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The Evolution and Identification of the Customary International Law of Armed Conflict

Updated: May 12, 2020

Author: Sir Michael Wood


Despite the many widely ratified treaties on the law of armed conflict (LOAC, also referred to as international humanitarian law (IHL)), customary international law remains of great importance in this branch of international law. So far as concerns international armed conflicts, customary international humanitarian law (CIHL) is of special importance in connection with states not party to Additional Protocol I of 1977.1 So far as concerns non-international armed conflicts, CIHL is of crucial importance for all states, since, for the most part, treaty provisions are rudimentary. The International Court of Justice has also had occasion to state that “a great many rules of humanitarian law applicable in armed conflict . . . are to be observed by all states whether or not they have ratified the conventions that contain them, because they constitute intransgressible principles of international customary law.”

The present contribution concerns the evolution and identification of CIHL. It does not deal with the substance of that law, on which there is a good deal of material, including in the impressive study under the auspices of the International Committee of the Red Cross (ICRC).3 It addresses in particular the relevance for CIHL of the UN International Law Commission (ILC)’s recent work on the topic Identification of customary international law.4 While working on the topic, the Commission had LOAC very much in mind and tried to take into account lessons learned in the field of CIHL.

Identification and evolution of the law are obviously not the same thing, but as the ILC noted when it changed the topic’s title fromFormation and evidence of customary international law toIdentification of customary international law, in that context they are in many ways closely related.5 In particular, the requirement—for identification of customary international law—to ascertain “a general practice” that is “accepted as law” reflects the fact that rules of customary international law evolve through a general practice andopinio juris. In other words, the two constituent elements of customary international law are also the twin criteria for its identification. The ILC’s commentary to draft conclusion 1 on the topic, adopted on first reading in 2016, thus notes:

Dealing as they do with the identification of rules of customary international law, the draft conclusions do not address, directly, the processes by which customary international law develops over time. Yet in practice identification cannot always be considered in isolation from formation; the identification of the existence and content of a rule of customary international law may well involve consideration of the processes by which it has developed. The draft conclusions thus inevitably refer in places to the formation of rules; they do not, however, deal systematically with how rules emerge, or how they change or terminate.


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