The Practical Guide to Humanitarian Law

« Calling things by the wrong name adds to the affliction of the world. » Albert Camus.

Customary International Law

Customary international law reflects certain practices that States follow in a repeated and consistent manner and that they accept as law ( opinio juris ). Defined by the International Court of Justice (ICJ) as “evidence of a general practice accepted as law” (Art. 38.1 of ICJ Statute), customary law is one of the oldest sources of international law, alongside the law codified in international conventions. In the international arena, States create legal norms not only by expressly stating their will through international conventions, which is referred to as conventional law, but also through their conduct. Written law represents only a small part of international law. Certain behaviors or practices followed in a repeated and consistent manner by States and accepted and recognized by them as legitimate and beneficial create precedents and gain the legal authority as customary law. Failure to respect such custom is therefore a violation of law. This violation, however, does not remove the customary character of the rule. Written conventional law therefore represents a small part of international law. Customary law addresses situations and cases not specifically provided for in treaty law and deals with problems of contradictory interpretation. Customary law plays a very important role in the law of armed conflict and humanitarian action because it codifies interactions and confrontations between States on the one hand and non-state actors on the other.

Customary Law Is a Law of Action

Customary international law originates from standards of behavior recognized and accepted as legitimate and beneficial. This conduct makes up the “precedents” that can be invoked as proof of such law. Conversely, repeated acts violating the law may result in the progressive erosion of international law if they are not denounced openly.

In the realm of humanitarian action, the conduct of State actors—as well as, increasingly, that of non-state actors—may therefore result in either the strengthening or weakening of international humanitarian law and principles. It is the duty of humanitarian actors to defend humanitarian customs through their actions and to denounce any failure to respect them.

Customary law often precedes written law. It may later be codified—for instance, in the form of a convention or a formal resolution adopted by the General Assembly of the UN or by the International Law Commission. This approach stands in contrast to the tradition of written law followed by most of the judicial systems based on Roman law, and it gives significant weight to the conduct followed by each actor in the international arena. As a law of action, customary law takes into account the notion of a non-state actor in situations of conflict. It partially fills the legal vacuum created by the asymmetry between States, which are actors of conventional international law, and non-state armed groups, which are parties to conflicts but are not signatories of international conventions.

Customary law also plays an important role in filling the voids left by written law, whether because it does not exist or because it cannot be applied—for instance, owing to the complex procedure of signing, ratifying, and possibly issuing reservations to an international convention. In fact, it is the second source of law to which the International Court of Justice refers in framing its decisions (Art. 38.1 of ICJ Statute). ▸ Hierarchy of normsInternational Court of Justice

In international humanitarian law, it is particularly important to avoid cases in which a person could be left without protection or assistance. This can happen if the relevant convention has no provisions for the situation in which the person finds him- or herself or if it is not in force in the country in question. The 1949 Geneva Conventions reiterate the fact that persons and situations not covered by the Conventions nonetheless remain covered by international customary law. This principle is found in all four Conventions and their first Additional Protocol. Commonly known as the “Martens clause,” it states that “in cases not covered by the [Geneva Conventions or Additional Protocol I] or by other international agreements, civilians and combatants remain under the protection and authority of the principles of international law derived from established custom, from the principles of humanity and from the dictates of public conscience” (API Art. 1.2, GCI Art. 63, GCII Art. 62, GCIII Art. 142, and GCIV Art. 158).

International customary law is as binding on States as the international conventions to which they are parties (as evidenced by Art. 38 of the International Court of Justice). The fact that a State has not signed an international convention has no bearing on its obligations under customary law.

Today, the four 1949 Geneva Conventions—as well as most provisions of the 1977 Additional Protocols—have gained the status of customary international law. This means that even States that have not ratified them must abide by their rules.

Content of Customary Humanitarian Law

The International Law Commission recognized in 1980 that the Geneva Conventions reflected the general principles that are the basis of humanitarian law. The Secretary-General of the UN reiterated this in his report on the establishment of the International Criminal Tribunal for the Former Yugoslavia (S/25704 of 3 May 1993). The Security Council approved this report in its Resolution 827 (5 May 1993). In 2005, the International Committee of the Red Cross (ICRC) published a study on the rules of customary international humanitarian law. This study spells out a comprehensive list of 161 rules of customary humanitarian law. This work has been done through an extensive reviewing process of State practices in this field. It was permitted to identify the rules accepted as binding by States in both international and non-international armed conflicts. This work shows a large convergence between rules applicable in international and non-international armed conflicts. It simplifies and facilitates the implementation of IHL and creates greater legal security as to the applicability of humanitarian law in a given context, since the authority and value of those 161 rules stand above signature or ratification process by States.

The complete list of the customary IHL study is presented in the entry ▸ International humanitarian law .

Fundamental guaranteesHierarchy of normsInternational Court of JusticeInternational humanitarian lawInternational lawNatural law, religious law and positive law


In its judgment in the Nicaragua v. United States of America case, the International Court of Justice noted that the violations of a rule of customary law do not remove the existence of its customary character. “The Court does not consider that, for a rule to be established as customary, the corresponding practice must be in absolutely rigorous conformity with the rule. In order to deduce the existence of customary rules, the Court deems it sufficient that the conduct of States should, in general, be consistent with such rules, and that instances of State conduct inconsistent with a given rule should generally have been treated as breaches of that rule, not as indications of the recognition of a new rule. If a State acts in a way prima facie incompatible with a recognized rule, but defends its conduct by appealing to exceptions or justifications contained within the rule itself, then whether or not the State’s conduct is in fact justifiable on that basis, the significance of that attitude is to confirm rather than to weaken the rule” (para. 186; Military and Paramilitary Activities in and against Nicaragua ( Nicaragua v. United States of America ), Merits, Judgment, ICJ Reports, 1986, p. 14).

The International Criminal Tribunals for the Former Yugoslavia and Rwanda have repeatedly affirmed their role in highlighting international customary rules relating to violations of humanitarian law.

In the Kayishema and Ruzindana Case (21 May 1999), the Trial Chamber I of the ICTR (para. 88) considered that the crime of genocide is considered part of international customary law and, moreover, a norm of jus cogens (peremptory norm). In the Rutaganda Case , the same Trial Chamber considered that the Genocide Convention is part of customary international law (para. 46). The jurisprudence of International Criminal Tribunals further explains how the Convention should be interpreted. ▸ Genocide

Regarding non-international armed conflicts, the Trial Chamber I of the ICTR, in the Akayesu Case (2 September 1998), considered that “the norms of Common Article 3 have acquired the status of customary law in that most States, by their domestic penal codes, have criminalized acts which if committed during internal armed conflict, would constitute violations of Common Article 3” (paras. 608–609, 616).

The Appeals Chamber of the ICTY ( Tadic Case , 2 October 1995) considered that Additional Protocol II is not universally recognized as part of customary law. But it considers that some provisions of this Protocol can now be regarded as declaratory of existing rules or as having crystallized emerging rules of customary law. However, the basic core of Additional Protocol II is reflected in Common Article 3 of the 1949 Geneva Conventions and therefore is a part of generally accepted customary law. This specifically includes its prohibitions on violence toward persons taking no active part in hostilities, hostage taking, degrading treatment, and punishment without due process (para. 117).

For Additional Information: Bruderlein, Claude. “Custom in International Humanitarian Law.” International Review of the Red Cross 285 (November–December 1991): 579–95.

Henckaerts, Jean-Marie. “Study on Customary International Humanitarian Law: A Contribution to the Understanding and Respect for the Rule of Law in Armed Conflict.” International Review of the Red Cross 857 (March 2005): 175–212.

Henckaerts, Jean-Marie, and Louise Doswald-Beck, eds. Customary International Law . 3 Vols. Cambridge: Cambridge University Press, 2005.

Koskenniemi, Martti, ed. Sources of International Law . Aldershot, UK: Ashgate, 2000.

Mendelson, M. H. “The Formation of Customary International Law.” Académie de Droit International , Recueil des Cours 272 (1998): 155–410.

Meron, Theodor. “The Continuing Role of Custom in the Formation of International Humanitarian Law.” American Journal of International Law 90 (1996): 238–49.

Schachter, Oscar. “New Custom: Power, ‘Opinio Juris,’ and Contrary Practice.” In Theory of International Law at the Threshold of the Twenty-first Century , edited by Jerzy Makarczyk, 531–40. Boston: Kluwer Law International, 1996.

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