25 Winter 2025 and the level of armament to hand will be relevant to the nature and intensity of response that it would be reasonable to expect, as well as the realistic possibilities of resorting to non-military means in the circumstances.17 There can be no doubt that Israel’s resort to self-defense was a necessary response to the October 7, 2023 Hamas attacks. Corn has described necessity as arising “only in response to a threat to a state’s security that is considered sufficient within the meaning of international law to justify an armed self-help response: an actual or imminent armed attack which leaves the target state with no other means of protection than the use of defensive force.”18 The October 7, 2023 Hamas attack against Israel easily satisfies this test. Proportionality International law recognizes two different usages of the term proportionality. The first usage embodies the rule, “well established in customary international law,” that self-defense must be “proportional to the armed attack.”19 The second usage, codified in the Fourth Geneva Conventions, reflects the jus in bello prohibition against inflicting civilian casualties that are excessive in relation to the expected military advantage of an attack.20 Origins of Proportionality Law One of the primary purposes of the proportionality limitation on a state’s Article 51 self-defense rights is to protect civilians. The modern doctrine of proportionality stems from Article 23 of the Annex to the 1907 Hague Conventions, forbidding belligerents from seizing or destroying enemy property unless “imperatively demanded by the necessities of war.”21 Shlomo Brody noted that early military ethicists defined proportionality as requiring military necessity. The ethicists also required that any destruction be effective and not wanton, and that any military gains not be “grossly disproportionate” to the extent of any destruction. Quoting M. W. Royse, Brody explained that if the above criteria were met, then “the act can hardly be condemned regardless of the amount of suffering and violence.”22 The proportionality doctrine saw further development in Additional Protocol I to the 1949 Fourth Geneva Conventions.23 Article 51(2) of the Protocol sets forth the international humanitarian law principle that “[t]he civilian population as such, as well as individual civilians, shall not be the object of attack. Acts or threats of violence the primary purpose of which is to spread terror among the civilian population are prohibited.” Article 51(5)(b) of Additional Protocol I prohibits indiscriminate attacks, including attacks “which may be expected to cause incidental loss of civilian life, injury to civilians, damage to civilian objects, or a combination thereof, which would be excessive in relation to the concrete and direct military advantage anticipated” [emphasis added]. But what does “excessive” mean? International law has struggled to answer this question. Proportionality Law Muddled: “Excessive” vs. “Clearly Excessive” Article 57(2)(b) of Additional Protocol I reiterates the requirement of “excessiveness” in determining whether an act of self-defense is disproportionate: [A]n attack shall be cancelled or suspended if it becomes apparent that the objective is not a military one or is subject to special protection or that the attack may be expected to cause incidental loss of civilian life, injury to civilians, damage to civilian objects, or a combination thereof, which would be excessive in relation to the concrete and direct military advantage anticipated [emphasis added]. 17. E. Wilmshurst, “The Chatham House Principles of International Law on the Use of Force in Self-Defence,” 55 INT'L AND COMPARATIVE LAW QUARTERLY 963, 96667 (2006). 18. G. Corn, “The Essential Link between Proportionality and Necessity in the Exercise of Self-Defense,” in NECESSITY AND PROPORTIONALITY IN INTERNATIONAL PEACE AND SECURITY LAW 86 (Claus Kreß and Robert Lawless eds., 2020). 19. Supra note 13, at 94. 20. Fourth Geneva Convention, 1949, 75 U.N.T.S. 287, art. 51 (2); see also supra note 15, at 1049. 21. Convention Respecting the Laws and Customs of War on Land, with Annex of Regulations, T.S. 539, Oct. 18, 1907. 22. Shlomo M. Brody, “How Israel Missed Its Chance to Eliminate the Leadership of Hamas,” TABLET MAGAZINE, Dec. 4, 2023, quoting M. W. Royse, AERIAL BOMBARDMENT AND THE INTERNATIONAL REGULATION OF WARFARE 137 (Vinal, 1928). 23. Supra note 20.
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