Global Risk

What Is the Sole Purpose of U.S. Nuclear Weapons?

09.16.21 | 7 min read | Text by Adam Mount

Read the full report PDF here. 

Summary

• Prior to assuming office, President Biden indicated that he would establish that “the sole purpose of our nuclear arsenal is to deter—and, if necessary, retaliate for—a nuclear attack against the United States and its allies.”
• Sole purpose should be understood as a central component of an integrated deterrence strategy that can effectively manage the risk of nuclear escalation in a limited conflict as well as the rising stability risks from nonnuclear weapons.
• Sole purpose could significantly reduce the risk of unintended escalation and increase the credibility of more flexible and realistic nonnuclear response options in a range of importance contingencies.
• In order to attain its intended benefits, declaratory policy must be reflected in force structure and planning.
• The president’s existing language on sole purpose provides considerable flexibility for the administration to define the doctrine, but does not itself provide clear guidance for strategy, force structure, or for related declaratory policies like “no first use.”
• Defining sole purpose is a critical task for the administration’s defense policy review.
  • As a central component of an integrated defense policy that will strengthen US deterrence and assurance credibility, sole purpose should be defined at the level of the NDS.
  • A sole type definition would state that the United States would consider nuclear use in response to a certain type of attack, having modest effects on a narrow set of plans but few effects on force structure.
  • A sole function definition would define what is and what is not a requirement of deterrence, potentially removing certain strategic or nonstrategic roles of nuclear weapons.

Depending on how it is defined, sole purpose could have transformational effects on nearly every aspect of nuclear weapons policy or relatively modest effects. It could accommodate or incorporate a range of related policy options, like a deterrence-only posture or no first use.

• Fully implementing a sole purpose policy is critical to attaining its benefits.
  • A simple declaratory statement is not a complete sole purpose policy. Because any statement is likely to be ambiguous, sole purpose should also consist of a set of presidential directives that determine how the policy will be affect force structure and planning.
  • By eliminating one or more of the requirements that structure US nuclear forces, a sole function definition could potentially have significant effects on a range acquisition decisions and plans.
  • If the president concludes that sole purpose has implications for force structure or force posture, the administration should ensure that these changes are made before the presidential term is concluded.
  • Following a decision to adopt sole purpose, civilian officials should review existing operational plans and concepts to ensure that they comport with the president’s guidance for escalation management of a conflict with a nuclear-armed adversary.
  • Embedding the policy in plans, force structure, and allied consultations is critical to achieving its benefits and reducing the risk that it is reversed by a future president, which would be highly risky.
• If defined, implemented, and communicated as a part of an effective integrated deterrence posture, sole purpose could strengthen assurance of allies.
  • Some allies will be understandably apprehensive about any shift in US nuclear weapons policy in the current environment.
  • Allies should be consulted closely as sole purpose is being defined, as it is released, and as it is being implemented.

In January 2021, President Biden assumed office after having made unusually explicit commitments to reduce the role of nuclear weapons in US national security strategy. In his primary articulation of his campaign’s foreign policy, Biden declared that “the sole purpose of the US nuclear arsenal should be deterring—and if necessary, retaliating against—a nuclear attack.”¹ Since assuming office, Biden has not repeated the pledge, though his initial national security guidance and his Secretary of State have reiterated the goal of reducing reliance on nuclear weapons.² As the Pentagon begins its review of nuclear weapons policy, Biden and his national security officials will have to determine whether to adopt sole purpose and, if so, what it means. The established language on sole purpose offers the administration considerable latitude in how it chooses to reduce reliance on nuclear weapons. Depending on how sole purpose is defined and implemented, it could have transformative consequences for nuclear force structure and strategy, or it could end up as a rhetorical commitment that has few practical effects at all.

Though the language dates back decades, there has never been a precise or agreed definition of sole purpose. The first published use of the phrase is in a piece Albert Einstein related to the eminent journalist Raymond Swing that was published in the Atlantic in 1947. Einstein argued while the United States must stockpile the bomb, it should forswear its use. “Deterrence should be the only purpose of the stockpile of bombs.” If the United Nations were granted international control over atomic energy, as President Truman had proposed, it should be “for the sole purpose of deterring an aggressor or rebellious nations from making an atomic attack.”³ Since the idea was popularized in the 1960s, sole purpose has become a persistent staple in ongoing debates about the role of nuclear weapons, but it has rarely been attached to a precise definition or a plan to implement it.

Sole purpose is more ambiguous than other declaratory policy proposals (such as no first use) because it purports to define, or constrain, the purpose of nuclear weapons. Depending on how the terms of the statement are defined and how the statement is implemented in practice, its effects could be broad, narrow, restrictive, permissive, or ambiguous. For example, President Biden’s sole purpose language could be construed to proscribe nuclear weapons from performing a wide range of functions or from being used in wide ranges of contingencies. Slight variations in the wording of a sole purpose declaration can produce dramatically different policies and be perceived differently by allies and adversaries, who will examine the policy closely. Depending on how sole purpose is defined and implemented, sole could reduce or eliminate requirements for each piece of the triad or for nuclear use in a variety of different contingency plans.

Sole purpose is one potential option in declaratory policy, that aspect of nuclear weapons policy that publicly communicates when and why the United States would consider the use of nuclear weapons. It can be combined with or can subsume a range of other potential declaratory policy options. Because the president has sole authority to order the use of a nuclear weapon, only the president can set limits on that power. Though changes in declaratory policy should consider the views of civilian national security officials, uniformed military officials, members of Congress, US allies, and the American public, the president should provide clear guidance on how to modify US declaratory policy. Like all presidents, President Biden should provide clear guidance to the officials conducting the national defense strategy about nuclear declaratory policy.

Because sole purpose could potentially be defined in many different ways, some definitions will be better or worse. Advocates or opponents should be clear about what constitutes a better or worse definition. The administration should not accept the argument that a good definition is one that preserves existing force structure or plans, maintains ambiguity for its own sake, or comports with the preferences of certain allies or services. This piece argues that a good definition of sole purpose is one that assists with the development and implementation of a credible, integrated posture by which the United States and its allies deter aggression and nuclear use; reflects the president’s preferences about how to manage escalation in limited conflicts with nuclear-armed adversaries as well as his assessment of the requirements of deterring a major strategic attack; reduces the risk of misperception and adversary nuclear first use incentives; and can be implemented in force structure and plans so that it is resilient to leadership changes in the United States. Because the president has expressed a preference to reduce the nation’s reliance on nuclear weapons, a good definition of sole purpose should help to do so in ways consistent with his preferences.

This piece examines the range of options available to officials working to define sole purpose and reduce reliance on nuclear weapons. It explores the practical implications of different definitions of sole purpose and the steps necessary to ensure that they are implemented in a way that is responsible, effective, and most likely to endure over time. There are two central arguments. First, sole purpose should not be understood as a nuclear declaratory policy but as critical component in an integrated deterrence strategy. Understood in this way, sole purpose is not only a valuable means of reducing the risk of nuclear escalation and of meeting US commitments to reduce reliance on nuclear weapons but because it is a substantive judgment about how US nuclear and nonnuclear forces can best manage escalation in a limited conflict with a nuclear-armed adversary. Second, an effective sole purpose policy cannot simply be a sentence in a paragraph on nuclear declaratory policy. If the administration is serious about attaining the benefits of sole purpose, the policy should be comprised of the declaratory statement, additional language to clarify and contextualize the policy, and a set of directives that communicate the president’s guidance for how the policy should affect force structure and plans.

Each of these arguments is critical for attaining the benefits of sole purpose and for maintaining an effective deterrence posture. Sole purpose will be a contentious idea under any circumstances. Allied governments, advocates of various aspects of the current nuclear weapons policies, and political opponents are understandably concerned about the president’s statements. Clearly defining the policy, articulating how it will strengthen an integrated deterrence policy, and moving forward with implementation will help to convince allies and many deterrence experts that sole purpose will increase rather than decrease deterrence credibility.