Guest Post: Charlie Kels on “Self-Defense Proportionality: In Defense of Self-Defense”

Today’s post is by a new Lawfire® writer, Charlie Kels.  I’ve admired his superb scholarship for quite some time, and his contribution today extends that record of excellence. 

In his post, Charlie takes on what strikes me as a very disturbing trend in the interpretation of the concept of proportionality. Its advocates—mainly academics but some others–conflate proportionality in self-defense with proportionality in targeting once a conflict is underway (jus in bello).

In an earlier essay, Charlie summarizes the misinterpretation this way:

To boil it down, what [an advocate of the misinterpretation] is saying is that even if a State is acting in self-defense with a lawful objective, and even if their forces abide by [the law of war] in military operations, they still have a legal obligation to stop fighting if civilian casualties are too high. (Emphasis added.)

Allow me to say definitively that this summary above is not the law and, furthermore, should not be the law – but some are pushing for this.

If it were the law, it would oblige a State, including one abiding by the law of war, to leave in place an attacker who presents an ongoing threat–to include one whose stated purpose is not only the annihilation of the victim State, but also the murder of the State’s people.

To require a victim State to stop fighting would provide that threat with the opportunity to regroup and refit so as to enable it to visit lethal violence on the victim State at the time and place of the threat’s choosing.  It would legally compel a State to put itself—and millions of its people—in a situation of continuing existential peril.

In other words, the propagators of this misinterpretation of proportionality would put in place a legal norm that would limit or even prohibit the ability of a victim State to extinguish a threat that has attacked it.  As Charlie explained in his earlier essay, the concept of proportionality does not operate that way:

[T]aking as a backdrop the general prohibition on the use of military force under international law, the jus ad bellum proportionality requirement limits the magnitude of defensive coercion to what is reasonably necessary to secure the lawful objective. It does not preclude a legitimate war aim of extinguishing the threat, as opposed to merely weakening or delaying it, depending upon the nature and severity of the armed attack that required the resort to self-defense in the first place. (Emphasis added.)

Parenthetically (and rather paradoxically) the result of the mistaken notion of proportionality being propagated–presumably in the hopes of protecting civilians–could actually, as I note below, put more civilians at risk in both the victim State and among the population the attacker controls. As such, there is no way to reconcile such a stance with a sensible military, legal, or moral approach. 

Indeed, it is not too much to say that this misinterpretation of proportionality is morally counterproductive.  Put plainly, it would incentivize attackers–and, yes, they can be this vicious–to maximize civilian casualties among the people with whom they live

The perverse logic is obvious: the more such civilian casualties adversaries can orchestrate, the more likely they can get the high numbers of casualties that would, under this emerging proportionality theory, create a ‘legal fortress’ that will protect them from lawful self-defense actions by a victim State.

If this defective interpretation of proportionality takes root, we will surely see more adversaries–especially in urban situations–burrow more deeply into civilian communities, do even more intermingling with noncombatants, employ yet more human shields, stop more civilians from evacuating danger zones, and use even more civilian objects for military purposes.  Such endangering of civilians is exactly the opposite of what international law seeks to accomplish.

Make no mistake about it, unconscionable belligerents will use these techniques to cause high civilian casualties among the people with whom they live if doing so supports a claim of ‘disproportionality’ that will legally oblige the victim State acting in self-defense to stop battling them.

To be clear, neither I nor Charlie nor other opponents to this theory are suggesting anything other than rigorous adherence to the existing law of war.  Rather, the issue here is the new interpretation of proportionality that, if embraced, could provide more opportunities for attackers to handicap victim States from acting in self-defense, and give attackers tremendous motivation to maximize casualties among civilians under their control. This approach is just wrong, legally and morally.

Of course, it is heartbreaking to see civilians killed and injured, and every reasonable effort needs to be made to mitigate the risk to them.  However,  the tragic reality of warfare in a densely-populated urban setting is that it inevitably puts civilians in danger, even when a State conducts its operations in full compliance with the law of war

Moreover, when a savage adversary deliberately puts the civilian population in peril as part of a depraved strategy, high civilian casualties are the likely result in virtually any lawful effort to uproot and eliminate such a belligerent. 

In fact, to countenance such a weaponization of civilians violates the international law norm found in the maxim ex injuria non jus oritur, which expresses the principle that “legal rights should not be understood to result from the commission of wrongful acts.” 

Put another way, as we hope and pray for a peaceful and just solution, the international legal community should not be giving belligerents, and especially terrorist organizations, reasons to put civilians in jeopardy. 

That’s my view, but Charlie superbly walks you through a thorough legal analysis.  He brilliantly–and meticulously–deconstructs this emerging proportionality theory that I believe would, despite the wishes of its advocates, put civilians at grave risk  You really need to read this one!

Self-Defense Proportionality: In Defense of Self-Defense

By Charles Kels

Most analyses of proportionality in the ongoing Israel-Hamas conflict have focused on the application of that principle to the conduct of hostilities, which is governed by the international law of armed conflict (LOAC), or the law of war. As Geoffrey Corn has explained so well in these pages, proportionality in LOAC is almost exclusively about mitigating the unintended-but-foreseeable harm to civilian persons and objects when military forces attack legitimate targets.

Recently, a new front in the proportionality debate has opened wherein some legal commentators claim that Israel’s overall war effort is disproportionate under the norms controlling initiation of military force (jus ad bellum). Pursuant to the jus ad bellum, a state’s use of force in self-defense in response to an armed attack must be both necessary and proportionate.

Earlier this month, Professor Adil Haque of Rutgers Law School published an essay in Just Security arguing that proportionality in self-defense requires balancing the use of force against overall civilian harm, irrespective of a state’s legitimate aims in resorting to defensive coercion. Based on this premise, the author asserted that Israel is legally compelled under the jus ad bellum to either accept an immediate ceasefire or dramatically scale back its operations in Gaza.

In response, I pointed out that civilian harm mitigation is generally a function of proportionality under LOAC and that a benefit-harm calculus does not accurately reflect established customary law on national self-defense. I advocated for clarity in discussions around proportionality, both as to its distinct legal meanings in different contexts and as to the differentiation between legal requirements and moral imperatives.

The author’s reply effectively doubled down on the original essay’s contention that a version of self-defense proportionality balancing the expected benefits of a military campaign against the civilian harm it causes constitutes settled international law and can be applied now as a legal mandate binding on Israel. It cited to states’ criticisms of Israel in United Nations (UN) Security Council debates regarding previous Israeli campaigns in Lebanon and Gaza since the mid-2000s as establishing “good law” on self-defense proportionality.

I believe this exchange may be indicative of a wider trend regarding interpretation of the jus ad bellum in academic circles that, if not effectively countered with clear explanations of state practice, threatens to enervate self-defense as a meaningful doctrine to inhibit aggression by both state and non-state actors. In particular, three main points of clarification are essential for analyzing self-defense proportionality in the context of the Israel-Hamas war:

I. Differentiating current law from aspirational law

At the outset, it bears emphasis that the traditional formulation of self-defense proportionality as weighing the magnitude and intensity of defensive coercion against the nature of the threat being faced is far and away the leading accepted articulation of the law by states and experts alike. In a comprehensive post at EJIL: Talk!, Marko Milanovic expressed deep sympathy with a more constraining version of the jus ad bellum but acknowledged its legal shortfalls:

Legally, however, the critical problem is that states (and scholars) have not unambiguously endorsed this third, balancing conception of proportionality in the law of self-defence. Many leading experts of the jus ad bellum deny that this type of proportionality, even if morally required, is also required legally (cf. Rule 72 of the Tallinn Manual 2.0, with the International Group of Experts not even noting the possibility that this type of proportionality is required). I certainly know what the law should be in this regard, but it is difficult to make the argument that the law already is where it should be.

In further discussion on this topic with Marty Lederman, the author clarified that the “bottom line” is “Israel can certainly argue that its actions are compliant with SOME version of proportionality, e.g. ends-means, and it’s difficult to argue that, as a matter of law, that version of proportionality is not the right one.”

Professor Milanovic’s essay referenced the seminal article on this topic by David Kretzmer, which discusses three types of self-defense proportionality: (1) “tit for tat,” or countermeasures balancing defensive force against the offending force; (2) a “means-ends” test balancing defensive force against legitimate aims; and (3) “narrow proportionality” balancing the benefits and harm of defensive force. Many scholars clearly favor the third option, while also readily admitting that the second option is likely the most descriptive in terms of the current state of the law.

International law certainly evolves over time based on state practice, but that does not provide license to characterize aspirational notions of the law as presently binding or applicable.

A 2018 Chatham House research paper, focused on presenting “existing law” to policymakers, described the prevailing ad bellum proportionality rule as tying the force used in self-defense to the precipitating “attack and to the aim of averting or stopping the attack.” It discussed incidental harm to civilians entirely within the LOAC context.

II. Differentiating legal practice from policy advocacy

Lawyers can and should be holistic counselors who incorporate moral and policy considerations into their legal advice. However, we have a corresponding obligation to distinguish legal and non-legal requirements, with the former often defining the outer bounds of acceptable conduct within which the client can operate and make informed choices.

For those advising commanders amidst time-sensitive decisions imperiling the lives of soldiers and civilians alike, accuracy and pragmatism are the attorney’s currency. (In that regard, it is worth mentioning that matter-of-fact assertions that Israeli forces are violating LOAC proportionality as a preliminary determination, without knowing more about the targeting process and the contemporary situation that informed it in specific cases, would be a frankly untenable approach to many military commanders and their legal advisors.)

In contrast to battlefield assessments, decisions regarding a state’s resort to armed force are typically made at the highest levels of foreign policy. Legal practitioners advising policymakers on self-defense options in response to an armed attack are operating in an environment where the victim state’s fundamental purpose of protecting its citizens is at stake.

Whether or not Article 51 of the UN Charter applies to armed attacks by non-state actors, that Article’s very presence is both a recognition and a reminder that states which are attacked will defend themselves, lest they forsake their reason for existence. The self-defense Article, moreover, enshrined existing customary law rather than diminishing it.

International law does in fact constrain state responses to aggression, but not in a way that will necessarily prove morally acceptable to all observers. National security lawyers endeavoring to factor international and domestic legal considerations into these matters can be quickly sidelined if their advice is not grounded and actionable. Determinations that a course of action is legally prohibited (as opposed to inadvisable for any number of reasons) must be rock-solid.

III. Differentiating legal proportionality from moral proportionality 

It is axiomatic that states make international law, and thus UN Security Council debates are clearly relevant and significant. However, it is difficult to make sense of the argument that state criticisms of Israel in which the term “disproportionate” is used in myriad contexts make settled customary law that can be applied in the here and now.

For example, in the 2009 Security Council debate over Israeli operations in Gaza, some of the condemnations of Israeli actions as disproportionate explicitly referenced LOAC norms as opposed to the jus ad bellum, whereas others appeared to express moral outrage as much as a legal assessment.

From a practical and historical perspective, there is also the reality that if the frequent criticisms of Israeli conduct at the UN were the ultimate touchstone of international law, then established customary law could essentially consist of a lengthy compendium of such denunciations.

Debates in the UN Security Council may provide compelling evidence of states’ positions on the matters under review, but to automatically prioritize critical statements uttered there over actual state practice and other formal explanations of associated responsibilities would seem to belie the very essence of opinio juris as how states act out of legal obligation. For example, in 2014 the Israeli government expressly detailed its views on proportionality in self-defense:

Israel’s use of force was also proportionate, given the need to repel the attacks and reduce the continuing threat posed by Hamas and other terrorist organizations operating out of the Gaza Strip. Israel responded to rocket fire throughout the month of June 2014 and in early July 2014 with limited strikes using precision-guided munitions. However, when faced with an escalation of violence that culminated on July 7 with more than 60 rockets fired at Israel in a single day, the Government of Israel was compelled to expand its use of force. Hamas’s continuation of rocket and mortar fire against Israel throughout the 2014 Gaza Conflict demonstrated the need for Israel’s sustained military action.

In this context it should be stressed that the number of civilian casualties of the adversary in the context of a military operation does not necessarily suggest that military action was disproportionate. Rather, under the Law of Armed Conflict, the proportionality of force used in self-defence depends upon the amount of force required to repel attacks and eliminate the continuing threat. Figures regarding the number of casualties (either looked at as a whole or as compared to losses incurred on the other side), do not, in and of themselves, point to a disproportionate use of force. In responding to Hamas’s attacks, Israel used no more force, for no longer a period than necessary to accomplish its objective: protecting Israel from incessant, illegal terrorist attacks.

As Professor Milanovic noted with respect to the Israeli position, “that’s clearly self-defence proportionality, and the ends-means (no more than necessary) version thereof.”

There’s no doubt that Israel’s actions have more detractors than supporters within the international community, but these kinds of reports assembled by state ministries provide considerably more detailed explanations of their perceived legal duties than varied uses of the word “disproportionate” among a litany of critiques by other states.

In a similar vein, one of the reasons why official guidebooks and digests like the United States Department of Defense (DoD) Law of War Manual are so important is because they enable states to spell out their understanding of legal requirements in considerable depth.

The DoD, for example, is on record in the Manual as interpreting self-defense proportionality in the mainstream sense of “weighing of the contemplated actions with the justification for taking action.” It states that “the proportionality of the measures taken in self-defense is to be judged according to the nature of the threat being addressed.”

As such, “assessing the proportionality of measures taken in self-defense may involve considerations of whether an actual or imminent attack is part of an ongoing pattern of attacks or what force is reasonably necessary to discourage future armed attacks or threats thereof.”

So long as Hamas continues to hold hostages and fire rockets into Israel, it is exceedingly difficult to determine, as a matter of the jus ad bellum, that Israel’s use of force in Gaza is disproportionate to its legitimate end of eliminating the ongoing threat posed by Hamas after the October 7 terrorist attacks.

As John Norton Moore has written, in a definition cited in Israel’s 2014 report, self-defense proportionality “is best understood as a requirement that responding coercion must be limited in intensity and magnitude to what is reasonably necessary to promptly secure the permissible objectives of defense.”

The contrasting vision of narrow proportionality advanced by an increasingly vocal minority of legal theorists may or may not be, to quote Professor Milanovic, “the kind of proportionality that we need” to reduce overall civilian suffering. But it is emphatically not the version of self-defense proportionality that we have.

For better or worse, the jus ad bellum does not do the work those demanding an immediate ceasefire in the Israel-Hamas conflict may want it to do. Accordingly, Professor Milanovic concludes that “this is one of those cases in which lawyers should not overly emphasize the law’s relevance or importance. This is one of those cases in which, in my view, ethics provides a clearer answer than the law.” Professor Lederman gets to the heart of the issue when he says “moral,” not customary or Charter, proportionality is what this debate is really all about.

For national security lawyers charged with guiding decisions around state uses of force and how they are conducted, the clarity of this distinction between actionable legal advice and advocacy for legal change is paramount.

The Israel Defense Forces, as in previous military campaigns, are on the proverbial clock as they seek to degrade and destroy as much of Hamas’s offensive capabilities as possible before international outcry spurs Israel’s allies to force its hand to the bargaining table—but this clock is political, moral, and strategic, not legal. Proportionality cannot square the circle to prevent victimized states from applying and sustaining overwhelming force in self-defense.

About the Author:

Lieutenant Colonel Charles Kels is a senior attorney for the Department of Homeland Security and a judge advocate (JAG) in the U.S. Air Force Reserve. His views do not reflect those of the Air Force or Departments of Defense or Homeland Security.


The views expressed are those of the author and do not necessarily reflect the official guidance or position of the the Department of Homeland Security (DHS), the U.S. Air Force, or any other part of the Department of Defense (DoD) or the U.S. Government (USG).  The appearance of external hyperlinks does not constitute endorsement by the USG or any other entity of the linked websites, or the information products or services contained therein. 

The views expressed by guest authors do not necessarily reflect my views, those of the Center on Law, Ethics and National Security, or Duke University. See also here. 

Remember what we like to say on Lawfire®: gather the facts, examine the law, evaluate the arguments – and then decide for yourself!

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