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. 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."
Sir Michael Wood,
The Evolution and Identification of the Customary International Law of Armed Conflict,
51 Vanderbilt Law Review
Available at: https://scholarship.law.vanderbilt.edu/vjtl/vol51/iss3/6