Law of War Considerations In Fielding Nuclear Forces

September 2016

By Justin Anderson

The status of nuclear weapons within international law was a subject of intense debate during last fall’s UN General Assembly First Committee session. State supporters of the humanitarian initiative on nuclear weapons pressed for resolutions asserting the illegality of nuclear weapons and sought to build support for the near-term negotiation of a global ban on nuclear arsenals.

They were strongly opposed by the five states permitted to possess nuclear weapons under the nuclear Nonproliferation Treaty (NPT).1

At Offutt Air Force Base in Nebraska, U.S. Navy Admiral. Cecil D. Haney (right), U.S. Strategic Command commander, and other command members monitor a Minuteman III intercontinental ballistic missile test launch at Vandenberg Air Force Base in California on May 20, 2015. [Photo credit: Jim Lamar/U.S. Strategic Command]Such clashes are likely to continue and prove intractable, shedding more heat than light on the relationship between the law of war and nuclear forces. The five NPT nuclear-weapon states (China, France, Russia, the United Kingdom, and the United States) will continue to view nuclear deterrence as vital to the security of themselves and key allies for the foreseeable future. Inconclusive debates in international forums over the general legality of nuclear weapons obscure a more important question at a time when the NPT nuclear-weapon states are overhauling their nuclear forces and, in the cases of Russia and China, upgrading and diversifying: for those states permitted to possess nuclear weapons, how does the law of war apply to their development, fielding, and deployment of nuclear forces? 

This article will discuss how core principles of the law of war,2 which represents the specific body of law that is lex specialis to the conduct of armed conflict, including the means and methods used to wage war, can apply to or be integrated with the decision-making processes of these five states with regard to their development, fielding, and deployment of nuclear forces.3 It provides guidelines and suggestions rather than specific “rulings” or prescriptions because the application and implementation of the law of war is best carried out through a series of assessments that evaluate weapons, their delivery means, and the rationale for their potential employment.4 It is informed by legal interpretations and policy positions associated with the United States but offered in hopes of catalyzing a broader discussion on how the law of war, which is intended to reduce the suffering and destruction of all types of conflicts and to facilitate the “restoration of peace,”5 applies to all states that elect to field nuclear forces.

Law of War Principles

Many participants in debates on the legality of nuclear weapons have discussed them as different and separate from other types of weapons, focusing on their destructive power. Yet for the NPT nuclear-weapon states, these weapons and the forces that operate them are not separate from international law, including the law of war. The “Department of Defense Law of War Manual” states this succinctly and directly: “The law of war governs the use of nuclear weapons, just as it governs the use of conventional weapons.”6

As such, the core principles of the law of war apply to the development, fielding, and deployment of nuclear forces. The manual identifies three “interdependent” core principles within the law of war: military necessity, humanity, and honor. Humanity itself encompasses three key principles: distinction, proportionality, and the prohibition against unnecessary suffering. This article seeks to address several of these principles as they relate to leadership decision-making on nuclear strategy, posture, and potential employment of nuclear weapons, to the development of nuclear delivery systems and weapons, and to the military assessment process known as weaponeering. 

The principle of military necessity states that, in times of war, the use of force is acceptable and justified in order to defeat an opponent. It qualifies, however, that the use of force is not unlimited; the manner in which it is employed should seek to bring about the end of a conflict as quickly as possible and with the least amount of destruction necessary. Francis Lieber, whose 1863 law of war instructions to the Union Army became the foundational document for many later writings and guidance documents on the topic, described the principle: “Military necessity admits of all direct destruction of life or limb of armed enemies, and of other persons whose destruction is incidentally unavoidable in the armed contests of the war…. [Yet,] military necessity does not admit of cruelty…. It does not admit of the use of poison in any way, nor of the wanton devastation of a district…and, in general, military necessity does not include any act of hostility which makes the return to peace unnecessarily difficult.”7

In weighing the legitimate use of force in wartime and the imperative of winning “quickly and efficiently” against the imperative of fighting in a manner that is not “cruel” or “wanton” or makes a future peace impossible, military necessity is often linked with the principle of proportionality.8 The latter principle asserts that the use of military force should not be “unreasonable or excessive.”9 In terms of application, proportionality asks leaders and war-fighters to consider the first- and second-order effects, including the potential effects on civilians, of proposed military actions. If these outweigh the benefits of the suggested action—they do little to advance the cause of victory and could result in significant civilian casualties and damage to civilian property—they should be rejected. 

One way to think about the relationship between the principles of military necessity and proportionality is to consider the example of an adversary military base that is important to their continued operations against you or that houses forces that can cause you grievous harm. This base is a legitimate target and can be totally destroyed, but proportionality considerations may mitigate or change how a leader or commander plans to destroy this facility and what means or methods are used. If a range of means are available, proportionality considerations will lead to the election of an option that is less likely to cause collateral damage. 

Military necessity and proportionality are critically important principles for decision-making by national leaders and their military advisers, as they survey the entirety of a conflict and promulgate the strategies and orders whereby all elements of national power are brought to bear on an adversary. The two principles require leaders to make decisions weighing the imperative of winning the war and directing the campaigns or major operations required to do so against the risks of causing collateral damage and prolonging conflict. The difficulty of obtaining, processing, and communicating accurate information due to the “fog of war” is an acknowledged limitation to effectively integrating consideration of these principles into decision-making processes at the highest levels.10

Acknowledging and attempting to address the tension between military necessity and proportionality are important for all potential uses of force, but they are particularly critical when it comes to the possible employment of nuclear force. What types of information and processes can ensure a measured evaluation of courses of action that includes consideration of these law of war principles? 

First, an assessment factoring in military necessity and proportionality should not begin during a nuclear crisis or conflict. The stress of the situation will likely result in a focus on quickly addressing the most immediate or pressing threats at hand, with little to no time or attention available to consider the potential second-order effects or broader implications of decisions made in the breach. Regardless of how an NPT nuclear-weapon state chooses to integrate law of war considerations into its national leadership decision-making calculus, this is an argument in favor of undertaking this integration in peacetime, well before any situation arises where nuclear employment may need to be considered. 

Second, when taken together, the principles of military necessity and proportionality frame a limited, discrete space for considering potential nuclear weapons employment. In a geopolitical environment where the NPT nuclear-weapon states face significant threats, including those from weapons of mass destruction (WMD), it is possible to envisage scenarios where the circumstances are dire and the demands of military necessity could lead to the consideration of employing nuclear force to destroy rapidly and totally, if necessary, these threats. Yet, the principle’s dual requirement to consider whether the means employed make the return to peace difficult or impossible, buttressed by the admonishment to minimize collateral damage to the extent possible, raise the bar for nuclear employment very high. 

Informed Choices

Determining the parameters of this space and the height of this bar is not a single, stand-alone assessment. It requires a series of nested, interrelated assessments that provide information allowing decision-makers to make informed choices that take into account military necessity and proportionality. These assessments should be undertaken shortly after a new leader begins their term in office, should draw on the experience and expertise of the state’s national security community, and should be supplied to the leader and their most senior advisers. 

Missileers train in the missile procedures trainer at Malmstrom Air Force Base in Montana, home to 150 Minuteman III intercontinental ballistic missiles on January 21, 2008. It is one of three U.S. Air Force bases that maintain and operate Minuteman III ICBMs. [Photo credit: John Turner/U.S. Air Force]These assessments would contribute to the development of a nuclear strategy, and their purpose would be wider than the law of war. They would also, however, provide information vital to an evaluation weighing the demands of military necessity against the limitations of proportionality. One set of assessments would focus on the military threats, such as weapons of mass destruction, which might justify consideration of nuclear strikes. This analysis would provide critical context with regard to military necessity. In a geopolitical environment where the NPT nuclear-weapon states face these types of threats, it is possible to envisage scenarios in which circumstances could lead to the consideration of employing nuclear strikes to destroy an adversary’s WMD-equipped forces (or associated command and control infrastructure) in order to prevent or halt a devastating attack against themselves or their allies. Yet, the parameters of these major threats—How immediate might they be employed? How potentially lethal?—should be assessed in detail.

Another set of assessments should focus on the first- and second-order effects of each nuclear weapon within the arsenal and offer a comparison of these effects against those of the state’s most powerful conventional weapons. Detailed descriptions of the effects of nuclear detonations against different types of targets will likely impress on decision-makers the weighty importance of all decisions involving potential nuclear employment (histories and memoirs, for example, demonstrate this to be the case with regard to U.S. presidents who received briefings on nuclear war plans during the Cold War).11

It is important to emphasize, however, that these assessments will also reveal the enormous difference, in orders of magnitude, between megaton, kiloton, and subkiloton weapons. They will also reveal an enormous difference between “small” nuclear weapons and whatever weapon represents the arsenal’s largest conventional munition. An additional, related set of assessments should focus on when and where the weapon detonates, such as the height of burst of a potential nuclear strike and the impact of shifting the aimpoint, i.e., whether hitting the center or the corner of an adversary’s military base. Here too, the assessments will demonstrate that the effects of strikes will vary dramatically depending on variables such as whether a weapon is detonated on the ground, a short distance from the earth, or high in the atmosphere. Similarly, effects will differ based on where a target is hit. 

These various types of effects assessment are important to informing considerations of proportionality, communicating the incredible power of nuclear munitions and the various effects of nuclear detonations such as heat, blast, and radiation. Yet, they will also demonstrate that the range of these effects is radically different based on the size and type of nuclear weapon employed and when and where it is detonated. In addition, they will elucidate the massive gap between the destructive power of nuclear and conventional munitions. These differences could figure significantly within the cost-benefit comparison between military necessity and proportionality should a target or targets be identified that must be destroyed in order to secure victory, prevent a devastating attack, or both.12

Furthermore, these assessments should consider how the adversary may react, addressing important questions regarding conflict escalation and war termination. These assessments will grapple with human psychology, strategic culture, and other issues that differ from the physics and engineering calculations behind assessments of the effects of nuclear detonations. In addition to being critical to nuclear strategy development, consideration of these factors is important to fully addressing the requirements of military necessity, which allows the use of force against legitimate strategic or tactical objectives but does not permit “any act of hostility which makes the return to peace unnecessarily difficult.” 

These considerations could serve to illustrate that the law of war does not always provide definitive answers with regard to strategic dilemmas. A national military or nuclear command headquarters, for example, may be a legitimate target in wartime, but would destroying it actually result in conflict escalation, up to and including nuclear employment, either because an adversary is now bent on revenge, loses command and control over its own forces, or both? The necessary tension inherent to the principle of military necessity is meant to prompt leaders and commanders wherever possible to thoroughly assess their options and consider likely outcomes. Its evaluative criteria, however, may help define decisions regarding nuclear forces—where the requirements of defending your population or forces may be paramount at the same time that uncertainty regarding adversary intentions and capacity for escalation may be acute—without facilitating them, even if the desire to adhere to the law is sincere. 

The law of war permits the use of force, including massive force, to achieve victory, but also emphasizes that attempting to achieve victory at any cost, with no thought for the consequences to civilians, is unlawful and harms the prospects of achieving a lasting peace. Yet, the challenge in correctly calibrating the use of force, particularly when confronted with adversaries capable of causing massive harm, is significant, acute, and sometimes unappreciated by advocates and analysts of international law. 

The assessments described above are important to a government’s development of its nuclear posture and its declaratory policy—that is, when, where, and how it will employ nuclear force. In addition to addressing questions of strategy and policy, they provide information that allows leaders to integrate law of war considerations into critical decisions and guidance documents. 

Principle of Distinction

The principle of distinction requires that the use of force be directed against military targets and not civilians or civilian objects, such as civilian buildings; as described by Lieber, “the unarmed citizen is to be spared in person, property, and honor as much as the exigencies of war will admit.”13 As a general rule, the more powerful and destructive the weapon, the greater the care and caution in ensuring that its design and form of employment permit it to be employed in accordance with this principle. 

Some activists and commentators assert that nuclear weapons can never be employed in a manner that does not violate the principle of discrimination, often pointing to radioactive fallout as an uncontrollable effect of nuclear detonations that will sicken and kill civilians far from the original target. These assertions often appear to assume that all nuclear weapons and detonations are alike in terms of their effects. As discussed above, however, how a weapon is employed can significantly change its effects. So too does its design and the design of its delivery means. This places significant weight across the spectrum of decisions associated with the development, fielding, and deployment of a nuclear force, with questions regarding discrimination first applied during the initial research and design stage of delivery systems and munitions.

A critical decision relevant to a weapon’s design is its yield. In its commission and design of nuclear munitions, an NPT nuclear-weapon state should carefully assess how many high-yield weapons it requires because a larger weapon is more likely to raise issues with regard to proportionality. This is also the point in time to consider whether to invest in weapons that have a variable yield, sometimes colloquially referenced as “dial-a-yield,” in order to provide greater flexibility in the event of actual employment. In short, with regard to the weapon itself, an important means of calibrating its effects can begin with design decisions.

Discrimination of effects is only possible if the delivery of the munition is itself discriminate. So, it is critically important for an NPT nuclear-weapon state to invest in the technologies—some organic to the delivery system, some resident within supporting systems—to ensure that a delivery system directly strikes its intended aimpoint and does not go astray. In the contemporary era, this almost certainly demands secure and accurate navigational guidance provided by space systems, for example. The degree of fidelity required to ensure that a nuclear-capable delivery system is discriminate is greater than that required for less destructive conventional systems. 

The above factors can help an NPT nuclear-weapon state field delivery systems and bombs or warheads that have the potential to be discriminate.14 Just as important, however, is how these delivery systems are used and how these weapons might be employed in a potential armed conflict. This should lead to law of war considerations being integrated into a state’s weaponeering processes. Weaponeering is defined by the U.S. Air Force as “the process of determining the quantity of a specific type of lethal or nonlethal means required to create a desired effect on a given target.”15 It refers to the integration of a series of evaluations by specially trained military personnel that considers factors such as the effects of different types of weapons and various types of strikes, including the question of how many weapons are necessary, whether delivered singly, in groups, or in sequence, to neutralize or destroy a target. 

The assessments discussed in the preceding section with regard to the size of a weapon’s yield and the location of the detonation are also included as part of weaponeering. Other topics taken into account include weather, terrain, and a target’s passive defenses, such as whether it is hardened and deeply buried. All of the above can have a significant impact on the effects of any type of weapon, including nuclear weapons. Weaponeering is a process and a skill set, requiring time and investment in order to ensure it is effective and can fully integrate law of war considerations.16

A Sandia National Laboratories engineer prepares for an impact test of hardware in the nose assembly of a mock B61-12 nuclear bomb on January 28, 2015 in New Mexico. The new guided version of the B61 gravity bomb, scheduled for production between 2020 to 2024, is intended to strike targets more accurately and is reported to have a variable explosive yield that could reduce the radioactive fallout. [Photo credit: Randy Montoya/Sandia National Laboratories]Weapons design and weaponeering are highly technical and specialized fields. Particularly during a period of time when an arsenal is being overhauled, upgraded, and perhaps expanded, ensuring that these processes integrate law of war considerations is critically important. They also require a robust interaction or feedback loop with the assessments that inform decision-makers discussed above; the degree to which a weapons designer can make a weapons system accurate, for example, should impact decision-maker views on whether these systems can be used in a discriminate and proportional way. 

In turn, decision-makers should provide guidance for how investments are made that, in addition to strategic considerations, result in the development of an arsenal that can implement and adhere to the law of war. A weapon may appear to fail tests of discrimination and proportionality, for example, until it can be matched with a delivery system whose accuracy improves on its predecessors (a high degree of certainty in reaching a target, for example, can also lead to the use of a smaller munition). In addition, weapons may be redesigned, replaced, or retired because their yield or other aspects of their design may raise discrimination and proportionality issues in light of changes to a potential adversary’s force that alter calculations of military necessity. Moreover, as noted above, a significant gulf exists between the destructive power of conventional and nuclear weapons; greater investment in high-end conventional capabilities may allow future non-nuclear munitions to fulfill roles and missions currently assigned to nuclear forces. 

Again, these points do not necessarily rule out the potential employment of nuclear weapons because the need to have a type of force capable of rapidly and totally destroying existential threats may be high. They are arguments, however, in favor of taking steps to integrate law of war considerations into the research, development, and acquisition phases for nuclear-capable delivery systems and nuclear weapons and into the education, training, and processes required to develop effective weaponeering cells within the military services. 

Principle of Honor

The principle of honor is a foundational concept that binds together all individuals who take up the profession of arms. As the Law of War Manual notes, “Opposing military forces should respect one another outside of the fighting because they share a profession and they fight one another on behalf of their respective States and not out of personal hostility.”17

This respect influences the treatment of wounded or captured combatants and, more broadly, the conduct of war, whereby military personnel do not demonize their opponents, viewing and treating them as professionals. This concept of honor extends to all warfighters, including those tasked with operating nuclear-capable systems and carrying out nuclear missions. 

This suggests that one way in which to “thicken” the relationship between the law of war and nuclear forces is to encourage professional interaction between those members of the uniformed militaries of the NPT nuclear-weapon states that are tasked with the unique and weighty responsibility of maintaining nuclear forces and preparing to employ them if called to do so in a future war. Aside from the scientists and engineers that develop nuclear weapons, no other class of professional is so well aware of the effects of different types of nuclear weapons employment and, in turn, is deeply wary of the costs and consequences of any conflict between two nuclear-armed states. 

Increased communication and interaction between these highly specialized forces is not a law of war requirement, but it could bolster the relationship between the law of war and nuclear forces by cementing respectful, professional relationships between these warfighters. The NPT nuclear-weapon states have held an annual conference since 2009 to discuss issues of mutual concern; future conferences could include semiformal sidebar meetings bringing together officers from these five states who have experience in commanding or serving as part of the crew of nuclear-capable delivery systems. 

Litmus Test

Many contemporary commentaries on nuclear weapons and the law of war discuss these weapons within a vacuum absent of any context regarding their development, fielding, or deployment. This can lead to abstract or erroneous analyses that contribute little to understanding or implementation of the law. For the NPT nuclear-weapon states, nuclear weapons are legitimate tools of self-defense and will remain so until such time as all states-parties to the treaty can negotiate a comprehensive disarmament treaty. This does not mean that these weapons or military operations involving these weapons are somehow unregulated. Nuclear weapons and the forces that are “organized, trained and equipped,” in U.S. parlance, to operate them are fully subject to the law of war. This article has offered some thoughts on what this means for these five states with regard to implementation of this body of law. It suggests that this implementation requires integrating consideration of key law of war principles within the broader decision-making processes that develop, establish, and sustain a state’s nuclear strategy, posture, and arsenal, including narrowing and restricting what these forces target and how they might be employed. 

How the NPT nuclear-weapon states conduct this integration with regard to their current nuclear forces and ensure it plays an important role in ongoing processes to upgrade their arsenals may differ, but it represents an important obligation for all five. Moreover, it serves as a litmus test for this body of law as a whole, providing these states with an opportunity to demonstrate that all weapons, including the most powerful, are subject to the law of war. 


1.   July 2016 marks the 20th anniversary of the International Court of Justice (ICJ) opinion on the “legality of the threat or use of nuclear weapons,” which continues to reflect these debates in a number of ways. Although the court expressed skepticism that nuclear weapons could be employed in a way that did not violate the laws of war, it concluded that it could not rule these weapons illegal in every circumstance and stated that the court “cannot lose sight of the fundamental right of every State to survival,” reflecting nuclear Nonproliferation Treaty nuclear-weapon states’ arguments that the existential threat posed by other state’s nuclear weapons justifies fielding their own nuclear forces for the purposes of deterrence. Legality of the Threat or Use of Nuclear Weapons, I.C.J. Reports, July 8, 1996, pp. 40-41,  

2.  The law of war is often referenced as the law of armed conflict or international humanitarian law. The latter body of law is not identical to the law of war. As noted by Christopher Greenwood, ICJ judge, “International humanitarian law thus includes most of what used to be known as the laws of war, although strictly speaking some parts of those laws, such as the law of neutrality, are not included since their primary purpose is not humanitarian.” Office of the General Counsel, U.S. Department of Defense, “Department of Defense Law of War Manual,” June 2015, p. 8, (hereinafter Law of War Manual).

3.  For a discussion of the law of war as lex specialis, see the Law of War Manual’s section 1.3.2. on “The Law of War’s Relationship to Other Bodies of Law.” Ibid.

4.  The term “employment” refers to nuclear forces conducting an operation that concludes with a nuclear detonation. In general, the U.S. armed forces tasked with nuclear deterrence responsibilities, including patrols and other operations that are not nuclear strike operations, will often utilize the term “use” to broadly encompass these and other non-kinetic activities. 

5.  Ibid., p. 15.

6.  Ibid., p. 393.

7.  Avalon Project, “General Orders No. 100: The Lieber Code,” 2008, arts. 15-16,

8.  Law of War Manual, p. 52.

9.  Ibid., p. 60.

10.  Ibid., p. 56.

11.  David E. Hoffman, The Dead Hand (New York: Doubleday, 2009), pp. 39-40.

12.  Col. Charles J. Dunlap Jr., “Taming Shiva: Applying International Law to Nuclear Operations,” The Air Force Law Review 42 (1997): 161-162.

13.  Avalon Project, “General Orders No. 100: The Lieber Code,” art. 22.

14.  The U.S. statement on the 1996 ICJ case considering the legality of the threat or use of nuclear weapons addressed a number of law of war issues, including the principle of distinction: “It has been argued that nuclear weapons are unlawful because they cannot be directed at a military objective. This argument ignores the ability of modern delivery systems to target specific military objectives with nuclear weapons, and the ability of modern weapon designers to tailor the effects of a nuclear weapon to deal with various types of military objectives.” U.S. Department of State, “Letter Dated 20 June 1995 From the Acting Legal Adviser to the Department of State, Together With Written Statement of the Government of the United States of America,” June 20, 1995, pp. 22-23,

15.  Curtis E. Lemay Center for Doctrine Development and Education, “Annex 3-60 Targeting: Weaponeering and Allocation,” January 10, 2014,

16.  Law of war considerations are included within U.S. Air Force weaponeering. “The output of weaponeering is a recommendation of [what is] needed to achieve desired effects while avoiding unacceptable collateral damage…. According to [the law of war], incidental damage to civilian objects must not be excessive in relation to the expected military advantage to be gained. Collateral damage criteria were established on this foundational principle.” Ibid.

17.  Law of War Manual, pp. 66-67.

Dr. Justin Anderson is a research fellow at the Center for the Study of Weapons of Mass Destruction at National Defense University. The views expressed in this article are those of the author and do not necessarily reflect the official policy or position of the National Defense University, the U.S. Department of Defense, or the U.S. government.