The Practical Guide to Humanitarian Law

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


One of the foundations of life in society is the principle that individuals must not use force to carry out justice themselves. The domestic laws of most States legislate that the only exception to this rule is the case of individual self-defense. This rule allows an individual to use force in response to an aggression that threatens his or her life or person. But this exception must be interpreted in a very stringent way and does not include the use of force in response to aggression against material goods. Besides, the threat must be real and the answer must be proportional to the threat.

The United Nations Charter prohibits the use of force in relations between States. Nonetheless, Article 51 of the UN Charter recognized the “inherent right of individual or collective self-defense if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security.” Individual self-defense complements the system of collective security set up by the Charter of the United Nations in 1945, according to which the Security Council may decide to use international armed force where peaceful means for settling a dispute between States have failed and where international peace and security is at stake.

In modern times, self-defense has therefore been the only legitimate reasons for a State to use force. This has led to extensive interpretations of the notions of self-defense and aggression. The United States used concepts including preventative self-defense and preemptive strikes during its “global war on terror.” However, the report of the UN Secretary-General’s High-Level Panel on Threats, Challenges, and Change concluded on 2 December 2004 that it did not favor the rewriting or reinterpretation of Article 51 of the UN Charter. This report admits that preemptive action against real and imminent danger may be necessary, but it excludes the use of preventive force where there is no credible evidence of the reality of the threat in question. In such cases, the authorization of the Security Council remains necessary.

In several judgments, the International Court of Justice (ICJ) has clarified the definitions of aggression and the legal requirements for the recourse to force by States in case of self-defense. Its judgments of 1986 and 2005, respectively Nicaragua v. United States of America and Democratic Republic of Congo v. Uganda , elaborate in depth between self-defense and aggression.

The Court distinguishes aggression from the other forms of threats to the national security of a State, which does not allow the latter to invoke self-defense and resort to force. It recalls the existence of a well-established rule of customary international law, whereby self-defense would only warrant measures that are proportional to the armed attack. It sets the requirements that allow self-defense and aggression to be invoked for acts committed by non-state armed groups operating under the control of a foreign State (infra Jurisprudence).

In 2010, an international definition of aggression was adopted during the Kampala Review Conference of the Rome Statute of the International Criminal Court. This should prevent States from making extensive interpretations of this notion to justify the use of force.


Self-defense is also an important notion in the framework of UN peacekeeping operations. Indeed, peacekeeping forces traditionally do not have the right to use force. With consent and impartiality, non-use of force was the third core principle that established traditional peacekeeping operations in the mid-1950s. This is the way to distinguish between peacekeeping operations and other international military operations authorized under Chapter VII of the UN Charter to use force to impose decisions upon a given State. Nonetheless, peacekeepers may have recourse to force in cases of personal self-defense or under very specific conditions, unless they have a mandate that expressly provides otherwise. This notion has been extended in recent UN operations, which have been more forceful than previous ones, to include the authorization to use force if the mission’s mandate itself is threatened, not just the persons of the peacekeepers. The resolutions allowing such extension refer to “extended” or “functional” self-defense.

In certain cases, members of the military have interpreted their mandate in humanitarian missions to include such forms of self-defense. Hence, in Bosnia, to ensure that the humanitarian convoys could be protected by force, the troops of the UN Protection Force (UNPROFOR) were authorized (under UN Security Council Resolution 776 of 14 September 1992) to use force in self-defense if armed persons attempted to prevent them from carrying out their mandate by force (Report S/24540 of the Secretary-General, 10 September 1992, and UN Security Council Resolution 776, 14 September 1992).

Similarly, on 4 June 1993, the Security Council adopted Resolution 836, which authorized UNPROFOR to use force to protect the civilians located in “safe areas.” It thus extended the definition of the situations in which UNPROFOR could fire to include responses to “bombardments against safe areas by any of the parties or to armed incursion into them, or in the event of any deliberate obstruction in or around those areas to the freedom of movement of UNPROFOR or of protected humanitarian convoys.” Regarding UNAMIR troops present in Rwanda during the genocide, only in May 1994 did the UN Security Council recognize that UNAMIR’s mandate empowered it to “take action in self-defense against persons or groups who threaten protected sites and populations, UN and other humanitarian personnel, or the means of delivery and distribution of humanitarian relief” (Resolution 918 of 17 May 1994).

Most of the current peacekeeping operations, called “complex peace operations,” have mandates which include “extended” self-defense.

This notion of extended self-defense was progressively replaced by the inclusion of standard clauses authorizing the use of force for the protection of civilian populations in the mandates of UN peacekeeping forces. These standard clauses are structured around three restrictive criteria that authorize the use of force without creating obligations on blue helmets nor rights or guarantees of protection for concerned populations. They authorize the use of force to protect civilians under imminent threat of physical violence, provided that these civilians are in the areas where peacekeepers are deployed. As an example, one can mention the UN Security Council Resolution 1925 of May 2010, which set the mandate for the peacekeepers deployed in Democratic Republic of Congo within the UN Stabilization Mission in DRC (MONUSCO). Article 11 of this resolution provides that “the protection of civilians must be given priority in decisions about the use of available capacity and resources and authorizes MONUSCO to use all necessary means, within the limits of its capacity and in the areas where its units are deployed, to carry out its protection mandate,” which is “the effective protection of civilians, including humanitarian personnel and human rights defenders, under imminent threat of physical violence, in particular violence emanating from any of the parties engaged in the conflict” (S/RES/1925 of 28 May 2010).

In practice, however, the rules governing the use of force in such situations are often interpreted restrictively by the field commanders, mainly because of the shortage of military resources at their disposal that are necessary to implement such decisions. When a gap exists between the mandate and the means, experience has shown that officers privilege the security of the peacekeepers over the respect for the mandate. This was illustrated during the trial before the Martial Court of the Belgian Colonel Marshall, in charge of the Belgian contingent of blue helmets in Rwanda in 1994. He was tried for his lack of foresight because he risked the lives of ten peacekeepers from the UN Assistance Mission for Rwanda (UNAMIR), on 6 April 1994, at the beginning of the genocide in Rwanda.

Collective securityInternational Court of JusticeInterventionPeacekeepingPublic orderSecurity Council of the UN


In two major decisions, the International Court of Justice (ICJ) specified the legal requirements of the recourse to armed force by States in case of self-defense; Military and Paramilitary Activities in and against Nicaragua ( Nicaragua v. United States of America ), Merits, Judgment, ICJ Reports 1986, p. 14; and Armed Activities on the Territory of the Congo ( Democratic Republic of the Congo v. Uganda ), Judgment, ICJ. Reports 2005, p. 168.

  • The ICJ established a close link between self-defense and aggression. It contended that in conformity with the UN Charter and customary law, only an armed attack (aggression) authorizes the recourse to armed force in case of individual or collective self-defense ( Nicaragua v. United States of America , paras. 49, 50)*.* The Court observed that “the normal purpose of an invocation of self-defence is to justify conduct which would otherwise be wrongful” ( Nicaragua v. United States of America , para. 74). The Court added that “the lawfulness of the use of force by a State in response to a wrongful act of which it has not itself been the victim is not admitted when this wrongful act is not an armed attack. In the view of the Court, under international law in force today—whether customary international law or that of the United Nations system—States do not have a right of ‘collective’ armed response to acts which do not constitute an ‘armed attack’” ( Nicaragua v. United States of America , para. 211).
  • The ICJ gives a definition of armed attack (aggression) that includes, under certain conditions, the acts committed by a State through non-state armed groups.

The Court sees no reason to deny that, in customary law, the prohibition of armed attacks may apply to the sending by a State of armed bands to the territory of another State, if such an operation, because of its scale and effects, would have been classified as an armed attack rather than as a mere frontier incident had it been carried out by regular armed forces. But the Court does not believe that the concept of “armed attack” includes . . . also assistance to rebels in the form of the provision of weapons or logistical or other support. Such assistance may be regarded as a threat or use of force, or amount to intervention in the internal or external affairs of other States. ( Nicaragua v. United States of America , para. 195)

The Court contended that “while the concept of an armed attack includes the despatch by one State of armed bands into the territory of another State, the supply of arms and other support to such bands cannot be equated with armed attack” ( Nicaragua v. United States of America , para. 247). In the case Democratic Republic of the Congo v. Uganda , the ICJ clarified this notion by affirming that “there is no satisfactory proof of the involvement in these attacks, direct or indirect, of the Government of the DRC. The attacks did not emanate from armed bands or irregulars sent by the DRC or on behalf of the DRC, within the sense of Article 3 (g) of General Assembly resolution 3314 (XXIX) on the definition of aggression, adopted on 14 December 1974. The Court is of the view that, on the evidence before it, even if this series of deplorable attacks could be regarded as cumulative in character, they still remained non-attributable to the DRC” ( Democratic Republic of the Congo v. Uganda, para. 146).

  • The ICJ distinguished aggression from other forms of threat to the national security of a State, which cannot allow it to invoke self-defense and resort to force.

Action taken in self-defence, individual or collective, might be considered as part of the wider category of measures qualified in Article XXI as “necessary to protect” the “essential security interests” of a Party. . . . But the concept of essential security interests certainly extends beyond the concept of an armed attack, and has been subject to very broad interpretations in the past. The Court has therefore to assess whether the risk run by these “essential security interests” is reasonable, and secondly, whether the measures presented as being designed to protect these interests are not merely useful but “necessary.” ( Nicaragua v. United States of America , para. 224)

Further, “Article 51 of the Charter may justify a use of force in self-defence only within the strict confines there laid down. It does not allow the use of force by a State to protect perceived security interests beyond the parameters. Other means are available to a concerned State, including, in particular, recourse to the Security Council” ( Democratic Republic of the Congo v. Uganda , para. 148).

  • Concerning the role of the Security Council in the recognition of the argument of self-defense, the ICJ affirmed that this is not a mandatory condition nor the condition of the lawfulness of the use of force in self-defense, but an element that can help to appreciate the reality of an aggression; “for the purpose of enquiry into the customary law position, the absence of a report [to the Security Council] may be one of the factors indicating whether the State in question was itself convinced that it was acting in self-defence” ( Nicaragua v. United States of America , para. 200). In the case Democratic Republic of the Congo v. Uganda , the ICJ specified that “Uganda did not report to the Security Council events that it had regarded as requiring it to act in self-defence” (para. 145). It underlines the fact that “while Uganda claimed to have acted in self-defence, it did not ever claim that it had been subjected to an armed attack by the armed forces of the DRC” (para. 146). “With regard to the characteristics governing the right of self-defence, . . . reliance is placed by the Parties only on the right of self-defence in the case of an armed attack which has already occurred, and the issue of the lawfulness of a response to the imminent threat of armed attack has not been raised. Accordingly the Court expresses no view on that issue” ( Nicaragua v. United States of America , para. 194).
  • The ICJ recalled the existence of a well-established rule in customary international law, “whereby self-defence would warrant only measures which are proportional to the armed attack and necessary to respond to it” ( Nicaragua v. United States of America , para. 176).

The International Criminal Tribunal for the Former Yugoslavia ruled in the Martic Case that self-defense cannot be used to justify a deliberate attack on civilian populations ( Martic Case , ITCY Appeals Chamber, IT-95-11-1, 8 October 2008, para. 268).

For Additional Information: Doswald-Beck, Louise. “International Humanitarian Law and the Advisory Opinion of the International Court of Justice on the Legality of the Threat or Use of Nuclear Weapons.” International Review of the Red Cross 316 (February 1997): 35–55.

Greenwood, C. “Self-Defence and the Conduct of International Armed Conflict.” In International Law at a Time of Perplexity , edited by Y. Dinstein, 273–88. Dordrecht: Martinus Nijhof, 1988.

Ochoa-Ruiz, Natalie, and Esther Salamanca-Aguado. “Exploring the Limits of International Law Relating to the Use of Force in Self-Defence.” European Journal of International Law 16, no. 3 (2005): 499–524.

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