On the Continuous and Concurrent Application of Ad Bellum and In Bello Proportionality

Abstract The principle of proportionality in international law operates both in the law on the resort to force—or jus ad bellum—and the law that governs how wars are fought, or jus in bello. On both levels, it seeks to constrain force in relation to a certain lawful objective. Yet, beyond this understanding, few other aspects concerning the interaction between ad bellum and in bello proportionality are clear. This chapter addresses two distinct yet interrelated aspects of this interaction. The first concerns the question whether ad bellum proportionality applies throughout an armed conflict, alongside proportionality under jus in bello. The second addresses the manner in which both levels of proportionality interact, assuming that they indeed apply concurrently. Concerning the first question, this chapter revisits the debate between the “static approach,” which argues that at least in some cases, ad bellum proportionality ceases to apply after the initial judgment on the resort to force, and the “continuous approach,” which holds that ad bellum proportionality applies continuously throughout the conflict. By uncovering and contesting the normative and theoretical assumptions that underlie the static approach, this chapter offers a defense of the continuous approach. Regarding the second question, this chapter explores the specific difficulties of concurrent application, as these arise under different conceptions of ad bellum proportionality. It concludes that although both levels of proportionality apply concurrently, and albeit they share some moral and conceptual similarities, we should not conflate between them. Rather, owing to the difficulties this chapter discusses, a functional separation between the spheres of proportionality should be maintained.