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States generally put more stock in each other’s capabilities and actions than in their declared intentions. Still, a state’s nuclear policies and forces require rationales to guide them. Declaratory policy articulates such rationales and intentions to one’s population and defense establishment, and to adversaries and allies, reflecting when the government thinks it could be prudent, effective, and justifiable to use nuclear weapons. Even if decisionmaking on capabilities sometimes has a logic of its own, declaratory policy should guide the acquisition and posturing of forces and the reduction of unnecessary capabilities.

There is no perfect or nonproblematic declaratory policy. It may be tempting to vaguely set a low threshold for when one would consider using nuclear weapons. However, experience shows that nuclear weapons do not deter all forms of aggression or coercion. If they did, additional states would want to acquire these weapons. Worse, overreliance on nuclear deterrence can create a strategic and moral hazard of decreasing leaders’ and polities’ resolves to prevent conflicts in the first place and to acquire conventional and other defenses to deter or defeat less-than-existential threats.

Because deterrence could fail, it would be folly to posit using nuclear weapons in situations where ensuing action-reaction dynamics would probably leave one (and one’s allies and partners) worse off than if nuclear weapons were not used. Conversely, it would be imprudent to promise not to use nuclear weapons when there might be no better alternative to doing so. The destructiveness of nuclear weapons and the uncertainty over whether and how escalation can be limited mean that nuclear use can be justified strategically and legally only when the threat to be deterred or stopped is proportionate to the likely consequence of their use. As the Department of Defense Law of War Manual declares, “the overall goal of the State in resorting to war should not be outweighed by the harm that the war is expected to produce.”1

Current Policy

Since 1994, NPRs have stated that the primary purpose of U.S. nuclear weapons is to deter nuclear attacks against the United States and its allies and partners, while contributing to deterrence of other extreme threats. According to the 2010 and 2018 NPRs, the United States “would only consider the employment of nuclear weapons in extreme circumstances to defend the vital interests of the United States, its allies and partners.”2

This formulation does not specify what interests are vital enough to warrant employing nuclear weapons in conflicts with Russia, China, North Korea, or anyone else, outside of a partial discussion of “extreme circumstances” or “vital interests” in the 2010 and 2018 NPRs. The 2018 review says extreme circumstances “could include significant non-nuclear strategic attacks,” including but not limited to “attacks on the U.S., allied, or partner civilian population or infrastructure, and attacks on U.S. or allied nuclear forces, their command and control, or warning and attack assessment capabilities.”3 The vagueness is purposeful, as the 2018 NPR states: “It remains the policy of the United States to retain some ambiguity regarding the precise circumstances that might lead to a U.S. nuclear response.”4

Ambiguity may help deter adversaries from actions less extreme than nuclear attack. It also spares presidents from making commitments based on hypothetical scenarios and preserves the flexibility to act based on the real-world situation at the time. Further, ambiguity can help alleviate allies’ fears that the United States might abandon them when they are under attack, or that Washington might entangle them in conflicts of its making.

Nevertheless, the ambiguity of “vital interests” or “extreme circumstances” paired with perceptions that the United States seeks offensive and defensive capabilities to preemptively attack adversary nuclear forces could make leaders and military commanders in Russia, China, or North Korea think that the United States would initiate nuclear use more readily than it actually would or should. More likely, Russia and China would use the arguably low declared U.S. threshold for first use to justify building up their countervailing nuclear forces. Either way, the result could make it more difficult to achieve crisis stability or arms control.

No First Use

In recent years, U.S. policymakers, encouraged by other countries and civil society groups, have considered whether to adopt a policy of no first use (NFU). In its most restrictive form, such a policy would pledge the United States to never use nuclear weapons first in a conflict.5 One reason for selecting this policy option is to reduce the salience of nuclear weapons nationally and globally.

Some of the most informed and thoughtful advocates of NFU argue that there are “few, if any” scenarios in which U.S. first use would constitute a credible threat.6 The authors agree with this line of thinking. However, allies, partners, adversaries, and U.S. policymakers understandably will (and should) focus on the word “few.” Does it mean that there are indeed some scenarios in which first use of nuclear weapons would be a viable last option for the United States to defeat an adversary’s strategic non-nuclear aggression or imminent nuclear attack? If so, what would Washington plan to do in these contingencies if it subscribed to NFU? For example, if North Korea, which has a handful of nuclear weapons that might or might not be able to penetrate U.S. missile defenses, were detected preparing to carry out orders to launch nuclear weapons against U.S. allies or the homeland, should the United States forswear the option of using nuclear weapons first to interdict such an attack if there was no other way to do so? Beyond the North Korean scenario, a few other hypothetical cases are evident, involving Russia and European allies and China and East Asian allies.

A no-first-use declaration without reduction of the weapons that are most tied to first use would be relatively meaningless to Moscow and Beijing.

Allies are an important audience for U.S. declaratory policy. They are more likely than adversaries to believe U.S. policy statements and plan accordingly. But allies are not uniform. Some may oppose any nuclear weapons use, particularly in and around their countries. Others may see NFU more broadly as a sign of U.S. withdrawal from its historic commitments to alliances. Still others—privately, at least—think NFU would weaken collective deterrence of Russia or China without securing any compromises or guarantees from them in return. Any consideration of declaratory policy change must involve sustained wide-ranging consultations with allies and examination of the significant potential for nuclear detonations occurring on or upwind from allied territory.

Perhaps more importantly, Russia and China, like the United States, pay more attention to capabilities than declared intentions. An NFU declaration without reduction of the weapons that are most tied to first use would be relatively meaningless to Moscow and Beijing. Yet the political capital that a president would expend to push NFU through the U.S. system and allied governments would leave little left to overcome traditional resistance to alter the force posture. The single most effective way to reduce destabilizing dynamics among the United States, Russia, and China, including risks of inadvertent escalation, would be to eliminate the types of weapons that increase predilections to undertake first strikes with or against nuclear forces. Silo-based ballistic missiles, especially those carrying multiple warheads, are prime examples of such first-strike capabilities. The political capital expended to persuade the U.S. system to accept NFU would be wasted if the ICBM force posture remained the same. This is one reason why changes in the U.S. force posture (as discussed in chapter 4) and an alternative formulation for a restrained U.S. declaratory policy are critical components of this assessment.

Sole Purpose

Another alternative is to declare that the “sole purpose” of U.S. nuclear forces is to deter or defeat adversaries’ uses of nuclear weapons. This formulation would leave open the possibility of employing nuclear weapons if it were the only way to preempt an imminent nuclear attack by a country like North Korea, for example. But if Russian or Chinese conventional forces were defeating U.S. and allied non-nuclear forces and threatening to inflict massive harm on their populations, then it would be imprudent to rule out proportionate use of nuclear weapons to stop them. Thus, a U.S. administration that adopted sole purpose would need to demonstrate that NATO and U.S. allies and partners in Asia were significantly improving their conventional military capabilities, their resilience, and their overall cooperation and cohesion.

Existential Threat Policy

Despite the flaws of alternatives such as NFU and sole purpose, U.S. declaratory policy should not remain the same. It needs to be clarified and made more consistent with international legal commitments made most recently by the Barack Obama and Donald Trump administrations.7 “If deterrence fails,” the 2018 NPR declares, “the initiation and conduct of nuclear operations would adhere to the law of armed conflict.”8 The threshold of “extreme circumstances” posited in the 2010 and 2018 NPRs is too ill-defined to adhere to the law of armed conflict.

We recommend that the United States should declare an existential threat policy (ETP). With this policy, the United States would consider the use of nuclear weapons “only when no viable alternative exists to stop an existential attack against the United States, its allies, or partners.” This formulation further defines “extreme circumstances” and “vital interests,” clarifying the magnitude of danger that would make it justifiable to counter with nuclear weapons.

Some ambiguity remains unavoidable—in this case, regarding what constitutes an existential attack. However, ETP would bring U.S. policy more in line with the law of armed conflict and demonstrate a more realistically restrained approach to the conduct of nuclear deterrence and war. This declared restraint could improve international security by encouraging U.S. and allied publics to deploy and rely on non-nuclear means to defeat all-but-existential threats.9

To clarify this thinking within the limits of a broad declaratory policy, the phrase “no viable alternative” is central to the concept, because the law of armed conflict requires that nuclear weapons employment must be necessary to defeat a given threat. If less destructive alternatives, such as conventional strike capabilities, are available, then they must be pursued first.10 Such an approach is prudent given the risks of escalation following any actor’s use of nuclear weapons. If there is no viable alternative, then the president and his or her advisers should be expected to prudently weigh political objectives they hope to achieve and choose a nuclear or non-nuclear attack accordingly.

The prudential threshold for nuclear use should be an aggression that threatens the existence—the viable functioning—of the United States or its allies or partners. No one, including the United States, would be wise and justified to use nuclear weapons in response to an injury that is less grave than a potential nuclear war. Obviously, nuclear attack on populations meets this criterion, as would a genocidal non-nuclear aggression. Other existential thresholds are harder to define. Any threat deemed existential obviously would meet the more ambiguous criterion of “extreme circumstances,” but lesser threats could too. Whatever criterion is used, it should involve threats that are proportionate to the existential risks of nuclear war.

A conventional occupation would be a complicated case. States have often survived such occupations, even by murderous regimes. In the most extreme example, Poland persisted as a state despite Nazi occupation in World War II and the loss of 15 to 17 percent of its 1939 population.11 If France or Poland had possessed nuclear weapons in 1939 or 1940, it is likely that a non-nuclear Germany would not have invaded either country. But if Adolf Hitler also had nuclear weapons, the probability of a German invasion would have been higher. And if a nuclear-armed Germany had invaded France or Poland, their decisionmakers would have faced excruciating choices. Depending on the number and yields of weapons in their possession, any initiator of nuclear use could have invited escalatory exchanges that could have left the victims of the initial aggression and Germany worse off than they were at the end of the war in 1945.

The Trump administration’s 2018 NPR highlighted the possibility of “significant non-nuclear strategic attacks” that could cause the United States to “not a priori rule out” using nuclear weapons.12 The administration provided some illustrative examples of such attacks, without clarifying a threshold of damage that must be met for the United States to consider a nuclear response. Well-connected observers suggest that a massively disruptive cyber attack on critical civilian infrastructure could meet the standard.13 Yet, cyber attacks are strategically attractive to adversaries and to the United States in part because they do not necessarily cause destruction or even irreversible damage, let alone widespread death.

To give a potential scenario, consider a cyber attack that was able to shut down the U.S. (and therefore international) economy on a scale commensurate to the economic damage wrought by the coronavirus pandemic. If the United States could with 99.9 percent certainty attribute the cyber attack to the Kremlin, would it be legally and strategically justified in responding to the attack by ordering a nuclear strike against Russia? What if China or North Korea were the villain in the same scenario? Nuclear retaliation would not stop the cyber attack or undo its damage, but it could trigger more death and destruction among belligerent and nonbelligerent nations alike. Even the threat to use a nuclear response to such a cyber attack against civilian infrastructure—one that did not cause damage commensurate with that caused by nuclear weapons—could “normalize” other states’ or nonstate actors’ use of nuclear weapons, including in response to U.S. cyber operations. Blurring cyber and nuclear thresholds also could encourage some states or nonstate actors to conduct “false flag” cyber attacks—for instance, by using leaked U.S., Russian, or Chinese malware—to catalyze conflict between the United States and Russia or the United States and China.

Would a non-nuclear attack that removed a government’s leadership pose an existential threat warranting nuclear retaliation? Intuitively, many would say “yes.” Certainly, leaders of nondemocratic governments who equate themselves with their state would. For that matter, if those leaders unleashed nuclear weapons to defeat such an attack, the United States and other victims of this nuclear action could be justified in using nuclear weapons in response. But law and common sense argue that nuclear use should be predicated on the scale of violence and destruction inflicted by an adversary, not merely on the “damage” to one’s own government.

Threats that do not harm societies on a scale proportionate to the destruction caused by even a limited nuclear war should be countered by non-nuclear means, even if procuring such means would be more costly than acquiring additional (or different) nuclear weapons.

Threats that do not harm societies on a scale proportionate to the destruction caused by even a limited nuclear war should be countered by non-nuclear means, even if procuring such means would be more costly than acquiring additional (or different) nuclear weapons. To try to invoke nuclear threats to deter such attacks amounts to a bluff and a moral hazard, insofar as belief in the power of nuclear deterrence could lead governments to avoid spending on more usable defensive capabilities. A society whose government was removed or whose constitution was subverted without massive destruction of life and property could survive and reform itself more readily than one whose population, resources, and infrastructure were decimated by nuclear war.

However, ETP should make clear that nuclear or extensive non-nuclear attacks on U.S. NC3 assets (on land or in space) could open the way for U.S. leaders to consider nuclear use. The judgment would depend crucially on whether the attacker was perceived to have the intention and capability to inflict further destruction on the United States and/or allies of a scale warranting nuclear response.

Critics may claim that ETP would invite Russia or China to act maliciously up to this declared nuclear threshold, which is higher than “extreme circumstances.” The prudent response here is to enhance non-nuclear defenses and NC3 rather than to expand the role of nuclear weapons, especially with the risks that regional conflicts could escalate inadvertently to nuclear war.

Of course, bolstering U.S. capabilities, whether nuclear or non-nuclear, often prompts alarm and countervailing reactions in Russia and China. To forestall these reactions and build international support for its position, U.S. declaratory policy should make clear its willingness to negotiate arms control and disarmament arrangements that enhance stability for all concerned. Chapter 6 explores this arms control challenge in greater detail.

Ultimately, if the United States wishes to retain or restore its international leadership in a global nuclear order, its declaratory policy should be one that Americans and others would find (relatively) acceptable if other states adopted it. If other states adopted an existential threat threshold, the United States and the international community would judge this to be more acceptable than a lower threshold would be.

The Law of Armed Conflict

Recent administrations have clearly declared that the United States would conduct all nuclear operations in accord with the law of armed conflict, but have not elaborated why or how they would do so. These important issues require further discussion.

Legal considerations affect decisions regarding the instances when it would be legal and appropriate to employ nuclear weapons during a conflict, targets against which nuclear weapons may be employed, and the types of nuclear weapons to be used. The law of armed conflict is primarily intended to protect civilians from the conduct of war between states. Such considerations date from the Hague and Geneva Conventions (1899 and 1904, and 1929 and 1949, respectively). This body of law has evolved positively since World War II, even as it has been frequently breached—and not only by states known for their disregard of international law. American military and civilian leaders have recognized the importance of such laws and devoted considerable attention to inculcating them throughout the armed forces, including in U.S. nuclear planning.14 Among other considerations, the morale of U.S. personnel who would be asked to conduct nuclear operations requires confidence that they would be asked only to carry out legal orders grounded in international humanitarian principles.

Legal considerations also should inform the yields and targets of nuclear weapons, as well as their numbers. In principle, yields should be no greater than that necessary to destroy a legitimate target. To the extent that using high-yield weapons on a target produces civilian casualties and environmental harm (including fire and radioactivity) in excess of what a lower-yield weapon would have produced, it is extremely difficult to say how the users of those weapons would comport with the law of armed conflict. Further, if the total number of weapons in an arsenal is so large that their detonation would cause global environmental, agriculture, and refugee crises of catastrophic dimensions, how could their use ever be justified as necessary and proportionate?

In practice, the United States and other nuclear-armed states deploy weapons of various vintages and technical capabilities. Most U.S. and Russian warheads, for example, were designed (and in the case of the United States, built) decades ago and have enormous destructive power to compensate for the relatively inaccurate delivery systems on which they originally were deployed. (High-yield warheads can destroy targets even when the weapons themselves do not precisely reach the desired aimpoint.) To replace now-overly destructive warheads with lower-yield but more precise ones would take some time and entail moderate costs. Critics would allege (fairly or not) that such technical changes amount to arms racing and planning for nuclear warfighting. Nevertheless, international law, environmental considerations, and common sense reinforce the basic principle that nuclear arsenals should include weapons with the lowest yields necessary to destroy legitimate targets.

Importantly, an arsenal and related policies that would be more likely to comport with the law of armed conflict could provide more credible and therefore more effective deterrence. A state that has worked through and publicly articulated why and how its declaratory policy is legal under the law of armed conflict presumably would be less self-deterred and its deterrent threats would therefore be more credible. This added credibility could inform adversary deliberations in deciding whether to escalate a conflict up to and including nuclear exchanges.

Several, perhaps contradictory, concerns arise from applying the law of armed conflict to the conduct of nuclear operations. Some argue reasonably that any use of nuclear weapons likely would lead to escalation in which the law of armed conflict would be violated. Thus, the established norm should be that any use of nuclear weapons would be illegal, which would help self-deter some actors. Paradoxically, however, such prohibitions could free other leaders and their populations from the progressive restraints that international law is designed to impose on the conduct of war. They could simply conclude that international law is irrelevant when it comes to situations as dire as nuclear war. This would be especially troubling to the United States and the United Kingdom, which have publicly committed to apply the law of armed conflict to their nuclear conduct. Rather than remove the restraint of law, civilization would benefit from persuading states to apply it and explain how they would do so. That said, the deterring potential of less restrained, illegal use of nuclear weapons would remain a material reality; the point is for governments to indicate in word and deed that they understand the reasons for restraints and to demonstrate as much as possible that they will uphold them.

To prevent the moral and strategic hazard of false adherence to the law of armed conflict, the United States and others, including nongovernmental organizations, should encourage further transparent international analysis and debate of these issues involving all nuclear-armed states.

The contrary argument is that any state that believes in the legal use of nuclear weapons will be more likely to unleash these weapons. This is related to the concern that states would be more likely to use “low-yield” weapons against legal military targets than they would be to use more destructive high-yield weapons. Their adversaries could then fear such use and be more inclined to preempt it with their own nuclear weapons. However, these adversaries—like U.S. leaders—should be expected to consider many factors beyond weapon yields in deciding when and how to use their nuclear weapons. In any case, the benefits of ignoring the restraining imperatives of international law are not likely to be greater than the risks of abiding by these imperatives, including the imperatives of proportionality and discrimination.

Instead of these concerns, the more likely risk is that states would profess to apply the law of armed conflict but would not change their nuclear forces and targeting doctrines to make them more consistent with this body of law. For example, states that plan to deliver high-yield weapons against targets in cities may be reluctant to reduce yields or change targets, especially as neither type of change is verifiable by outsiders. To prevent the moral and strategic hazard of false adherence to the law of armed conflict, the United States and others, including nongovernmental organizations, should encourage further transparent international analysis and debate of these issues involving all nuclear-armed states. Such analysis and debate should extend to whether and how states conducting nuclear war should be held accountable for the consequences, which would vary depending on weapon yields and targets.

This leads to the issue of positive security assurances. In 1995, the five permanent members of the United Nations (UN) Security Council, or P5, specifically committed to direct Security Council action if any NPT party is threatened with nuclear weapons, to “include efforts to settle the dispute and restore international peace and security as well as provision of technical, medical, scientific or humanitarian aid and compensation from the aggressor for loss, damage or injury from the attack.”15 In 1995, the P5 adopted this commitment in UN Security Council Resolution 984 to persuade non-nuclear-weapon states to agree to an indefinite extension of the NPT. This positive security assurance reflects nuclear-weapon states’ responsibility for the unintended consequences of nuclear use, and possibly for assisting unwitting victims of a nuclear conflict.

However, since the passage of Resolution 984, little effort has been made to develop contingency plans or other demonstrations of commitments to uphold its letter and spirit. Official records and the security studies literature show scant evidence that any nuclear-armed states have planned and developed capabilities to conduct such preventive interventions in conflicts to which they are not otherwise a party. Nor have nuclear-armed states said anything about compensation for any loss, damage, or injury that belligerent or nonbelligerent nations might suffer from nuclear war. Many of the roughly 150 states that do not rely on nuclear deterrence—directly or through extension from allies—find this state of affairs deeply unjust.16 This lack of action has heightened their antipathy toward nuclear-weapon states and made them more likely to support the Treaty on the Prohibition of Nuclear Weapons, which has clear provisions for assisting victims of nuclear attack.

The recent tradition of “P5 solidarity” on such issues has achieved little of value. If Russia, China, or other states are unwilling to clarify commitments they made to non-nuclear-weapon states in 1995, it is in U.S. interests to expose their reticence. All nuclear-weapon states bear responsibility to address the very real potential costs of nuclear use.

The United States should propose that the P5 declare, as the United States and the United Kingdom do, that they would adhere to the law of armed conflict when conducting nuclear operations. If the other members of this group—the five nuclear-weapon states under the NPT—refuse to do so, the international community deserves to know why.

The United States should also continue its 2018 NPR negative security assurance: the United States “will not use or threaten to use nuclear weapons against non-nuclear weapons states that are party to the NPT and in compliance with their nuclear non-proliferation obligations.”

The United States should affirm its willingness to offer a positive security assurance to protect and marshal assistance to non-nuclear-armed states that are nonbelligerents in any conceivable scenario in which U.S. nuclear use may occur.

Notes

1 Department of Defense Law of War Manual, 86.

2 See “Report on Nuclear Employment Strategy of the United States, Specified in Section 491 of 10 U.S.C., U.S. Department of Defense,” Department of Defense, June 12, 2013, p. 4, https://fas.org/wp-content/uploads/2013/06/NukeEmploymentGuidance_DODbrief061213.pdf; and 2018 NPR, 21.

3 2018 NPR, 21. “The United States would only consider the employment of nuclear weapons in extreme circumstances to defend the vital interests of the United States, its allies, and partners. Extreme circumstances could include significant non-nuclear strategic attacks. Significant non-nuclear strategic attacks include, but are not limited to, attacks on the U.S., allied, or partner civilian population or infrastructure, and attacks on U.S. or allied nuclear forces, their command and control, or warning and attack assessment capabilities.” For “clarification” of this statement, see Bureau of Arms Control, Verification and Compliance, “Strengthening Deterrence and Reducing Nuclear Risks,” 1. See also George Perkovich, “Critiquing the State Department’s Nuclear Posture Clarification,” Carnegie Endowment for International Peace, May 6, 2020, https://carnegieendowment.org/2020/05/06/critiquing-state-department-s-nuclear-posture-clarification-pub-81722.

4 2018 NPR, 22.

5 Some analysts privately suggest that preemptive nuclear use may be permitted under a U.S. NFU policy in certain limited circumstances. For example, if the United States received intelligence information warning that an adversary was preparing a nuclear attack against the United States, then the United States would be justified in using nuclear weapons to prevent that attack—if doing so was the only means available at the time.

6 Steve Fetter and John Wolfsthal, “No First Use and Credible Deterrence,” Journal for Peace and Nuclear Disarmament 1, no. 1 (2018): 102–14.

7 “Report on Nuclear Employment Strategy of the United States.”

8 2018 NPR, 23. The United States has been more transparent about its legal considerations than any other nuclear-armed state, and military lawyers are part of the targeting staff at the U.S. Strategic Command headquarters in Omaha, even as legal doctrine on these issues remains unsettled. See Air Force Operations and the Law, 2014, p. 300, https://www.afjag.af.mil/Portals/77/documents/AFD-100510-059.pdf.

9 For example, U.S. government lawyers often have relied on the contested doctrine of “belligerent reprisal”—which successive instruments of international law have weakened over the years. Lieutenant Colonel Ted Richard and Sean Watts, “The International Legal Environment for Nuclear Deterrence,” Just Security, March 27, 2017, https://www.justsecurity.org/39281/international-legal-environment-nuclear-deterrence/. A comprehensive review of the history of U.S. nuclear targeting and the law of belligerent reprisal is found in Theodore T. Richard, “Nuclear Weapons Targeting: The Evolution of Law and U.S. Policy,” Military Law Review 224, no. 4 (2016): 862–978. Belligerent reprisal is assessed in Anthony J. Colangelo, “The Duty to Disobey Illegal Nuclear Strike Orders,” Harvard National Security Journal 9, no. 2 (2018): 84–120; and Scott D. Sagan, “The Commitment Trap: Why the United States Should Not Use Nuclear Threats to Deter Biological and Chemical Weapons Attacks,” International Security 24, no. 4 (2000): 85–115, https://www.jstor.org/stable/2539316. For broader explorations of legal issues, see Valentin Jeutner, Irreversible Norm Conflicts in International Law (Oxford: Oxford University Press, 2017); Timothy J. Heverin, “Legality of the Threat or Use of Nuclear Weapons: Environmental and Humanitarian Limits on Self-Defense,” Notre Dame Law Review 72, no. 14 (2014), 1290; Richard Falk, “Nuclear Weapons, International Law and the World Court: A Historic Encounter,” American Journal of International Law 91, no. 1 (1997): 64–75, Colangelo, “The Duty to Disobey Illegal Nuclear Strike Orders”; Stuart Casey-Maslen, “The Use of Nuclear Weapons as a Reprisal Under International Humanitarian Law,” in Nuclear Weapons Under International Law, ed. Gro Hystuen, Stuart Casey-Maslen, and Annie Golden Bersagel (Cambridge, UK: Cambridge University Press, 2014), 182; and John Burroughs, “Looking Back: The 1996 Advisory Opinion of the International Court of Justice,” Arms Control Today, July/August 2016, https://www.armscontrol.org/ACT/2016_07/Features/Looking-Back-The-1996-Advisory-Opinion-of-the-International-Court-of-Justice.

10 Jeffrey G. Lewis and Scott D. Sagan, “The Nuclear Necessity Principle: Making U.S. Targeting Policy Conform With Ethics and the Laws of War,” Daedalus 145, no. 4 (2016): 62–74.

11 Adam Easton, “Poland Still Counts Losses From WW2 Invasion,” BBC, August 31, 2019, https://www.bbc.com/news/world-europe-49523932.

12 Bureau of Arms Control, Verification and Compliance, “Strengthening Deterrence and Reducing Nuclear Risks,” 1.

13 David E. Sanger and William J. Broad, “Pentagon Suggests Countering Devastating Cyberattacks on Nuclear Arms,” New York Times, January 16, 2018, https://www.nytimes.com/2018/01/16/us/politics/pentagon-nuclear-review-cyberattack-trump.html.

14 See for example, the Department of Defense Law of War Manual.

15 Judy Aita, “Security Council Gives Assurances in Case of Nuclear Attack,” U.S. Information Agency, April 11, 1995, https://fas.org/nuke/control/npt/news/950411-386876.htm.

16 Practical problems abound in the provision of effective security guarantees and legal accountability and redress. Each of the five primary security guarantors in the UN Security Council can veto calls for action by the council. They also have nuclear weapons and comparatively strong conventional military forces. If one or more of these nuclear powers were involved in the given crisis or conflict, they likely would veto proposals to use force or impose sanctions to compel them to deescalate and avert nuclear use. Other states could take preemptive military or political-economic measures such as sanctions, but it is difficult to imagine how these measures would influence events except in coalition with at least some of the permanent members.