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Biden Must Be Clear About What Nuclear Weapons Are For

The U.S. president can clarify his belief in “sole purpose” through a new review process.

By , a senior fellow and the director of the Defense Posture Project at the Federation of American Scientists.
Anti-nuclear activists protest in Berlin.
International Campaign to Abolish Nuclear Weapons activists protest outside of the North Korean Embassy in Berlin on Sept. 13, 2017. Ome Messinger/Getty Images

President Joe Biden has assumed command of the U.S. nuclear arsenal at a particularly turbulent time. The Trump administration, consistent with former President Donald Trump’s own fixation on the destructiveness of nuclear weapons, accelerated programs to develop a new generation of warheads and delivery vehicles, sought a new low-yield warhead and a new sea-launched cruise missile, and added vague new language to U.S. declaratory policy that expanded the role of nuclear weapons in deterring nonnuclear attacks. As the Department of Defense is struggling with an enormous “bow wave” of modernization costs across the force, these efforts sometimes diverted funds away from critical conventional priorities. The overall effect was to increase the nation’s reliance on its nuclear forces at the expense of more credible conventional deterrence options.

During the campaign, Biden pledged to reduce, rather than increase, reliance on nuclear weapons. He stated, “the sole purpose of the U.S. nuclear arsenal should be deterring—and if necessary, retaliating against—a nuclear attack,” and he promised to review the U.S. policy that reserves the right to use nuclear weapons first in a conflict. His initial national security guidance and his secretary of state have reiterated the goal of reducing reliance on nuclear weapons

But what does “sole purpose” mean in practice? The term originated in the 1960s, and the idea even earlier, but it is not clear how the concept would apply to today’s world. It could be a transformative change that has sweeping consequences for U.S. nuclear force structure and deterrence strategy, or it could be just rhetoric. The answer will depend on how the president defines sole purpose—and whether he replaces the antiquated Nuclear Posture Review with a process that can determine how conventional and other tools can safely reduce reliance on nuclear weapons.


In its 2010 Nuclear Posture Review, the Obama-Biden administration explicitly considered a move to sole purpose, which it regarded as a statement about the types of attacks that nuclear weapons would deter. That document stated that the United States was “not prepared at the present time to adopt a universal policy that deterring nuclear attack is the sole purpose of nuclear weapons,” because they still play a role in deterring a nuclear, chemical, or biological attack in a “narrow range of contingencies.” Sole purpose in this sense is a statement that nuclear weapons pertain to a sole type of attack: Nuclear weapons would deter nuclear attacks—not conventional, chemical, and biological attacks.

Seven years later, in his last days in office, then-Vice President Biden reported that “the president and I strongly believe we have made enough progress that deterring—and if necessary, retaliating against—a nuclear attack should be the sole purpose of the U.S. nuclear arsenal” and that they were “confident we can deter … nonnuclear threats through other means.” But they did not make the change; they left it for a Hillary Clinton administration that never came.

Since the 2010 Nuclear Posture Review, Biden’s definition of sole purpose has evolved. His phrasing in the 2017 speech and in subsequent campaign statements pertains not only to the type of attack but the function of nuclear weapons—which is deterrence and, if necessary, retaliation for nuclear attacks. This definition seems to say that nuclear weapons don’t just pertain to a sole type of attack—they have a “sole function” in responding to nuclear attacks.

As he and his National Security Council draft guidance for the next defense policy reviews in the next months, they should give a clear and explicit definition of sole purpose. What is the opposite of a sole purpose policy? Is it a policy that nuclear weapons also deter nonnuclear attacks, or is it a policy in which nuclear weapons also perform functions other than deterrence?

Deterrence is the obvious use of nuclear weapons, but there are other potential functions. One option is that nuclear weapons are useful for tactical warfighting—for striking battlefield targets for military advantage in a limited conflict. Another is that weapons are useful for strategic warfighting—for limiting damage to the United States or an ally by conducting large-scale strikes against an enemy’s nuclear forces before they can be launched. Another is that nuclear weapons are uniquely valuable for signaling U.S. resolve in response to a nuclear attack. Another is that they might prevent a nuclear-armed adversary from developing new strategic capabilities, or hedge against that risk. Another is that nuclear weapons might be uniquely capable in assuring allies of U.S. intentions to defend them from nuclear or nonnuclear attacks. All these and more were mentioned in the 2018 Nuclear Posture Review as critical roles of U.S. nuclear weapons.

It might indeed be the case that deterrence depends on having a warfighting arms capability, a damage limitation capability, a nuclear option for signaling resolve, a hedge against racing, and a strong nuclear assurance. But it also might not. In defining sole purpose, the president will be defining the requirements of deterrence.

When the president and National Security Council issue guidance to the Pentagon for the next defense reviews, they could define the sole purpose of nuclear weapons as a sole function, as appropriate to a sole type of attacks, or they could decline to provide a specific definition. If the president does not provide a clear definition of sole purpose, a Pentagon review would likely water down the doctrine until it represents only a modest shift that pertains to a handful of chemical and biological weapons targets.

In either of the latter two cases, force structure and planning are likely to remain largely unaffected. Though the president could choose to cut or delay an acquisition program or two, the arsenal would likely remain large, diverse, postured for a range of functions, prepared for use at a moment’s notice, and central for plans to control escalation of a limited conflict. If it was not accompanied by visible changes to force structure or plans, adversaries would doubt that U.S. policy had changed, limiting its benefits for strategic stability. Lastly, a sole purpose declaration of this type could be easily rescinded by a future president. Its primary consequences would be political: acclaim from disarmament groups, disapproval from Republicans, and some tension with allies. Sole category, or an undefined sole purpose, would likely not reduce reliance on nuclear weapons.

A “sole function” statement, in contrast, could have transformative effects on both force structure and operational planning. A nuclear arsenal that is not postured for warfighting might be significantly smaller, less diverse, and less expensive. Eliminating of certain functions would effectively increase reliance on advanced conventional forces. While conventional options are already available for most targets relevant to a limited conflict, confining nuclear weapons to a sole function could force planners to prioritize the development of conventional responses to an adversary’s aggression or coercive first use of a nuclear weapon, helping to avoid a situation where a president feels constrained by America’s reliance on nuclear weapons. It would fit well with a commonsense statement that a U.S. president would not use nuclear weapons if they had viable conventional options available.

Like other declaratory policy, sole purpose is consequential to the extent that it affects force structure and operational planning. For a sole purpose statement to achieve Biden’s objectives—strengthening strategic stability and reducing U.S. reliance on nuclear weapons—it will have to be clearly defined in terms of the functions of nuclear weapons, accompanied by clear guidance for how the Pentagon should implement the shift, and receive sustained investment of time from senior political appointees to review acquisitions plans and operational concepts to ensure they comport with the president’s preferences.

As the president provides clear guidance about the purpose of nuclear forces, he should also direct changes to how the Pentagon structures its policy review to ensure the changes are implemented safely and effectively. Since the end of the Cold War, each new administration has conducted both a Nuclear Posture Review and a broader defense review. In the most recent cycle, the 2018 review argued that new nuclear options were critical for deterrence in a limited conflict with a nuclear-armed adversary, while the concurrent National Defense Strategy described a strategy predicated on multiple layers of conventional forces. But there was little or no sense of how the two strategies would relate to each other. This partitioned process cannot provide clear guidance to planners developing operational concepts for managing escalation or to services trying to rationally appropriate finite funding between nuclear and conventional capabilities.

The purpose of the Nuclear Posture Review is not to find the best way to deter the threats the United States faces but to explain what nuclear weapons might deter. A separate nuclear review doesn’t help the U.S. government find the right tools for the job; it helps it find a job for this particular tool. It encourages excessive reliance on nuclear weapons by setting nuclear requirements in isolation from a broader strategy.

The United States needs a single integrated review that develops a common strategy for deterring limited conflict with nuclear-armed adversaries with nuclear, conventional, cybersecurity, and space capabilities.

Part of an integrated strategy review would be examining conventional-nuclear integration, which was a leading priority for the Trump administration but remained only a slogan that was attached to efforts to integrate nuclear and conventional forces to reflect integrated adversary strategies, ensure that nuclear forces could carry out signaling and employment missions during a limited conflict, and procure dual-capable strike and command-and-control systems. None of these is a good reason to integrate nuclear and conventional forces. The Biden administration should review these efforts, and the operational plans that follow from them, to ensure integration of conventional and nuclear forces strengthens strategic stability and reduces, rather than increases, reliance on nuclear weapons.

In practice, an integrated review is the only way to safely reduce reliance on nuclear weapons, because it provides a format to develop a strategy that determines the roles and capabilities of both nuclear and conventional forces. As reliance on nuclear forces declines, a corresponding increase in reliance on conventional forces may require complementary or compensatory changes to conventional posture. Importantly, it would allow the United States to discuss these measures with allies and impress upon them an important fact: Because nuclear weapons cannot defend them, they must cooperate with the United States on combined strategy to manage escalation in a limited conflict with nuclear-armed adversaries. Ultimately, this will be more reassuring than reliance on tenuous signals of nuclear assurance.

Biden comes into office with strong preferences on nuclear weapons. But it will not be enough to issue a statement and let the chips fall where they may. Sole purpose can strengthen U.S. deterrence posture—but only if senior officials are willing to devote the time and attention required to achieve its transformative potential.

Adam Mount is a senior fellow and the director of the Defense Posture Project at the Federation of American Scientists. He holds a Ph.D. in government from Georgetown University. Twitter: @ajmount