the nature and function of necessity and proportionality and the purposes of the right of self-defence that frame and underpin the subsequent analysis.

Necessity

Chapter 2 focuses on the meaning and content of necessity. It argues for the first time that there are two different ‘types’ of necessity. In so doing, this author proposes a novel taxonomy to distinguish between them. This distinction addresses, and better explains, the two principal and distinct concerns of necessity that are reflected in state practice, International Court of Justice jurisprudence and scholarship: (a) is military force required at all in the circumstances (an issue of ‘general necessity’), and (b) if so, where must such force be directed (an issue of ‘specific necessity’)? Without general necessity, the applicability of both specific necessity and proportionality is moot. Drawing on core principles of international humanitarian law, Chapter 2 provides a clearer and more workable understanding of necessity that highlights both the weaknesses in, and opportunities for, its operation.

Proportionality

Chapter 3 examines proportionality. As with Chapter 2, it employs primary and secondary sources of international law to provide a more coherent and granular realization of this requirement and how it operates in practice. In particular, it offers a nuanced analysis of the various factors and interests that inform the review of ‘how much’ force states may use in their defence and whether other states, international organizations, courts and scholars might view such action as excessive. Chapter 3 draws upon the laws of state responsibility to consider how a diversity of interests are affected by the exercise of self-defence. Applicable rules of international humanitarian law are also examined, including how they interact with the jus ad bellum on a regime level and might affect a review of jus ad bellum proportionality in specific instances.

Conclusions

Chapter 5 completes this book by setting out its conclusions. It provides a summary of the theory and practice relating to necessity and proportionality and considers the strengths and weakness of the current law. This author suggests that the analytical framework set out in this book offers the opportunity for a doctrinal switch that recognizes necessity and proportionality, and not the armed attack, as the core regulating requirements of the exercise of self-defence. The chapter rounds off the exploration of this subject by considering improvements to the reporting and monitoring of claims of self-defence and possible options for future legal development.

Necessity and Proportionality and Armed Attacks by Non-State Actors

Chapter 4 builds on the analysis of the previous chapters by considering how necessity and proportionality are adapted to apply to defensive action against NSAs. With a particular focus on international terrorism, including the ongoing Global Coalition intervention in Syria against Daesh and other terrorist groups, the potential and limitations of necessity and proportionality are brought to the fore. In particular, this chapter examines the position of the ‘host state’ (being the state in whose territory military action is taken), which includes a review of the controversial ‘unwilling or unable’ doctrine. The duration and geographical scope of the right of self-defence are also considered in this context. A worrying trend is identified regarding how states appear to take a more permissive attitude towards anti-terrorist operations. This state practice has serious implications for the meaningful application of proportionality, whilst highlighting the potential for specific necessity to act as a restraint on state action.