O'Meara, Christopher; (2018) Necessity and Proportionality and the Right of Self-Defence in International Law. Doctoral thesis (Ph.D), UCL (University College London).

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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.

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