Abstract
The jus ad bellum is one of the most important areas of international law, with the prohibition on the use of force forming the cornerstone of the UN Charter. However, a lack of doctrinal clarity, especially regarding the jus ad bellum proportionality principle, undermines its ability to meaningfully regulate the use of force by States in self-defense. There is further lack of understanding as to how the jus ad bellum interacts with the jus in bello, in particular as between the two proportionality principles found in both bodies of law. The orthodox view rejects any such interaction, for fear of compromising the equal application principle of jus in bello. This article takes on both these challenges. It first engages in a deep-dive of jus ad bellum proportionality and the various competing models of application canvassed in existing literature, and proposes a two-stage hybrid model with differing considerations applying during the planning and execution phases of operations. It is hoped that this unique intervention provides States a more principled mechanism by which to understand and apply their proportionality obligations. The article then grapples with the equal application principle, showing that while it does doctrinally preclude the jus ad bellum from influencing jus in bello distinction and proportionality, the converse does not apply. As the paper explains, challenging orthodoxy, it is entirely appropriate for jus in bello violations, in particular those relating to causing significant civilian harm, to impact assessments of jus ad bellum proportionality, without imperilling the equal application principle.
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