Logo: to the web site of the Swedish Defence University

It has long been contended that jus in bello has to be kept strictly separated from jus ad bellum, despite both being concurrently applicable to regulate situations of armed conflict. Very persuasive arguments, pertaining to the humanitarian nature of jus in bello and to the logical reciprocity of the application of jus in bello, have been developed to support this claim. However, is it not in the very nature of law to demand more than moral and logical arguments to govern the relationship between two distinct branches of international law?

The principle of separation inherently calls for a legal basis, a foundation that entrenches it in international law. Indeed, fundamental rules and principles of international law call into question the absoluteness of the separation. However, no argument in contemporary doctrine proved sufficient to justify, legally, the principle of separation. Maybe the answer is to find elsewhere, in a new understanding of the relationship between jus ad bellum and jus in bello. In a new understanding that does not presuppose an absolute separation between both bodies of law but accepts that they interact and mutually influence each other.

In this research, I propose to consider the relationship between jus ad bellum and jus in bello as being explicitly governed by a rule of jus in bello. Specifically, I contend that sufficient elements, including state practice and opinio juris, support the existence of a customary rule of jus in bello that prevents the illegal use of force under jus ad bellum from affecting the duty of any belligerents to equally respect their obligations under jus in bello. While a more exhaustive analysis is needed, there is certainly a case to be made that such a rule exists.