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Understanding a Nation’s Right to Defensive Force During Turbulent Times

About the Author: Dr. T. Markus Funk is a former federal prosecutor, USDOJ Section Chief in Kosovo, and law professor at institutions including Oxford University (where he obtained his PhD) and the University of Chicago. Markus is the author of a number of books and scholarly articles on self-defense and international law, including Rethinking Self-Defence: The 'Ancient Right's' Rationale Disentangled (Hart Publishing, 2021), as well as Victims' Rights and Advocacy at the International Criminal Court (Oxford University Press, 2nd ed. 2016). Now in private practice with the international law firm of PerkinsCoie LLP, you can reach Markus at or follow him @TMarkusFunk1. Portions of this article are adapted, with permission, from a shorter piece published by The Hill.

U.S. Ambassador Bathsheba "Sheba" Nell Crocker joined the Ambassador of Israel to condemn the Hamas terrorist attack in front of the United Nations. Photo by United States Mission Geneva available here.

When, how, and why a nation has the right to deploy deadly defensive force is one of the most widely discussed and passionately debated legal questions of the day. On October 22, 2023, President Biden joined leaders from Western-allied countries reaffirming Israel’s right to deploy deadly defensive force against Hamas, while also emphasizing the need to protect civilians. China later followed suit by similarly endorsing Israel’s “right to self-defense.” However, other nations, as well as certain U.N. personnel, disagree with these claims of justifiable defensive force.

As the current situation highlights, any exercise of trans-border force is bound to ignite fierce, frequently ideologically driven debates among the public, as well as legal scholars and the governments they advise. Unfortunately, the prevalent tendency among scholars to overcomplicate international law topics, including the subject of a nation’s right to self-defense, does a disservice to the general public during a time when clarity is needed more than ever.

There will of course always be competing factual disputes over whether certain combatants are using force to save life, as opposed to taking life. But the underlying international law of self-defense is not as complex as common academic treatments, diplomatic discourse, or media appearances by the legal commentariat portray it to be.

Moving from the general to the specific, the best way to understand when a country can rely on justified defensive force under customary international law and the United Nations Charter is through the lens of the more relatable and intuitive individual right to self-preservation under domestic U.S. state law.

The United Nations Charter’s Default Prohibition Against the Use of Force

Often referred to as the “ancient right” or the “first civil right” in the context of individuals, the practice of the ancient tribes - and later nations - concerning the use of defensive force has a long and tangled history.

A country’s right to self-defense formally developed through customary practices between nations. Today, the right of nations to engage in self-defense is formally enshrined in the United Nations Charter.

The Charter is similar to US domestic self-defense law in that both seek to reduce overall societal violence. Indeed, Article 2(4) of the Charter explicitly prohibits the use of threatened or actual force against the territorial integrity or political dependence of any state. The Charter’s drafters, in setting out this default presumption against force, echoed the International Military Tribunal at Nuremberg's 1946 admonition that initiating an aggressive war is "not only an international crime; it is the supreme international crime."

The UN Charter's Significant Self-Defense Carve-Out

Although Article 2 creates this broad prohibition against the use of force, Article 51 permits a nation to use defensive force "if an armed attack occurs." The right to deploy defensive force, moreover, is not limited to the defending state's territory; though like all things international law not a matter of universal agreement, the prevailing view is that it can also be used to prevent or stop acts of violence against the emanations of the state, such as embassies and armed forces.

Further, customary international law, including UN Security Council Resolution 1373, provides that a state can exercise the right to self-defense against non-state actors such as terrorist groups operating in the territory of another state. That said, it must be evident, and provable, that the recalcitrant state is unable or unwilling to deal with the non-state actors itself.

Finally, even after a nation engages in what it claims to be a defensive force, Article 51 requires the defending nation to report "immediately" such use of force to the UN Security Council so that the Security Council can take measures aimed at restoring international peace and security.

Understanding Article 51 and the Customary International Law of Self-Defense Through the Domestic Lens

Overview of the Common Self-Defense Elements. An straight-forward way to understand the scope of self-defense under international law is to examine how US states treat self-defense claims. As we will see, the core elements of self-defense under US state law – which, contrary to the received wisdom in many media outlets and institutions of higher learning, in fact is representative of the majority of domestic self-defense laws worldwide – mirror those found in the UN Charter and the customary international law governing nations, after which the Charter is modeled.

More specifically, under both US state self-defense laws and international law, an individual or country can successfully raise a self-defense claim by establishing that (1) the defender was the subject of an unprovoked violent attack; (2) the defender used an objectively reasonable degree of force to thwart the attack; and (3) the defender's fear of serious bodily injury or death was both honestly held and objectively reasonable. We examine each of these basic elements in the following section.

The Attack. Both domestic and international law require that the defender, whether individual or nation, subjectively (that is, honestly) and objectively (that is, reasonably) believe they are facing an unlawful (unprovoked) violent attack. The attack, moreover, must involve the use of armed force or violence, rather than mere economic damage.

Necessity. Under both domestic and international law, the element of "necessity" makes defensive force available only when the central bodies - whether the coercive machinery of the justice system or the United Nations - cannot offer the necessary protection, and no other means of protection is available. The moral principle animating this element is that all human life, even the life of a violent criminal or of individuals engaged in terroristic violence, is valuable and deserves protection, except in circumstances where the defender has no alternative but to resort to defensive force.

In the domestic context, the less able the police are to protect citizens, the stronger the justification becomes for an act of self-defense being, as a matter of fact, necessary. The same is true in the international context, where many observers have become skeptical about the UN Security Council’s ability to effectively defend nations against attacks.

Timing/Imminence of the Attack. Turning to the always-controversial issue of timing of the defensive force, both US domestic and international law require the attack to be either occurring or about to occur (that is, the attack must be "imminent").

Timing, Part II: Anticipatory Self-Defense. Although responding to an ongoing unjustified attack is largely uncontroversial, intense debates persist in both domestic and international legal spheres over what exactly constitutes an "imminent" (that is, an "about-to-happen") attack. Yet, there is broad agreement that the criterion of imminence must be interpreted against the backdrop of the type and seriousness of the threat posed. Put differently, the more dangerous the threatened attack, the more proactive the permitted defensive action.

Some states, including the governments of the US (in the context of the invasions of Iraq and Afghanistan), Russia (regarding the invasion of Ukraine), and Israel (regarding, among other situations, the Second Lebanon War) favor immediate, proactive self-help when they deem it necessary. They, accordingly, have called for a broader reading of Article 51's permission to use defensive force.

Under their more expansive interpretations of the Article, countries are permitted to engage in anticipatory self-defense (alternatively called "preemptive self-defense," "preventative war," "interceptive self-defense," "incipient self-defense," and "precautionary self-defense") against what they consider terrorist groups and the rogue states that sponsor them. Under their reading, defensive force is justified even when no formal armed attack has occurred or provably will occur in the immediate future.

Representatives of these countries will also point to the reality that all self-defense, whether between countries or individuals, is by definition "anticipatory" in that its aim is to avoid future harm. In the domestic context, after all, the defender need not wait until the attacker actually pulls the gun’s trigger or thrusts the uplifted knife down before deploying justified defensive force. They argue, with some legal and common-sense support, that any more restrictive reading of Article 51 would require states to engage in acts of suicidal abstention.

The 2002 US National Security Strategy argues for a broader interpretation of "imminence" this way:

For centuries, international law recognized that nations need not suffer an attack before they can lawfully take action to defend themselves against forces that present an imminent danger of attack. Legal scholars and international jurists often conditioned the legitimacy of preemption on the existence of an imminent threat – most often a visible mobilization of armies, navies, and air forces preparing to attack. (Emphasis added)

But this broad interpretation of the international self-defense authorization, which also evokes the US Supreme Court's 1921 admonition in Brown v. U.S. that "detached reflection cannot be demanded in the presence of an uplifted knife,” predictably also has its vocal critics.

Led by prominent proponents, including the International Court of Justice, advocates for a more restrictive view of Article 51 claim powerful states – that is, those states most capable of launching effective cross-border attacks – are prone to abusing the right. They argue for restrictive legal prerequisites, including mandates that threatened/perceived attacks must reach a certain level of gravity and that the country claiming self-defense must identify the source of the threat by clear and convincing evidence.

Parallels Between International Law and Domestic Battered Intimate Partner Situations. The US government's call for a greater leeway to engage in anticipatory self-defense finds interesting parallels in the arguments advanced in the domestic battered intimate partners context. The debate in this domestic arena focuses on whether battered intimate partners should receive broader justification for deploying defensive force against their tormentors.

The traditional rule, still operative in most domestic contexts, was that the battered intimate partner has the option of going to the police or moving out, rather than killing, say, the sleeping drunk and abusive partner. Stated differently, the conventional/prevailing view is that this kind of purportedly anticipatory defensive force used by the battered partner is not, as a matter of fact or law, necessary because other options for avoiding perceived future harm are available. Further, critics of providing broader leeway contend that the anticipated attack from the sleeping intimate partner, if it happens at all, could be hours away, meaning that the attack is also not imminent.

The counterargument is that this traditional understanding of battered intimate partners fails to adequately recognize the complex dynamics of violent relationships and that jurors, therefore, should have the benefit of expert witness testimony about abuse.

Deploying an analogous line of reasoning, those nations wishing to use force against, for example, terrorist groups located in another country are confronted with calls that they should be required to first bring their claims to the UN Security Council. If they do not, so the argument goes, their use of force in all but the most extreme cases should be deemed unnecessary under international law. Those in favor of such a limitation of defensive force will, as a fallback, also claim that, as with the sleeping abusive spouse, there is insufficient tangible proof that, for example, the terrorist group is as a matter of fact about to launch a terror attack, thus also negating the imminence requirement.

In both domestic and international cases, the perceived attack may not technically be imminent. But the core of the counterargument is that a failure to act immediately may make such an attack inevitable and significantly increase its likelihood of causing more serious damage. Further, those favoring a broader defensive right will point out the unfortunate reality that a resort to the authorities (in the case of a nation, the United Nations Security Council; in the case of an individual, the police) may be technically possible, but in the real world it is unlikely to be effective under the exigencies of the moment.

Proportionality. Finally, under both domestic and international law, the force used, taken as a whole, must not be excessive in relation to the need to avert the attack or bring it to an end. The consequences of the force used, moreover, must not be disproportionate to the anticipated harm. Representing an alternative perspective on necessity, this requires that the level of force used must not be greater than that of the force necessary to end the attack or remove the threat.

“Innocent shields.” President Biden and European Commission President Ursula von der Leyen have joined many other world leaders alleging that Hamas uses civilians as human shields by embedding themselves in civilian communities and taking civilians hostage. Though the details may in some circles be subject to dispute, what is not controversial is that an aggressor’s use of innocent shields is prohibited under domestic and international law and constitutes a serious war crime.

Though subject to varying interpretations, the balance of authority, moreover, holds that the presence of human shields does not prevent defensive force. Rather, an aggressor’s use of innocent human shields factors into the above-referenced proportionality calculations — the defending country must take “active precautions (that is, do everything feasible under the circumstances) to avoid harming the civilian shields. If, however, the civilians are found to have used their civilian status to impede the defender’s actions they, like colluding shields in the domestic context, lose these protections.

Parting Thoughts

Claims of self-defense, whether made by individuals or by countries, are inherently fact­ intensive, often evoking strong emotional responses. Commentators, including those opining on the present deteriorating geopolitical climate, unfortunately tend to overcomplicate the discussions on the international law of self-defense. Whether because of nature or of academic nurture, they too frequently develop the bad habit of shrouding their analysis in layers of nearly impenetrable legal theories.

Never more so than in times of crisis, providing the public with an uncluttered explanation of foundational rights, and as relevant here the right to self-preservation under international law, is critically important. Drawing out the intuitive parallels between international and domestic self-defense, and in so doing avoiding unnecessary legal formalism, represents a step in the right direction.



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