Customary international humanitarian law

Customary international humanitarian law is a body of unwritten rules of public international law, which govern conduct during armed conflict.

Customary international law, like international treaty law, is recognized as a primary source of public international law. While international treaties are written agreements by which States establish certain rules, customary international law consists of unwritten rules which derive from “general practice accepted as law”. Therefore, for a rule of international custom to be established, two elements are required: “an objective one, the repeated behaviour of States ... and a subjective one, the belief that such behaviour depends on a legal obligation (opinio juris sive necessitatis)”. The objective element is also often referred to as State practice; the subjective element as opinio juris.

International humanitarian law (IHL), also known as the law of war or the law of armed conflict, is the area of public international law which aims, “for humanitarian reasons, to limit the effects of armed conflict. It protects persons who are not or are no longer participating in the hostilities and restricts the means and methods of warfare”.

War is as old as mankind and so, in a sense, is IHL. Behaviour during armed conflict “has always been subject to certain principles and customs”, based on the practices of armies around the world. Since the mid-19th century, however, many such rules have been codified by States in international treaties, like the Hague Regulations and Geneva Conventions. Not withstanding this increasing codification of IHL, customary rules remain relevant in contemporary armed conflicts.

Customary international humanitarian law complements the protection provided by international humanitarian treaty law in situations of armed conflict. International treaty law only binds States which are party to a particular treaty; customary international law, on the other hand, is, in general, binding on all States. And while some international humanitarian law treaties, such as the 1949 Geneva Conventions, are today universally ratified, this is not the case for all treaties. Here, customary international humanitarian law can be used to fill gaps in the protection provided in situations of armed conflict.

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