Armed Conflict Foreign Relations & International Law Terrorism & Extremism

Calibrating Proportionality and Self-Defense in Gaza

Chile Eboe-Osuji
Thursday, December 7, 2023, 1:00 PM
Two fundamental concepts of international law of armed conflict, according to a former ICC president, who gives comfort to neither Hamas nor the Israeli government in their operations since Oct. 7.
Smoke rises out of a high-rise tower in Gaza City, October 2023. (Palestinian News & Information Agency (Wafa) in contract with APAimages,; CC BY-SA 3.0 DEED,

Published by The Lawfare Institute
in Cooperation With

The recent outbreak of war between Israel and Hamas since Oct. 7 has thrust two words into the public sphere. They are “proportionality” and “self-defense.” These two words are legal concepts.

Some legal scholars well versed in international law have made helpful interventions to explain the meaning of these terms. However, there is room for further clarification. 

Words matter. Let us consider here, for instance, the current general practice of every state’s naming its military bureaucracy as “Ministry [or Department] of Defense”—instead of “Ministry [or Department] of War” (as was the general practice before World War II). Doing so psychologically conditions those who are not well versed in the law of war into assuming that the state is engaged in “defense,” whenever it is engaged in an armed conflict, especially close to home. One only needs the further seemingly insignificant step of slipping the tiny word “self-” in front of that usage of “defense,” and the idea of self-defense gets confused in ways that can have catastrophic humanitarian consequences. To be clear, there is nothing wrong with the nomenclatures of “Ministry of Defense” or “Defense Forces.” What is important, rather, is to avoid the slip of thought that instinctively sees a state as fighting a war of “self-defense” whenever the state is engaged in an armed conflict.

The following is an explanation of this author’s own understanding of the legal concepts of “proportionality” and “self-defense”—informed by decades of practicing, researching, and teaching the law of armed conflict.

Proportionality in International Humanitarian Law

There is a general agreement among legal scholars that the principle of proportionality is one of the key norms of international humanitarian law (IHL). Briefly explained, IHL comprises rules of international law that, for humanitarian reasons, seek to constrain the effects of armed conflict, animated by the overriding principle that there are necessary limits to the means and methods that states may use in fighting wars. In that regard, IHL primarily seeks to protect persons who are not participating in the hostilities (such as civilians and humanitarian workers) or persons whose initial participation had come to an end (such as wounded or sick combatants or those who laid down their arms in surrender or those who had been captured during battle and taken into captivity). 

The rule of proportionality is one of the central rules of IHL. A fuller appreciation of that rule requires the context of its origins to be kept in mind. That context is the need to modulate the uncontroversial (though depressing) reality of warfare. This reality means that combatants of the one side may lawfully shoot and kill combatants of the other side. That reality is uncontroversial in the sense that international law does not forbid opposing combatants from killing each other amid a raging battle. However, even in that context, IHL imposes proportionality. IHL does not allow those warring combatants to destroy or annihilate the combatants on the opposing side—just because they can. As a study group of the International Law Association correctly noted in 2018, even a proper case of “self-defense cannot justify ‘all-out’ war to destroy the enemy.”


There is troubling information tending to suggest that elements within both the Hamas movement and the Israeli government might be motivated by the mindset to annihilate the opposing side. It is thus important to stress that international law does not allow combatants of one side to exterminate their opponents. Evidence of acts and intention in that regard can amount to an international crime—yes, including genocide.

Some experts in international criminal law have suggested that both Hamas and the Israeli government may be investigated for genocide for their operations in the current conflict. It is beyond the scope of this paper to enter into the merits of that suggestion. It is enough to say, as a general proposition, that when warring sides are consumed by any combination of rage and revenge and frustration and fear, and military power is on their side, they can very easily blunder into genocide without knowing it. The hubris of gun power can misguide people in that way. The jurisprudence of international law seldom contains an instance in which those convicted of genocide had thought to themselves, let alone vocalized the thought, “I’m going out to commit ‘genocide’ today.” What is more, the law of genocide does not require the killing of an entire racial, religious, national, or ethnic group. 

The International Criminal Tribunal for the former Yugoslavia convicted both Radovan Karadžić and Ratko Mladić of the crimes of genocide, extermination, and murder for international crimes committed in Srebrenica, which involved the killing of between 7,000 and 8,000 Bosnian Muslim men and boys, and the forcible transfer of over 20,000 Bosnian Muslims who “did not have a genuine choice but to leave” Srebrenica. One lesson in these cases is that any military operation in which there is a high death toll runs a serious legal risk, if that death toll is not directly explained by convincing evidence that the deceased were combatants who died in the active exchange of gunfire between equally matched sides. And the additional evidence of a mindset of annihilation comes with the additional hazard of a charge of genocide. (It should be kept in mind that international law does not recognize the plea of provocation in relation to those high crimes.) 


Returning now to proportionality, the original—and still operative—meaning of the rule of proportionality is that belligerent combatants are not free to destroy or annihilate their armed opponents. That orientation of the principle—that is, in relation to combatants—is readily apparent in both the Geneva Convention of 1864 and the St. Petersburg Declaration of 1868, the first international instruments that gave the principle of proportionality the essence that continues to guide modern IHL. 

The principle of proportionality relative to combatants should be clear enough in the text of Article 6 of the Geneva Convention. It requires that “[w]ounded or sick combatants of whatever nations they may be, shall be collected and cared for” (emphasis added). If you are required to collect and care for someone, it means you are not permitted to kill them. Assuming the proposition (relative to combatants) to be even debatable in the 1864 Geneva Convention, it is inescapable in the 1868 St. Petersburg Declaration. It insists that “the only legitimate object which States should endeavour to accomplish during war is to weaken the military forces of the enemy.” And for that purpose, “it is sufficient to disable the greatest possible number of men.” That being the case, it is forbidden to employ against opposing combatants methods or means of warfare that “uselessly aggravates their sufferings … or render their death inevitable” (emphasis added). 

It is obvious, then, that the principle of proportionality was designed, in the first place, to protect enemy combatants from the use of any weapon or method of warfare that goes beyond weakening or disabling those combatants to the point of defeat or surrender—if it is possible to do so without killing them. That has been the rule in international law since 1868 at least.


Another norm that underscores this principle of proportionality—as directed at the protection of combatants—is the old norm that prohibits any side to the armed conflict from declaring that “no quarter will be taken/given.” As far back as 1861, the rule was captured in the Lieber Code, the first known effort to distill principles of customary international law of war, compiled by Francis Lieber, a law professor at Columbia University. Article 60 of the Lieber Code provides that it is “against the usage of modern war to resolve, in hatred and revenge, to give no quarter. No body of troops has the right to declare that it will not give, and therefore will not expect, quarter” (emphasis added). Lieber allowed only one exception to that rule: A commander might direct his troops to give no quarter only “when in great straits, when his own salvation ma[de] it impossible to cumber himself with prisoners” (emphasis added). But even that strictly limited exception has not been recognized in modern international law since 1899, when extensive law of war treaties were adopted for the first time during the First Hague Peace Conference. Article 23(d) of the Hague Regulations (annexed to the 1899 Hague Convention (II) with Respect to the Laws and Customs of War on Land) simply provides that it is “especially prohibited … [t]o declare that no quarter will be taken” (emphasis added). No exceptions are recognized either in that treaty or in the equivalent provisions in the Hague Regulations (annexed to the 1907 Hague Convention (IV) respecting the Laws and Customs of War on Land). It has been that way in international law since.

This rule, from the time of the Lieber Code, forbids fighting a war with the declared aim of wiping out the combatant adversary. It is necessarily a rule of proportionality, because every war will have wounded and sick combatants and combatants who surrender. They are not to be killed. That is the real purpose of the rule of proportionality: to protect enemy combatants from unnecessary killing or needless cruelty. 

But that is not to say that the rule of proportionality does not also protect civilians. It is only to say, first, that civilians are not the rule’s primary raison d’être, in the manner that the rule is often expressed these days in legal or political discourse. (As we will soon see, civilians have their own special rule of protection.) To routinize the proportionality rule in the discourse about civilian protection tends to diminish how the rule best serves everyone in a conflict, even civilians. This is because making civilians the primary focus of the rule of proportionality tends to obscure the cardinal principle of distinction. This principle, as its naming suggests, insists that a distinction must be made at all times between combatants and civilians: to the end that only combatants may be attacked during war and civilians spared from attacks and their effects. This is the first and the most sacrosanct rule of IHL.

Just as the rule of proportionality was designed primarily to protect combatants, the rule of distinction concerns itself primarily with the protection of civilians. The two rules should not be conflated. That conflation necessarily occurs when it is thought that the killing of “innocents” is “the price of waging a war.” The resulting error is that it concentrates the psychological focus primarily on the proportionality of civilian casualty, rather than concentrating the primary inquiry on whether the sanctity of the principle of distinction had really been observed. In the outcome, the sacred principle of distinction is overlain by proportionality concerns about civilian casualties—as if to say that civilians are fair game during war as long as the numbers are kept relatively low enough. 

The obverse of the above problem benefits civilians in the sense that focusing the principle of proportionality on the need to protect adversarial combatants will have an added advantage for civilians by magnifying the importance of the principle of distinction. This is in the sense that a scrupulous observance of the rule of proportionality—as something designed primarily to protect combatants—affords a stronger reason to be even more scrupulous about it in relation to civilians.

I had always taught my students to look at the principle of proportionality in that way. I recently happily discovered that Baroness Rosalyn Higgins expressed that view long before me—almost 30 years ago—from the bench of the International Court of Justice. In her dissenting opinion in the Nuclear Weapons Advisory Opinion, she first reproached the all-too-familiar tendency to “conflate” the norms that hold that “harm to civilians as a means of securing victory over the enemy was not a legitimate right of war; and that even in seeking to disable the military not every method was lawful.” She continued:

[P]rohibitions against means of conflict that cause unnecessary suffering is directed towards the fulfilment of the second, progressive, limb—namely, that even in seeking to disable the military forces of the enemy, there is a limitation upon the means that may be employed. These provisions are not directed at the protection of civilians—other provisions serve that purpose. It is in any event absolutely prohibited to attack civilians, whether by nuclear or other weapons. Attack upon civilians does not depend for its illegality upon a prohibition against “superfluous injury” or aggravating the sufferings of men already disabled. [Emphasis added.] 

I worry that the conflation that Higgins reproaches in her opinion may lend integrity to the depressing refrain that “collateral” civilian casualties are an accepted outcome in every war. That refrain tends to muddy the gross immorality of the war crime entailed in the failure to make every possible effort to respect the “absolute[] prohibit[ion]” against subjecting civilians to attack “in any event,” as Higgins put it.

Self-Defense in International Humanitarian Law

Beyond “proportionality,” another international law concept that has fallen victim to confusion of thought is the concept of “self-defense.” 

The ongoing Israel-Hamas war is not the first time the plea of “self-defense” has been invoked in an armed conflict. Often both sides of a conflict claim self-defense. In the current conflict, that claim has been made on the side of Hamas. At the same time, Israel and its own allies have consistently claimed that Israel’s operations in Gaza in response to the Hamas attack of Oct. 7 amount to “self-defense.” 

The plea of self-defense has specific tests in international law, as I will discuss below. The claim of self-defense made on the side of Hamas doesn’t warrant extended discussion—the reason for the short shrift is readily apparent. There is no legal theory of self-defense that could possibly accommodate the Hamas gunmen’s attack and kidnapping of Israeli civilians on Oct. 7. Perhaps, those who seek to legitimize Hamas violence against Israel have “reprisals” in mind. Reprisals are acts of self-help that entail violation of international law but are undertaken as a countermeasure of last resort against an opponent’s own earlier violations of international law. While international law doesn’t encourage resort to reprisals, it tolerates them under strict conditions. One of those conditions is that on no account may civilians and civilian objects be subjected to acts of reprisals, illustrated by the 1977 Additional Protocol 1 to the Geneva Conventions, specifically Articles 20 and 51-56. This prohibition is recognized as a rule of customary international law, which binds even states that are not a party to Additional Protocol I. This means that Hamas militants are not entitled to subject Israeli civilians to violent actions in reprisals against violations of international law by Israeli authorities.

Besides the implausible claim of self-defense on the side of Hamas, the more serious question is whether the manner of Israeli military operations in Gaza since the days following the Hamas attack meet the legal tests of self-defense.

Relevant Jurisprudence of the International Court of Justice

Certain conclusions of the International Court of Justice in the Construction of a Wall Advisory Opinion (2004) make a useful starting point for an examination of the validity of the self-defense claim in the current Israel-Hamas war. For one thing, the ICJ observed that a valid claim of self-defense is not available to Israel in its armed conflicts with Palestinians, because Israel “exercises control in the Occupied Palestinian Territory.”

The automatic rejection of the plea of self-defense at the instance of an occupying power in relation to measures it takes against insurgents brings to mind the doctrine of actio libera in causa, which entails a limitation to the plea of self-defense if not its entire negation in particular cases. That Latin phrase and the doctrine it stands for are adequately explained in the following quote:

[J]urisdictions recognizing the defense also limit it so that persons cannot rely on it to escape liability if they have culpably created the conditions under which its conditions are satisfied. This is often reflected in legal doctrines that explicitly bar the defense to those who have acted as the first aggressor; persons who start an altercation with another, in order to provoke an attack to which they will have to respond self-defensively, cannot then escape liability by appealing to self-defense as a justification. Likewise, if persons provoke others into attacking them, they will then be barred from appealing to self-defense to justify their conduct.

A recent judgment of the Supreme Court of Canada in R. v. Khill (2021) sheds further light on the doctrine in question, which the Supreme Court of Canada more accessibly termed “person’s role in the incident.” In that case, the Court upheld an appeal judgment that overturned the acquittal of a defendant who pleaded self-defense. The conviction was overturned because the trial judge had failed to instruct the jury to consider the role of the accused in the encounter in which he used deadly force claiming self-defense. The judgment is indeed a useful discussion of the “person’s role in the incident” concept. More broadly, the discussions in the judgment should help to calibrate a proper understanding of “reasonableness”—a referent term similarly encountered in international law in relation to self-defense.


The salience of the actio libera in causa doctrine as a consideration that denies or limits the plea of self-defense to an occupying power is apparent in thinking through the reasons that international law cared enough to regulate military occupation so extensively. (See, for instance, Section I of Part III of the Geneva Convention No. IV.) For one thing, contemporary international law axiomatically recognizes the right of occupied peoples to “fight … alien occupation … in the exercise of their right of self-determination, as enshrined in the Charter of the United Nations and the Declaration on Principles of International Law concerning Friendly Relations and Co-operation among States in accordance with the Charter of the United Nations.” (See 1977 Additional Protocol I to the Geneva Conventions of 1949, Article 1(4).

Although military occupation used to be a method of acquisition of territory in the past, international law was reformed after World War I to reject that practice, inspired by Woodrow Wilson’s vision of the right of self-determination to all “nations great and small.” It was thus that Germany was divested of all its occupied territories in Europe and colonies beyond. Notably, since the adoption of the Kellogg-Briand Pact in 1928 and the resulting acceptance of the Stimson Doctrine in 1931, international law has rejected the idea of acquisition of sovereignty over an occupied territory. That doctrine is enshrined in Article 11 of the Montevideo Convention on the Rights and Duties of States (1933). Consequently, the International Law Association captured the principle in paragraph 5 of their 1934 Budapest Articles of Interpretation of the Briand-Kellogg Pact. (See U.S. Attorney General Robert H Jackson’s address to the Inter-American Bar Association, Havana, March 27, 1941 at page 355.) Hence, since occupation is no longer a right in modern international law, the occupying power has no right of self-defense in relation to the occupation.

Furthermore, international humanitarian law imposes extensive obligations upon the occupying power, in order to ensure that the occupied peoples suffer no abuses, as Section I of Part III of the Geneva Convention No. IV makes clear. 

Military occupation is therefore fraught with circumstances that can easily trigger considerations of actio libera in causa. It thus makes it easier to see why self-defense should apply more readily when a state is within its natural or recognized territory minding its own business and got attacked there, but not so much when a state is using military force to occupy a territory that is not its own, over the objection of the indigenous population—for whom international law has recognized a right to resist violation of their right to self-determination.

However, it is important to add that no theory of international law makes it permissible or defensible, in the name of resistance to alien occupation, to attack civilian citizens of the occupying power, especially those on their own territory. An occupying power faced with such a predicament is not without options in international law. We will examine those later.

The discussion at this point is only to explain the basic proposition in the ICJ’s Construction of a Wall Advisory Opinion, which says that self-defense is not available to an occupying power. In the nature of things, this can only be understood as a general rule. It is readily conceivable that the attacks against the civilian citizens of an occupying power in their own territory can rightly occasion countermeasures of self-help including self-defense—under the strictures of that doctrine (discussed below). There is, nevertheless, much value in the general rule laid down in the Construction of a Wall Advisory Opinion, lest it allows an occupying power to plead self-defense in relation to acts of resistance involving use of force directed purely at the occupying power’s structures of military occupation itself, and not its civilian nationals in their own territory.

That said, it is also important to stress once more that international law does not deny an occupying power the right to protect itself from insurgency attacks emanating from within the occupied territory. But the lawfulness of such right to security is based on considerations other than those of self-defense.

Notably, the ICJ fully recognizes that international law does not stand against Israel taking measures to protect its citizens—to the contrary, it has a duty to do so, said the ICJ—but the measures taken must follow applicable international law. The ICJ expressed that proviso as follows:

The fact remains that Israel has [had] to face numerous indiscriminate and deadly acts of violence against its civilian population. It has the right, and indeed the duty, to respond in order to protect the life of its citizens. The measures taken are bound nonetheless to remain in conformity with applicable international law. [Emphasis added.]

Although the proviso contemplates other aspects of conformity with international law—including applicable principles of IHL—it is still helpful to take a closer look at the real meaning of self-defense, since the ICJ did not explain it in the advisory opinion.

The Self-Defense Tests

In its report on aggression and the use of force, a study group of the International Law Association observed that “[w]hile an armed attack is a necessary trigger for the exercise of lawful self-defense, it is not in itself sufficient—once the right is triggered it is then further regulated by various legal constraints.” In an explanation of that observation, the study group further explained: “Once self-defense has been triggered by an armed attack, any ensuing action taken in self-defense must adhere to the requirements of necessity and proportionality” (emphasis added).

The general orientation of those provisos has merit in international law. But even those provisos beg the question in a material way. This is in their assumption that the “ensuing action” entails a right of “self-defense” that needs to be “further regulated by various legal constraints.” The circumstances of a particular case may not always bear out that assumption. It may well be that the “ensuing action” following an initial attack does—or does not at all—entail the right of self-defense. It all requires a look at the tests of self-defense.


The accepted tests of self-defense were afforded to international law in 1842 as an outcome of the Caroline Crisis. Notably, U.S. Secretary of State Daniel Webster formulated those tests—as part of his rejection of the British claim of self-defense on that occasion.

The diplomatic crisis arose when British soldiers raided the steam vessel named the Caroline on Dec. 29, 1837, as it was moored on the docks of Fort Schlosser in New York, on the American side of the Niagara River. Following the attack, the raiding soldiers set the boat ablaze, cut it loose from its moorings, and set it adrift over the Niagara Falls three miles downriver. That operation put an end to the Caroline’s service to a rabble of Canadian rebels who had been fomenting insurrection against the British colonial administration in Upper Canada (present-day Ontario province) with the aid of some headstrong Americans endeared to turbulent adventure (and whom U.S. authorities could not control). 

The British government claimed that considerations of self-defense justified the attack and destruction of the vessel. In exchanges with Lord Ashburton, the British plenipotentiary handling the matter for his government, Webster articulated the test and trammels of the plea of self-defense that have served international law ever since. He wrote:

It will be for [Her Majesty’s] Government to show a necessity of self-defense, instant, overwhelming, leaving no choice of means, and no moment for deliberation. It will be for it to show, also, that … even supposing the necessity of the moment authorized them to enter the territories of the United States at all, did nothing unreasonable or excessive; since the act justified by the necessity of self-defense, must be limited by that necessity, and kept clearly within it. It must be shown that admonition or remonstrance to the persons on board the “Caroline” was impracticable, or would have been unavailing; it must be shown that daylight could not be waited for; that there could be no attempt at discrimination, between the innocent and the guilty; that it would not have been enough to seize and detain the vessel; but that there was a necessity, present and inevitable, for attacking her, in the darkness of the night, while moored to the shore, and while unarmed men were asleep on board, killing some, and wounding others, and then drawing her into the current, above the cataract, setting her on fire, and, careless to know whether there might not be in her the innocent with the guilty, or the living with the dead, committing her to a fate, which fills the imagination with horror. A necessity for all this, the Government of the United States cannot believe to have existed. [Emphasis added.]

As we will soon see, these are not liberal tests. But it doesn’t mean states are left helpless to endure attacks against them when these strict tests are not met. States go to war for various reasons. The complex of wars often defies straightforward theories of causes, objectives, or both—some of which may be valid and others not—besides the binary legalities of aggression (unlawful) and self-defense (lawful). It is true that the “just war” theory has lost much of the value it had in an era when states were left purely to their own devices of self-help, with no international framework in place to facilitate the 20th century imperatives of peaceful settlement of international disputes, which was the primary reason for being of the League of Nations and now the United Nations. It remains possible, however, to sympathize, or even accept, the validity of a state’s objectives in engaging in a particular war for a cause other than self-defense. One such cause could be the need to ensure lasting security against a neighbor with a demonstrated inclination to indulge in attacks regardless of the cogency of the assailant’s own cause for the initial attacks. The validity of either side’s continued engagement in a particular war may then be considered on its own merits and particular circumstances. Not every permissible war is explained, let alone justified, by the dictates of self-defense.


Properly understood, what the self-defense theory does is make legally excusable (at least) the actions of someone who, in a moment of the most acute emergency, was left without alternative means, and without a moment of reflection, but to protect himself or herself by using means and methods that would ordinarily be unlawful. The action that would ordinarily be unlawful is the action of striking at someone—the assailant. Indeed, even striking back at an assailant would ordinarily be unlawful, in the absence of self-defense, because the lawful response to a concluded violent attack is to call the police or commence legal proceedings for reparation. (I shall return to this later, in the context of the importance of the injunction upon states to seek to resolve all disputes through peaceful means.)

Understood from that perspective, the opportunity to engage in self-defense is of extremely limited duration. It does not operate beyond the particular, immediate emergency of being under an attack that needed to be repelled as it was happening or about to happen. And the doctrine allows only enough force necessary to repel that ongoing or imminent attack.

One of the reasons that self-defense must be extremely limited in duration is because of the need to limit, in turn, the collateral damage that may be done to innocent persons in the turmoil and heat of the victim’s frantic efforts to repel an ongoing or imminent attack against him or her. This aspect reveals the fatal hazards that “self-defense” entails for civilians when the time span for legitimate self-defense is indifferently extended beyond the strict and narrow limits permitted under the Caroline tests. It is one thing to take countermeasures of self-defense that result in unintended harm to the civilian populations while the assailant-provocateurs’ attack was ongoing or imminent; it is quite another matter to engage in extended military operations that result in sustained unintended terror and harm to civilians when the assailant-provocateurs’ attack has ceased—even for the moment.


The foregoing analogy holds steady in international law because the mere use of force by one state against its neighbor does not always entitle the neighbor to use force in return. Since the adoption of the Kellogg-Briand Pact of 1929, international law has required that “the settlement or solution of all disputes or conflicts of whatever nature or of whatever origin they may be, which may arise among them, shall never be sought except by pacific means” (emphasis added). For its part, the UN Charter has fully retained this imperative of peaceful (or pacific) settlement of international disputes (PSID), by the combined operation of Articles 2(3) and 33(1). 

Some commentators appear bogged down by an analytical obstacle they see in Article 2(4) of the UN Charter, which provides that all UN members “shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the Purposes of the United Nations.”

Some observers interpret this provision to mean that the UN Charter’s injunction against use of force engages only when all the parties to a particular conflict undoubtedly qualify as states. So, the question is posed, does the injunction against use of force operate in the Israel-Hamas war, when there is controversy about the recognition of Palestine as a state? A negative answer to that question necessarily asks analyses to ignore the following: that “recognition itself is not a formal requirement of statehood”; that the “political existence of the state is independent of recognition by other states”; that 139 states have recognized Palestine as a state; and that the UN General Assembly recognized Palestine as state. Any analysis that ignores those considerations—while concluding that Article 2(4) of the UN Charter has no application in the Israel and Palestine conflict—cannot be stable in the relevant respect. 

There is the additional difficulty that any construction of the UN Charter, in a way that limits the injunction against use of force exclusively to armed conflict between states, will do serious disservice to the objects and purposes of international law in several ways. First, it directly ignores the significance of Article 2(3) of the UN Charter, the focus of which is peaceful settlement of international disputes “in such a manner that international peace and security, and justice, are not endangered.” That focus is undistracted by questions of statehood. Indeed, the current Israel-Hamas war elevates the significance of that provision to its highest profile. This entails more than the unsettling repercussions of the war on the streets and university campuses of Europe, North America, Australia, New Zealand, and elsewhere very far from the Middle East. More than that, the very deployment of U.S. aircraft carriers to the region immediately upon the outbreak of that war is the clearest evidence of the risk that the Israel-Hamas conflict may plunge the region and the world into a wider armed conflict.

Second, in enjoining states against use of force, it is not certain that Article 2(4) of the UN Charter contemplates only state-to-state use of force. It is certainly true that the provision enjoins states to not use force “against the territorial integrity or political independence of any state” (emphasis added). But the further instance in which the provision also enjoins states against use of force is expressed in the words “or in any other manner inconsistent with the Purposes of the United Nations” (emphasis added). Considering that the very first purpose of the UN as expressed in Article 1(1) of the charter is to “maintain international peace and security,” the conclusion becomes elementary that the use of force injunction operates in every circumstance in which that purpose is engaged—whether or not the situation entails state-to-state confrontation.

Third, the preoccupation with Article 2(4) and its perceived requirement of statehood also reprises the era when international law was seen as concerned only with states as its exclusive subjects. International law has since outgrown that limitation.

And, finally, the concern ignores the related consideration that the purposes of the UN also include “international cooperation in solving international problems,” including those of a “humanitarian character, and in promoting and encouraging respect for human rights” (see Article 1(3) of the charter). The humanitarian and the fundamental human right to life dividends of the injunction against use of force ought not be denied to the potential victims of a given war notwithstanding that one of the parties to the conflict may not qualify as a state.

All that is to say that there is no sensible theory of international law that will normatively preclude the PISD injunction from the matter of the relations between Israel and Palestine of which Hamas is a part. 


The injunction to settle all disputes peacefully, so comprehensively set out in the Kellogg-Briand Pact, does not necessarily carve out conflicts provoked by an initial use of force from the order of conflicts for which peaceful resolution must first be sought—if it was possible and reasonable in a given situation to avoid a military response to an initial use of force. No doubt, catastrophic humanitarian outcomes must be factored into the reasonableness of staying the hands of a military response to an initial use of force. It is not a sign of weakness—especially on the part of a side that could bring overwhelming military might to bear but chooses to exercise restraint. It is, rather, a sign of heightened aptitude of civilization in general and the rule of international law in particular in its prohibition of the use of force.


One vexing question that occurs in the discourse concerning the prohibition of use of force and the related PSID injunction has to do with the significance of Article 51 of the UN Charter, which preserves self-defense as an inherent right of states. Article 51 does not present an unusual conundrum, once it is understood that the preservation of that right—as understood in customary international law—does not detract from the PSID injunction in the first place, any more so than in the domestic system where the criminal code forbids the use of force while still preserving for citizens the right of self-defense under strict conditions. To the contrary, the strictures of self-defense in international law accentuate the normative value of the PSID injunction—to the extent possible and reasonable, recognizing that pacific resolution of disputes may not always be possible (as discussed immediately below). This is because self-defense is a strictly constrained exception to the prohibition against use of force—permissible only in an extreme emergency (that limits the necessity of the self-defensive actions within that specific temporal scope of that persisting emergency) that leaves no room whatsoever for resorting to the PSID methods.

The Miscarriage of Pragmatism

Once more, it must be stressed, the discussion in this piece does not suppose that a state’s reason for continuing in an ongoing war is necessarily invalid in international law unless that reason is entirely explained by self-defense. Acts of an assailant that amount to extreme provocation (acts that might have justified self-defense during the limited time when those acts were ongoing or objectively imminent) can present an opportune psychological moment (on both sides) to take sustained security countermeasures against future attacks. It is also possible that a state involved in an armed conflict quite simply has no other avenue of hope for pacific resolution. It may be that the opponent is a state that will not respect the injunctions of the ICJ, as a means of pacific settlement of international disputes; or the opponent may be a non-state actor over whom the ICJ has no jurisdiction in the first place. And here, it must be observed that a careful review of Articles 4, 5, and 9 of the Draft Articles on Responsibility of States for Internationally Wrongful Acts contemplates the possibility that the conduct of Hamas, as the administrative authority in Gaza, can sensibly anchor ICJ jurisdiction for purposes of international responsibility of Palestine, if Palestine is made a formal party to judicial proceedings that examine the actions of Hamas in the same way the U.S. was the proper party in the LaGrand Case, though the violations in question involved the conduct of officials of the state of Arizona. At any rate, it is far more reasonable, as a general proposition, for Israel to bring PSID claims against Palestine first for the conduct of Hamas (where the circumstances permit) than to engage in extended military operations in and against Gaza—resulting in massive loss of Palestinian lives—in reaction to the conduct of Hamas.


Contrasted to any other instance of self-help where a state is constrained to use force in a lawfully excusable way (such as in a situation of reprisals discussed above), the difference with the analysis of self-defense remains the need to see an ongoing armed conflict for what it really is. If there had been a circumstance of self-defense, it helps to know when the considerations of self-defense begin and end, and when other legitimate considerations other than self-defense have taken over. Those other considerations may still sustain the validity of a war effort, but they will be assessed according to their own circumstances and merits, not to be confused with the acute emergency measures that the law of self-defense excuses.

As noted earlier, the ICJ was alive to that consideration in its Construction of a Wall Advisory Opinion. That explains the proviso the court offered in saying that although Israeli authorities may not claim self-defense in the occupied territories, they are still under a duty to protect their citizens but that the measures they take must “remain in conformity with applicable international law.”

That proviso should adequately address the anxieties of pragmatists who unwittingly tend to invite self-defense into a normative space from which Daniel Webster specifically banished it in 1842. One sees that pragmatist mindset in the opinion of the ILA study group:

Advocates of the narrower approach argue that the notion that self-defense can only be used to end the current attack resonates best with the aims of the UN Charter, by minimising the possibilities for lawful recourse to force. There is, however, also significant support and practical reason to accept that the UN Charter should be read as accepting that self-defense measures may take into account the need to ensure that the attacker has not simply momentarily refrained from operations while the attacks are in fact set to continue in the near future. It would appear therefore that while self-defense cannot justify “all-out” war to destroy the enemy, the forcible measures can include the need to defend the State from the continuation of attacks, and not only repel the attack of the moment. [Emphasis added.]

There is evident value in that argument, but that value lies elsewhere than in the doctrine of self-defense. Notably, the angle of pragmatism in that view is directly inconsistent with the Caroline doctrine of self-defense. The dissonance in the pragmatist argument appears unwitting because its protagonists do not explicitly announce that they are consciously revising the doctrine of self-defense in customary international law so that it says something different. They suggest, instead, that their pragmatist theory of self-defense remains within the original Caroline construct. 

The pitfall for the pragmatist approach is that Webster’s construct had specifically considered and rejected the idea that self-defense should accommodate measures taken, in the words of the ILA study group, “to ensure that the attacker has not simply momentarily refrained from operations while the attacks are in fact set to continue in the near future. It would appear therefore that … self-defense … measures can include the need to defend the State from the continuation of attacks, and not only repel the attack of the moment.” That, precisely, was the essence of the British argument in the Caroline crisis. Webster flatly rejected it.

“No moment for deliberation” and “no choice of means”

For self-defense to succeed, Webster required that there must be “no moment for deliberation.” That requirement is significant because it addresses the element of mens rea, or intent—the element of fault that attracts criminal culpability to a wrongful action. But that element of fault is negated if the victim was constrained to take the wrongful action in self-defense. For present purposes, it is not necessary to engage in the philosophical distinction between “justification” (suggesting a positive entitlement to do what was done) versus “excuse” (suggesting merely reprieve from blameworthiness for having done what was done). It is enough to say that the party pleading self-defense did not with malice aforethought, to use the old expression, engage in the wrongful conduct that is the action taken in self-defense—the circumstances left the party no moment to reflect—“no moment for deliberation,” as Webster put it—on that action for its rights and wrongs. 

 It is this requirement—that there be “no moment for deliberation”—that makes self-defense more suitable for discrete actions (the “narrower view,” as the ILA study group called it). Contrary to the explanations suggested by some commentators, the requirement of “no moment for deliberation” will in most cases preclude a proper application of self-defense to an entire war (the pragmatist view) in which military operations are deliberated upon in war cabinet or command staff meetings, before reacting to a provocateur’s prior use of force that is no longer ongoing. In other words, given Webster’s insistence that self-defense permits “no moment for deliberation,” the validity of self-defense claim diminishes in inverse proportion to the incidence of planning of any operation deployed in reaction to a prior use of force. Planning is deliberation. 

The test of “no moment for deliberation” is evidently related to that of “no choice of means” and is accentuated by it. For it is in the course of deliberation that a choice is made as to means and methods of use of force in reaction to the prior incident.

The ICJ appears to have validated the foregoing understanding of the doctrine of self-defense in the Oil Platforms case, in which the court observed that “the requirement of international law that measures taken avowedly in self-defense must have been necessary for that purpose is strict and objective, leaving no room for any ‘measure of discretion’” (emphasis added). 

Once more, it doesn’t mean a state cannot fight a war lawfully if any aspect of that war involves any planning on their part. It only means that the legality of such wars will be assessed on a basis other than self-defense. That brings us to another stricture of the Caroline doctrine of self-defense.

The Need to Prevent an Assailant’s Escape From Punishment or Retribution

There is a certain detail in Webster’s statement of the law of self-defense that, properly understood, helps to put the mainframe of his analysis in its proper context. Although appearing as an afterthought at the end of the paragraph preceding the familiar main text of his discussion (the passage quoted earlier), it is significant that Webster had effectively begun his discourse with these words: “If there have been cases in which individuals, justly obnoxious to punishment, have escaped, this is no more than happens in regard to other laws.”

The prefatory significance of that text, according to Webster, is that the prospect of an assailant’s escape never gave direction to the law of self-defense. What does that mean? It means that self-defense is justified only by the necessity of repelling an ongoing or imminent attack. Self-defense doesn’t serve the need to respond to an attack that has already taken place or that has been neutralized in the meantime. If someone is attacking you and continues to do so—or is about to attack you—in circumstances in which you have no time to call the police to intervene, and the only way available for you to stop the ongoing or imminent attack is to defend yourself by deploying a countervailing force against the assailant, then that countering force is fully justified by self-defense. But self-defense is not available to you if the assailant punches you and runs away, and then you chase him down and beat him to a pulp because you want to make sure that he doesn’t escape retribution or pose a danger of future attacks—that is what Webster was saying in that seemingly innocuous text.

Indeed, Webster knew that the British colonial administration of Upper Canada had a legitimate interest in suppressing the insurrection for which the Caroline had been used to ferry personnel and materiel. Webster also knew that the U.S. government (in all its neutrality) had been unable to control the actions of the American swashbucklers who were swelling the ranks of the Canadian rebels, let alone control the rebels themselves. The difficulties that the insurrectionists had presented to Canada “were increased by the course of the insurgents, who, when defeated, sought refuge in the United States, where they endeavored to recruit their forces”—up to a point of increased following from about 200 or 300 men to “about 1,000 men, well armed.” Notably, Lord Ashburton, in his letter to Webster on July 28, 1842, accepted the tests of self-defense proposed by Webster, while insisting that the circumstances in which the Caroline was destroyed in U.S. territory fully met those tests. As Ashburton put it:

This force, formed of all the reckless and mischievous people of the border, formidable from their numbers and from their armament, had in their pay and as part of their establishment this steamboat Caroline, the important means and instrument by which numbers and arms were hourly increasing. I might safely put it to any candid man acquainted with the existing state of things, to say whether the military commander in Canada had the remotest reason on the 29th of December to expect to be relieved from this state of suffering by the protective intervention of any American authority. How long could a Government, having the paramount duty of protecting its own people be reasonably expected to wait for what they had then no reason to expect? 

Webster remained unmoved in his position that the British were not entitled to the claim of self-defense in the circumstances of the case. The crisis was resolved only when the U.S. government preferred to accept as satisfaction the British acknowledgment of the error of carrying out the operation inside U.S. territory and apologized for it.

It is thus that the plea of self-defense does not apply to accommodate measures taken to prevent the recurrence of future attacks. Those measures may be legitimate for reasons other than self-defense. But the principles that grant them legitimacy are not as forgiving as the doctrine of self-defense. The further requirements that self-defense or any other countermeasure must satisfy as common elements of legality are the requirements of reasonableness and proportionality.

The Proportionality-Reasonableness Requirement in Self-Defense

Proportionality and reasonableness can receive separate treatments. For present purposes, however, it is convenient to treat them together, since proportionality is ultimately an element of reasonableness. 

Proportionality-reasonableness is a standard legal requirement. It modulates the lawfulness of action in a variety of legal contexts beyond criminal law—notably constitutional law and administrative law. That standard serves as a legal backstop to ensure that people do not abuse a right, privilege, or licence that the law allowed them in the first place. In criminal law, the proportionality-reasonableness standard serves to ensure, for instance, that a reaction which objectively amounts to an overkill (either at inception or eventually) is not permissible in plea of self-defense.

Unsurprisingly, the proportionality-reasonableness standard also composes a major element of the Caroline doctrine. In that regard, Webster insisted that “even supposing [that] the necessity of the moment” warranted the measures taken in self-defense, that necessity still permits “nothing unreasonable or excessive; since the act justified by the necessity of self-defense, must be limited by that necessity, and kept clearly within it” (emphasis added).

The ICJ has affirmed this element in its repeated observation that “in customary law ‘whether the response to the [armed] attack is lawful depends on observance of the criteria of the necessity and the proportionality of the measures taken in self-defense.’”

In its judgment in R. v. Khill, the Supreme Court of Canada gave a very useful analysis of the requirement of reasonableness in the assessment of self-defense claims. Among those explanations is that reasonableness imports an objective assessment in the appreciation of both the circumstances and the force claimed as self-defensive. Regarding reasonableness of the circumstances, “the question is not what the accused thought was reasonable based on their characteristics and experiences, but rather what a reasonable person with those relevant characteristics and experiences would perceive.” Regarding reasonableness of the force claimed as self-defensive, the reasonableness inquiry “operates to ensure that the law of self-defense conforms to community norms of conduct” (emphasis added). But for this guardrail, the “law of self-defense might otherwise ‘encourage hot-headedness and unnecessary resorts to violent self-help.’” Notably, the Supreme Court hung the law of self-defense on that ultimate peg of reasonableness, as Webster did in 1842. As the Supreme Court put it:

To the extent self-defense morally justifies or excuses an accused’s otherwise criminal conduct and renders it non-culpable, it cannot rest exclusively on the accused’s perception of the need to act. Put another way, killing or injuring another cannot be lawful simply because the accused believed it was necessary. Self-defense demands a broader societal perspective. Consequently, one of the important conditions limiting the availability of self-defense is that the act committed must be reasonable in the circumstances. [Emphasis added.]

The sentiments of the Supreme Court of Canada are also identifiable on the international plain, to the effect that the validity of claims of self-defense are not simply to be left to the party pleading it. In 1928, for instance, at the same time that the Kellogg-Briand Pact was being negotiated, the League of Nations delivered to its members certain questions in preparation for a conference on the codification of international law. One question regarding the circumstances in which a state might be absolved from responsibility for an internationally wrongful act was: “What are the conditions which must be fulfilled when the state claims to have acted in self-defense?” Quincy Wright reported at the time that “no replies suggested that a state could make any action ‘self-defense’ merely by calling it such.”

Bringing the analysis full circle, the proportionality-reasonableness requirement—even in self-defense—highlights the importance of that requirement in those circumstances in which a state’s use of force for the protection of its citizens may be valid, though the measures taken may not satisfy the strict emergency standards of self-defense. Those otherwise lawful measures become decidedly unlawful when they breach the proportionality-reasonableness backstop.

Unfortunately, it seems, Israeli government officials appear to have repeatedly reached back to the Allied bombings of German and Japanese cities during World War II as models of conduct that might justify similar bombings in Gaza in 2023. For instance, a New York Times news analysis by Michael D. Shear, David E. Sanger, and Edward Wong reported the following information that allegedly emerged in discussions between U.S. officials and Israeli leaders: “It became evident to U.S. officials,” it is reported, “that Israeli leaders believed mass civilian casualties were an acceptable price in the military campaign. In private conversations with American counterparts, Israeli officials referred to how the United States and other allied powers resorted to devastating bombings in Germany and Japan during World War II—including the dropping of the two atomic warheads in Hiroshima and Nagasaki—to try to defeat those countries.”

The mindset in question is surprisingly disheartening. This is for the simple reason that it ignores the monumental international law reforms that occurred in the wake of World War II, in an effort to ensure that the rampancy of inhumane conducts that occurred as part of that war were not repeated in later years. The foremost among those law reforms include the following: a convention was adopted in 1948 to prevent and punish the crime of genocide; a universal declaration was adopted in 1948 guaranteeing certain basic rights for human beings, including the right to life and security of the person; and a set of four conventions were adopted in Geneva in 1949 and revised in 1977 to enhance and emphasize, among other things, the norms which insist that civilians are not to be subjected to indiscriminate attacks (including the kinds of bombings that the Allies conducted against Germany and Japan during World War II). It is quite simply wrong to ignore these developments in 2023, while contending that states had conducted themselves in comparable ways in 1945, let alone point to such conducts as paradigms of proportionality and reasonableness.


The law’s intervention (using its substance and procedures) in the regulation of human interactions assumes a primary premise that rejects the idea that the “end justifies the means.” In no circumstance is that rejected idea starker than in the arguments that both Hamas and the Israeli government and their respective sympathizers and allies have used to justify the Hamas attack on Oct. 7 and the Israeli government’s reaction during the period after. A primary claim has been self-defense. Those claims are not readily sustained on either side of the conflict. 

The self-defense claim is easier to dismiss in relation to the Hamas operation of Oct. 7. Even the alternative claim of reprisals is seriously undermined because of the targeting of Israeli civilians on Oct. 7.

For their part, the claims of the Israeli government and their allies fare no better. Although the horror of the Oct. 7 attack rightly attracted instant and overwhelming sympathy, the law’s train still runs on main lines laid on grounds of defined principles. Regarding self-defense, those lines of principle were laid during the Caroline incident in 1842 and have guided international law ever since.

In addition to the exorbitant loss of civilian Gazan lives that the military reaction of the Israeli government has entailed, their claim of self-defense runs into additional obstacles presented by public pronouncements of its top leadership. These include the promise of “mighty vengeance,” the “Amalek” reminder, the determination to “wipe Hamas off the earth,” the mindset that no Gazan is innocent of the Oct. 7 attack because they elected Hamas into office in 2006 (notwithstanding that Hamas won less than 45 percent of the votes in Gaza). Seen in their best possible light, those pronouncements suggest motivations that fall entirely outside the tests of self-defense that Daniel Webster formulated in the Caroline incident in 1842. 

It would be wrong to suggest that those who were horrified by the plight of civilians in Gaza in the wake of Israeli strikes did not care for the innocent Israeli civilians who were killed earlier on Oct. 7. It is also wrong to suggest that anyone who called for a cease-fire or “humanitarian pause” necessarily stands against the neutralization of Hamas as a military organization capable of carrying future attacks against Israeli civilians. There is no known principle of international law that insists on the structural survival of any organization that would plan and launch an attack against a civilian population, killing so many and taking others hostage in the manner seen on Oct. 7—and then endanger a civilian population they call their own, by carrying out military operations from among them. Still, the doctrine of self-defense, properly understood, is not designed for the purpose of ensuring that an assailant is permanently disabled to attack again in future—especially not when those aims are pursued with such catastrophic consequences to lives of civilians, including children who were obviously not the assailants.

Indeed, it goes without saying that international law specifically forbids causing serious harm to innocent civilians as a cost they must pay even for the legitimate project of dismantling an organization willing either to take civilian lives or to provoke their loss. 

In the final analysis, some of the troubling questions that impede a view of the loss of civilian lives and facilities in Gaza as a reasonable price to pay to ensure the dismantling of Hamas include the following. In a regime of international law that requires protection of civilian lives without discrimination, is it acceptable for the Israeli government to expend the lives of Palestinian civilians in such high numbers out of a need to protect the lives of Israeli civilians (who unquestionably deserve protection from wrongful attacks by the Hamas organization that the Israeli government has vowed to dismantle)? Is the project of destroying Hamas even feasible? Or does the project only entail the momentary dissolution of a militant organization without dissolving their raison d’être? Is it acceptable to kill so many Gaza civilians before the realization settles that the project may not be feasible after all? Is there objective clarity that the project is the only or the best solution to the problem of Israeli security that is at the center of the Israeli-Palestinian difficulty, before giving that project the urgency that is occasioning such a heavy toll on civilians? In other words, has the Israeli government demonstrated that it has offered Hamas (or Palestinians, for that matter) a genuine carrot of peaceful, dignified co-existence, such that the only solution now left is the heavy stick of destruction of an organization that is incorrigibly diabolical—at a high cost to the Gaza civilian population?

Chile Eboe-Osuji served as the fourth president of the International Criminal Court from 2018 to 2019 and as a judge of the Appeals Division during the same period. He is the Distinguished International Jurist at the Lincoln Alexander School of Law at the Toronto Metropolitan University.

Subscribe to Lawfare