24 No. 73 JUSTICE an international border, but also “the sending by or on behalf of a State of armed bands, groups, irregulars or mercenaries, which carry out acts of armed force against another State of such gravity as to amount to” (inter alia) an actual armed attack conducted by regular forces, “or its substantial involvement therein.” This description . . . may be taken to reflect customary international law. 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.13 Necessity and Proportionality Article 51 of the UN Charter contains no express limitations on how the right to self-defense must be exercised. However, customary international law and international humanitarian law require self-defense to be exercised subject to two conditions: necessity and proportionality. As the ICJ noted in the Nuclear Weapons Advisory Opinion: The entitlement to resort to self-defence under Article 51 is subject to certain constraints. Some of these constraints are inherent in the very concept of selfdefence. Other requirements are specified in Article 51. The submission of the exercise of the right of self-defence to the conditions of necessity and proportionality is a rule of customary international law . . . a use of force that is proportionate under the law of self-defence, must, in order to be lawful, also meet the requirements of the law applicable in armed conflict which comprise in particular the principles and rules of humanitarian law.14 Malcolm Shaw noted that “necessity” and “proportionality” are not well defined in international law. Instead, determining the boundaries of necessity and proportionality “will depend on the circumstances of the case.”15 Indeed, there appears to be much conflation of the necessity and proportionality principles, leading to further confusion and ambiguity.16 But no such ambiguity exists as applied to Israel’s wholly necessary and lawfully proportionate response to the October 7, 2023 Hamas attacks. Necessity The Chatham House Principles on International Law describe the necessity principle as follows: The criterion of necessity is fundamental to the law of self-defence. Force in selfdefence may be used only when it is necessary to end or avert an attack. Thus, all peaceful means of ending or averting the attack must have been exhausted or be unavailable. As such there should be no practical non-military alternative to the proposed course of action that would be likely to be effective in averting the threat or bringing an end to an attack. Necessity is a threshold, and the criterion of imminence can be seen to be an aspect of it, inasmuch as it requires that there be no time to pursue non-forcible measures with a reasonable chance of averting or stopping the attack. Necessity is also a limit to the use of force in self-defence in that it restricts the response to the elimination of the attack and is thus linked to the criterion of proportionality. The defensive measure must be limited to what is necessary to avert the attack or bring it to an end. In applying the test of necessity, reference may be made to the means available to the State under attack; the kinds of forces 13. Military and Paramilitary Activities in and against Nicaragua (Nicar. v. U.S.), Judgment, 1986 I.C.J. 14 (June 27) 103-04 ¶ 195. 14. Legality of the Threat or Use of Nuclear Weapons, Advisory Opinion, 1996 I.C.J. 226 (July 8) 244-45 ¶ 40-42. 15. Malcolm Shaw, INTERNATIONAL LAW 1002 (9th ed., 2021). 16. J. McMahan, “Necessity and Proportionality in Morality and Law,” in NECESSITY AND PROPORTIONALITY IN INTERNATIONAL PEACE AND SECURITY LAW 5 (Claus Kreß and Robert Lawless eds., 2020).
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