Necessity and Proportionality and the Right of Self-Defence in International Law Christopher O’Meara Faculty of Laws, University College London Submitted for the degree of PhD 27 July 2018 1 I, Christopher O’Meara, confirm that the work presented in this thesis is my own. Where information has been derived from other sources, I confirm that this has been indicated in the thesis. 2 ABSTRACT When states use force extraterritorially, they invariably claim a right of self- defence. They also accept that its exercise is conditioned by the customary international law requirements of necessity and proportionality. To date, these requirements have received little attention. They are notorious for being normatively indeterminate and operationally complex. As a breach of either requirement transforms lawful acts of self-defence into unlawful uses of force, increased determinacy regarding their scope and substance is crucial to how international law constrains military force. This thesis addresses this fact. It examines the conceptual meaning, content and practical application of necessity and proportionality as they relate to the right of self-defence following the adoption of the UN Charter. It provides a coherent and up-to-date description of the lex lata and an analytical framework to guide its operation and appraisal. It does this by undertaking the first comprehensive review of relevant jurisprudence, academic commentary and state practice from 1945 to date. Although the operation of necessity and proportionality is highly contextual, the result is a more determinate elaboration of international law that bridges theory and practice. This greater normative clarity strengthens the law’s potential to exert a pull towards compliance. Necessity determines whether defensive force may be used to respond to an armed attack, and where it must be directed. Proportionality governs how much total force is permissible. This thesis contends that the two requirements are conceptually distinct and must be applied in the foregoing order to avoid an insufficient ‘catch-all’ description of (il)legality. It also argues that necessity and proportionality must apply on an ongoing basis, throughout the duration of an armed conflict prompted by self-defence. This ensures that the purposes of self-defence are met, and nothing more, and that defensive force is not unduly disruptive to third party interests and international peace and security. 3 IMPACT STATEMENT As an original contribution to existing scholarship, this thesis constitutes a significant development of the academic understanding of international law relating to self-defence. Whilst confirming and updating the existing literature, it also confronts a number of conclusions made by authors in the field. It presents a coherent account of applicable theory, which includes a novel taxonomy that captures more accurately the operation of international law and provides a clearer explanation of the differences between often conflated concepts. It tackles deficiencies and gaps in existing knowledge and advances the subject in a methodologically rigorous fashion. As well as undertaking a critical review of relevant jurisprudence and academic commentary, this thesis provides the first comprehensive review of state practice since 1945 relating to necessity and proportionality. This developed understanding of international law has significant practical utility beyond academia. For example, the analytical framework set out herein has the potential to benefit future jurisprudence. Should a claim of self-defence be bought before an international court or tribunal, it serves as a judicial guide to necessity and proportionality and the use of defensive force more generally. Moreover, this thesis constitutes a tool for states to justify and legitimate their actions and for other states and international organiZations to review more effectively defensive action. It assists, therefore, with improved and more transparent decision-making, whilst operating to limit potential abuse by states that assert an overly broad right of self-defence. This thesis also represents an important academic resource. Normatively, its arguments and conclusions provide a basis for how jus ad bellum scholars should view the right of self-defence and can develop it in the future. It provides a scholastic tool, therefore, for approaching some of the long-standing debates in the jus ad bellum relating to, inter alia, a right of self-defence against non- state actors, responding to imminent armed attacks and whether there is a gravity threshold that triggers the right of states to defend themselves. A more 4 coherent conception of necessity and proportionality allows for a novel way to tackle these controversial subjects and advance the literature on them. This author intends to disseminate this work via workshops, seminars and conferences, as well as journal articles and blog posts. Engaging with policy makers and lawyers in governments and international organiZations, in addition to military lawyers and non-governmental organizations, allows for academia and practice to be bridged in a meaningful and useful way. Such engagement will be on a national and international basis. This thesis clarifies the scope and content of a notoriously indeterminate area of international law and strengthens its potential to regulate how states use force in self-defence. Working with these key stakeholders allows this potential to be realised. 5 ACKNOWLEDGEMENTS Writing this thesis has been a transformative process. It has taken me from being a practising lawyer into the wonderful world of academia. This could not have happened without the support of an array of noteworthy people who deserve my sincere gratitude. First and foremost, I owe so much to Dr Kimberley Trapp, my remarkable primary PhD supervisor. Dr Trapp has inspired me to be a scholar and an educator, and I am indebted to her for developing and refining my thinking, insisting on precision and for being a source of patient wisdom and friendly encouragement. I thank her for going the extra mile and being so generous with her time, for her unfailing good humour, passion for the project and beautifully detailed feedback. I am lucky indeed to have such a supportive guide and mentor. I thank Professor Roger O’Keefe, my secondary PhD supervisor, for his frank and incisive comments, delivered with his unique style and valuable rigor. I am also grateful for the backing of my fellow PhD candidates and colleagues at University College London. I thank the Faculty of Laws for generously funding my research, and all the staff that have helped me along the way. Thanks to Dr Steven Vaughan for his guidance regarding structuring my thesis and emphasising its strengths, as well as his counsel for what comes after, namely finding that precious academic job. My research has also benefitted from my time as a Visiting Researcher at the University of Leiden and at Harvard Law School. Throughout the months spent at these marvellous institutions, I spent many hours sharing my work with students, visitors and faculty members, all of whom have aided its progression. Amongst many others, I am grateful to Dr Erik Koppe, Professor Jack Goldsmith, Professor Ashley Deeks, and Professor Allan Rock. I am immensely appreciative of the support of my family and friends, and for trying their very best to understand a little of ‘what I do’. They have always 6 wanted the very best for me and have been there to help me along my chosen path. Special mention goes to Dino Fontes and Claire Jervis who kindly offered to proofread sections of my thesis. I am thankful for their speedy review and attention to detail. Last, but by no means least, I want to thank my husband, Mike. Mike spurred me on to return to university, and I am lucky to have had him by my side during these years of study. I am grateful for his encouragement and steadfast support (in every sense of that word!), for his patience, love and understanding, and for doing everything humanly possible to help me succeed. I would have not have undertaken this thesis without him, and I can never thank him enough. This thesis is dedicated to Mike. 7 TABLE OF CONTENTS ABSTRACT .................................................................................................... 3 IMPACT STATEMENT ................................................................................... 4 ACKNOWLEDGEMENTS .............................................................................. 6 TABLE OF CONTENTS ................................................................................. 8 LIST OF ABBREVIATIONS .......................................................................... 12 CHAPTER 1: INTRODUCTION .................................................................... 13 1 Introduction ........................................................................................... 13 1.1 Context and provenance ................................................................ 13 1.2 Purpose of this thesis ..................................................................... 18 1.2.1 Jurisprudence of the International Court of Justice 20 1.2.2 Scholarship 24 1.2.3 State Practice 25 1.3 Thesis structure, arguments and initial observations ..................... 27 1.3.1 The nature and function of necessity and proportionality 28 1.3.2 The purposes of the right of self-defence 32 a) Halting, repelling or preventing an armed attack ........................ 32 b) An overriding defensive purpose ...............................................