Rule 14. Proportionality in Attack

State practice establishes this rule as a norm of customary international law applicable in both international and non-international armed conflicts.

The principle of proportionality in attack is codified in Article 51(5)(b) of Additional Protocol I, and repeated in Article 57.[1] At the Diplomatic Conference leading to the adoption of the Additional Protocols, France voted against Article 51 because it deemed that paragraph 5 by its “very complexity would seriously hamper the conduct of defensive military operations against an invader and prejudice the inherent right of legitimate defence”.[2] Upon ratification of Additional Protocol I, however, France did not enter a reservation to this provision. At the Diplomatic Conference leading to the adoption of the Additional Protocols, Mexico stated that Article 51 was so essential that it “cannot be the subject of any reservations whatsoever since these would be inconsistent with the aim and purpose of Protocol I and undermine its basis”.[3] Also at the Diplomatic Conference, several States expressed the view that the principle of proportionality contained a danger for the protection of the civilian population but did not indicate an alternative solution to deal with the issue of incidental damage from attacks on lawful targets.[4] The United Kingdom stated that Article 51(5)(b) was “a useful codification of a concept that was rapidly becoming accepted by all States as an important principle of international law relating to armed conflict”.[5]

The principle of proportionality in attack is also contained in Protocol II and Amended Protocol II to the Convention on Certain Conventional Weapons.[6] In addition, under the Statute of the International Criminal Court, “intentionally launching an attack in the knowledge that such attack will cause incidental loss of life or injury to civilians or damage to civilian objects … which would be clearly excessive in relation to the concrete and direct overall military advantage anticipated” constitutes a war crime in international armed conflicts.[7]

A large number of military manuals lay down the principle of proportionality in attack.[8] Sweden’s IHL Manual, in particular, identifies the principle of proportionality as set out in Article 51(5) of Additional Protocol I as a rule of customary international law.[9] Numerous States have adopted legislation making it an offence to carry out an attack which violates the principle of proportionality.[10] This rule is supported by official statements.[11] This practice includes that of States not, or not at the time, party to Additional Protocol I.[12] When the ICRC appealed to the parties to the conflict in the Middle East in October 1973, i.e., before the adoption of Additional Protocol I, to respect the principle of proportionality in attack, the States concerned (Egypt, Iraq, Israel and Syrian Arab Republic) replied favourably.[13]

In their submissions to the International Court of Justice in the Nuclear Weapons case and Nuclear Weapons (WHO) case, numerous States, including States not, or not at the time, party to Additional Protocol I, invoked the principle of proportionality in their assessments of whether an attack with nuclear weapons would violate international humanitarian law.[14] In its advisory opinion, the Court acknowledged the applicability of the principle of proportionality, stating that “respect for the environment is one of the elements that go to assessing whether an action is in conformity with the principles of necessity and proportionality”.[15]

While Additional Protocol II does not contain an explicit reference to the principle of proportionality in attack, it has been argued that it is inherent in the principle of humanity which was explicitly made applicable to the Protocol in its preamble and that, as a result, the principle of proportionality cannot be ignored in the application of the Protocol.[16] The principle has been included in more recent treaty law applicable in non-international armed conflicts, namely Amended Protocol II to the Convention on Certain Conventional Weapons.[17] In addition, it is included in other instruments pertaining also to non-international armed conflicts.[18]

Military manuals which are applicable in or have been applied in non-international armed conflicts specify the principle of proportionality in attack.[19] Many States have adopted legislation making it an offence to violate the principle of proportionality in attack in any armed conflict.[20] In the Military Junta case in 1985, the National Appeals Court of Argentina considered the principle of proportionality in attack to be part of customary international law.[21] There are also a number of official statements pertaining to armed conflicts in general or to non-international armed conflicts in particular that refer to this rule.[22] The pleadings of States before the International Court of Justice in the Nuclear Weapons case referred to above were couched in general terms applicable in all armed conflicts.

The jurisprudence of the International Criminal Tribunal for the former Yugoslavia and a report of the Inter-American Commission on Human Rights provide further evidence of the customary nature of this rule in non-international armed conflicts.[23]

No official contrary practice was found with respect to either international or non-international armed conflicts. Alleged violations of the principle of proportionality in attack have generally been condemned by States.[24] The United Nations and other international organizations have also condemned such violations, for example, in the context of the conflicts in Chechnya, Kosovo, the Middle East and the former Yugoslavia.[25]

The ICRC has reminded parties to both international and non-international armed conflicts of their duty to respect the principle of proportionality in attack.[26]

Several States have stated that the expression “military advantage” refers to the advantage anticipated from the military attack considered as a whole and not only from isolated or particular parts of that attack.[27] The relevant provision in the Statute of the International Criminal Court refers to the civilian injuries, loss of life or damage being excessive “in relation to the concrete and direct overall military advantage anticipated” (emphasis added).[28] The ICRC stated at the Rome Conference on the Statute of the International Criminal Court that the addition of the word “overall” to the definition of the crime could not be interpreted as changing existing law.[29] Australia, Canada and New Zealand have stated that the term “military advantage” includes the security of the attacking forces.[30]

Upon ratification of Additional Protocol I, Australia and New Zealand stated that they interpreted the term “concrete and direct military advantage anticipated” as meaning that there is a bona fide expectation that the attack would make a relevant and proportional contribution to the objective of the military attack involved.[31] According to the Commentary on the Additional Protocols, the expression “concrete and direct” military advantage was used in order to indicate that the advantage must be “substantial and relatively close, and that advantages which are hardly perceptible and those which would only appear in the long term should be disregarded”.[32]

Numerous States have pointed out that those responsible for planning, deciding upon or executing attacks necessarily have to reach their decisions on the basis of their assessment of the information from all sources which is available to them at the relevant time.[33] These statements were generally made with reference to Articles 51–58 of Additional Protocol I, without excluding their application to the customary rule.