Proportionality under Jus ad Bellum and Jus in Bello. Clarifying their Relationship

Proportionality is a condition provided under both jus ad bellum and jus in bello. Based on a particular interpretation of state practice and international case law, recent legal literature argues that the two notions of proportionality are interrelated in that proportionality under jus in bello is included in the assessment of proportionality under jus ad bellum. This article seeks to refute such a position and, more generally, to clarify the relationship between the two notions of proportionality. The main argument of the article is in line with the traditional position regarding the relationship between jus ad bellum and jus in bello. It is argued that, although sharing common features and being somewhat interconnected, the notions of proportionality provided under these two separate branches of international law remain independent of each other, mainly because of what is referred to in this article as the ‘general versus particular’ dichotomy, which characterises their relations. Proportionality under jus ad bellum is to be measured against the military operation as a whole, whereas proportionality under jus in bello is to be assessed against individual military attacks launched in the framework of this operation. This article nonetheless emphasises the risk of overlap between the assessments of the two notions of proportionality when the use of force involves only one or a few military operations. Indeed, in such situations, the ‘general versus particular’ dichotomy, which normally enables one to make a distinct assessment between the two notions of proportionality, is no longer applicable since it becomes impossible to distinguish between the military operation as a whole and the individual military attacks undertaken during this operation.