Although the United States has, in recent years, unequivocally accepted the notion that international humanitarian law (IHL) applies to its nuclear operations, there’s a catch. To date, the U.S. government has not declared that it no longer reserves a purported right to target civilians by way of reprisal, in response to an unlawful attack against U.S. or allied civilians. As we have argued elsewhere, and as Adil Haque recently called on the Biden administration to do, it is time for the United States to acknowledge that customary international law today prohibits targeting civilians in reprisal for an adversary’s violations of the law of war. The Biden administration is conducting a nuclear posture review, which will provide an opportunity to clarify its position. Even without reliance on the doctrine of belligerent reprisal, the United States can credibly deter illegal attacks against civilians through responsive strikes that do not target the adversary’s civilians and that comply with IHL.

The Application of IHL to the use of Nuclear Weapons

The United States has not always been clear or consistent regarding the application of IHL to the use of nuclear weapons. For instance, in 1977, when the United States signed Additional Protocol I to the 1949 Geneva Conventions Relating to the Protection of Victims of International Armed Conflict (Protocol I), it declared that the rules established by the Protocol “were not intended to have any effect on and do not regulate or prohibit the use of nuclear weapons.” In the intervening decades, however, international consensus has emerged that IHL would apply to any use of nuclear weapons. By 2013, the Obama administration, explicitly embraced the notion that all nuclear weapons use plans “must …be consistent with the fundamental principles of the Law of Armed Conflict.” The United States reaffirmed, in the Trump administration’s 2018 Nuclear Posture Review, that any U.S. nuclear military operations “would adhere to the law of armed conflict.” This means that the United States accepts, among other IHL obligations, that its nuclear targeting policies must apply the principle of distinction (that it may not direct attacks against the civilian population or civilian objects), and the principle of proportionality (that it may not launch attacks that would cause incidental civilian harm that is excessive in relation to the concrete and direct military advantage anticipated from the attacks). The United States also accepts that it is legally required, under the principle of precaution, to take all feasible measures to minimize incidental damage to civilian populations and civilian objects.

And yet there is an enormous caveat lurking behind the seemingly categorial U.S. statement that it “will not intentionally target civilian populations or civilian objects” in its nuclear operations.  The United States has not to date foresworn the possibility that it might direct attacks against the civilian population, or otherwise launch attacks that cause disproportionate civilian harm, by relying on the customary international law doctrine of belligerent reprisal. This doctrine permits, “in exceptional cases,” acts that would otherwise violate IHL “when used as an enforcement measure in reaction to unlawful acts of an adversary.” Despite its relative obscurity, even among international lawyers, in 2016, then STRATCOM Judge Advocate General lawyer Theodore Richard argued that the doctrine of belligerent reprisal remains “an important part of nuclear weapon policy and deterrence theory.”

It is time for the United States to acknowledge that whatever the law may have allowed decades ago, it is no longer permissible under customary IHL to intentionally target civilians by way of reprisal. State practice and the associated expressions of legal obligation (opinio juris) support our position that it is no longer legally permissible to make civilians the object of attack with nuclear (or other) weapons by way of reprisal. Even a state that sustains an illegal attack against its civilian population must respond in a way that comports with IHL. The United States should end its “calculated ambiguity” about the doctrine of belligerent reprisal and make categorical its recognition that it may not intentionally target civilian populations or civilian objects.

The Customary International Law Roots of Belligerent Reprisal

The longstanding doctrine of belligerent reprisal provides a means of self-help by which states that are victims of IHL violations committed by their adversaries can, by engaging in proportionate violations of IHL, induce their adversaries to comply with the law. Belligerent reprisals thus are meant to promote compliance with the law of war. The doctrine is as controversial as it is longstanding, however. Apart from the prudential concerns that the use of belligerent reprisals may lead to escalating violence, belligerent reprisals have been morally condemned as a “barbarous” means of seeking compliance because “they are not directed specifically at the individual authors of the initial violation.” The scope of belligerent reprisals has accordingly been subjected to ever-tightening restrictions, aimed primarily at protecting vulnerable persons in wartime. The 1949 Geneva Conventions, for instance, expressly prohibit reprisals against persons protected by those Conventions, namely, sick and wounded soldiers in the field (art. 46); sick, wounded, and shipwrecked sailors (art. 47); prisoners of war (art. 13); and civilians who find themselves “in the hands” of enemy forces (art. 33). There is little doubt that the prohibition on reprisals against these classes of persons protected by the 1949 Geneva Conventions has acquired customary international law status (Rule 146).

Protocol I extended the prohibition on reprisals. Article 51(6) provides that “[a]ttacks against the civilian population or civilians by way of reprisals are prohibited.” The protection against reprisals in Protocol I goes beyond that in the Fourth 1949 Geneva Convention governing civilians because it is not limited to the treatment of those “in the hands” of a party to a conflict, e.g., persons in occupied territory, but applies to all civilians. Although the United States has not become a party to Protocol I, it has long accepted that key IHL provisions in that treaty – including the principles of distinction, proportionality, and precaution – reflect codifications of binding customary international law. These principles accordingly apply to all U.S. military operations, including nuclear operations.

The United States has in the past asserted that unlike the principles of distinction, proportionality, and precaution, the prohibition on reprisals against civilians in Protocol I was not a customary international law rule, and that the United States accordingly retains the right to target civilians by way of reprisal. In justifying this position in 1987, then-State Department Legal Adviser Abraham Sofaer stated: “To formally renounce even the option of such attacks, however, removes a significant deterrent that presently protects civilians and other war victims on all sides of a conflict.” The United States reiterated this position on reprisals in 1995 in its submission to the International Court of Justice (ICJ) in connection with proceedings in the Legality of the Threat or Use of Nuclear Weapons Advisory Opinion.

Belligerent Reprisals Today

Whatever the United States may have asserted twenty-five years ago, the prohibition on targeting civilians by way of reprisal today is a customary international law rule that applies even to states that are not parties to Protocol I. It consequently also applies to nuclear operations.

We recognize, of course that merely saying that customary international law prohibits belligerent reprisals against civilians does not make it so. At the same time, merely denying that the prohibition on targeting civilians by way of reprisal has acquired customary international law status does not resolve the matter, either. Following Michael’s Reisman’s characterization of the “ceaseless dialectic” by which international law develops, we must evaluate the legal status of the purported prohibition on belligerent reprisals against civilians by considering not only what states do, but also their assertions about what they and other states may or may not lawfully do.  What is striking in this regard is the utter absence of any support among states in recent history for the existence of a right of states to intentionally target other states’ civilians, even in reprisal for attacks against civilians by the adversary.

In its 2005 encyclopedic study on customary IHL, the International Committee of the Red Cross (ICRC) observed that while “the vast majority of States have…committed themselves not to make civilians the object of reprisals,” the ICRC could not yet conclude that a customary rule specifically prohibiting reprisals against civilians had “crystallized” because of “very limited” contrary practice. Yet the record cited by the ICRC is hardly sufficient to refute the existence of widespread and consistent practice informed by opinio juris – the standard for establishing a customary international law rule –  condemning reprisals against civilians. Part of the “limited practice” the ICRC refers to is the fact that when Protocol I was negotiated, five states – Egypt, France, Germany, Italy, and the United Kingdom – made statements that seemed to affirm the right of belligerent reprisal.  A careful reading of the record, however, reveals that the statements of most of these states – Egypt, France, Germany, and Italy – merely affirmed the right to react to serious and repeated violations of the law of armed conflict with means admissible under international law.  In view of the widespread consensus today about the illegality of reprisals against civilians, such statements hardly represent a clear assertion that these states believe that international law permits them to direct attacks against civilians. In any event, Egypt subsequently asserted before the ICJ in the Legality of the Threat or Use of Nuclear Weapons case that the prohibition on reprisals against civilians reflects customary international law (para. 43), and recent military manuals of France and Germany prohibit reprisals against civilians, citing Article 51(6) of Additional Protocol I.

The other form of practice that might give rise to doubts about the customary international law status of the prohibition on reprisals targeting civilians is the conduct of states in wartime.  Michael Newton, for example, refers to “the many instances of state practice … that would indicate recourse to reprisals in fact if not in phraseology.” But such practice is insufficient to undermine the customary international law character of the prohibition on reprisals against civilians unless the states that engage in such attacks claim that they are legally entitled to do so – that is, their practice alone does not evidence the other element required to create customary international law, opinio juris. The argument that reprisals against civilians are permitted under customary international because states sometime employ them is no more persuasive than the argument that customary international law permits targeting of medical facilities because warring parties sometimes target them, or that there is no customary international law prohibition against torturing prisoners because states sometimes torture prisoners.

The only recent international armed conflict in which states appear to have claimed the right to target one another’s cities in reprisal for such attacks by the other side is the “War of the Cities” phase of the 1980-88 Iran-Iraq war. Although neither Iran nor Iraq were parties to Protocol I, the international community hardly acted if it believed those states were legally permitted to target civilians by way of reprisal. To the contrary, the U.N. Security Council in Resolution 540 explicitly condemned “all violations of international humanitarian law” committed by the parties to the conflict and “call[ed] for the immediate cessation of all military operations against civilian targets, including city and residential areas”; in Resolution 598, the Council again deplored “the bombing of purely civilian population centres.” In its own unilateral statement, the U.S. State Department – rather than defending the right of states to target civilians pursuant to the doctrine of belligerent reprisal – said the United States “deplores the tragic and needless loss of both Iranian and Iraqi lives, especially through attacks on civilian populations” and urged both states “to respect their obligations under international conventions designed to mitigate the human suffering of warfare.” In short, there is widespread practice, backed by opinio juris, to support the notion that the prohibition of attacking civilians in reprisal has acquired customary international law status. We are unaware of a single instance in which a state has invoked the doctrine of belligerent reprisal as a basis for directing attacks against civilians that has not generated harsh condemnation by the international community.

A Persistent Objector?

Could the United States claim that it qualifies as a persistent objector to the now-crystallized customary international law prohibition on reprisal attacks against civilians?  The record of U.S. behavior of the past decades would not support such a claim. The most recent assertion by the United States that Protocol I’s prohibition on reprisals against civilians is not a customary international law rule dates back to its assertion before the ICJ in 1995 – over 25 years ago – that Article 51(6) of Protocol I was a “new rule” that had “not been incorporated into customary international law.”  Since then, rather than affirming the right to target civilians in reprisal, U.S. statements of nuclear targeting policy have categorically declared that the United States will not intentionally target civilians; the U.S. statement at the 2015 NPT Review Conference similarly declared that U.S. nuclear plans “will apply the principles of distinction and proportionality and will not intentionally target civilian populations or civilian objects,” with no caveats.

The Department of Defense Law of War Manual, updated in 2016, simply recounts Legal Adviser Sofaer’s 1987 U.S. assertion that the prohibitions on belligerent reprisals against civilians in Additional Protocol I are “counter-productive and that they remove a significant deterrent that protects civilians and war victims on all sides of a conflict.” But the Manual does not state expressly – as the United States has demonstrated it knows how to do in other contexts – that the prohibition on reprisals set out in Protocol I does not reflect or contribute to the development of customary international law.

Although there is no precise formula for determining how persistently a would-be persistent objector must object to an emerging rule, where a rule is clearly supported by an overwhelming majority of states, David Colson persuasively argues that “the persistent objector must continually make its position known to ensure that the law does not find tacit consent through a relatively short period of silence.…[T]he more isolated a State becomes in its legal perspective, the more active it must be in restating and making clear its position.” The United States record of objection to the prohibition on targeting civilians by way of belligerent reprisal falls far short of that standard.

Deterring Attacks Against U.S. and Allied Civilians

Some strategists share the fears of former Legal Adviser Sofaer that acknowledging the impermissibility of belligerent reprisals would remove a deterrent that presently protects American and allied civilians from unlawful attacks by U.S. adversaries. We doubt that adversaries will be more likely to launch attacks illegally targeting civilians if the U.S. declares that reprisals directed against civilians are impermissible. As we have noted, a nuclear posture that rules out the option of directing attacks against civilians would be more ethical than one that retains it; such a nuclear posture would be more likely to be followed by U.S. military leaders and thus would be more credible and more effective. Moreover, the proper application of IHL would still permit costly nuclear or conventional responses to any nuclear attack against U.S. or allied civilians, including attacks directed against legitimate adversary military and leadership targets. This prospect is more likely to deter the autocratic leaders of U.S. adversaries than would the threat of attacks against their civilians.

More fundamentally, a decision about whether to renounce the option of directly targeting civilians, even by way of reprisal, should not be driven only by strategic assessments about the impact on deterrence.  As we conclude in our article in the Spring 2021 issue of International Security:

[T]he fact that an adversary has violated the law does not provide a sufficient legal or moral reason to violate the rights of noncombatants merely because they are nationals of the breaching state. When North Vietnam tortured American prisoners of war, few argued that the U.S. morally should or legally could torture North Vietnamese prisoners held by U.S. forces. When ISIS bombed cafes in Paris, beheaded prisoners, or sexually abused American aid workers, few argued that the U.S. morally should or legally could bomb markets in Syria, behead ISIS prisoners, or sexually abuse women supporting ISIS. It would be appropriate for the U.S. to stop threatening to deliberately kill millions of innocent civilians, even in the name of deterrence, when it rightly no longer threatens to perpetrate similar illegal acts against individual innocent civilians.

In short, for legal, ethical, and strategic reasons, it is time for the United States to affirmatively recognize the customary international law prohibition on targeting civilians by way of belligerent reprisal.

IMAGE: WASHINGTON, DC – MARCH 08: U.S. President Joe Biden delivers remarks as Secretary of Defense Lloyd Austin listens in the East Room of the White House March 8, 2021 in Washington, DC. (Photo by Alex Wong/Getty Images)