Although the Nuclear Posture Review is ongoing, we can be confident that the deterring nuclear attack on the United States and its allies will remain the central role of U.S. nuclear weapons. This is not the only role, but it is the most important one. Effective nuclear deterrence is intrinsically tied to the other roles nuclear weapons play in U.S. strategy. If the United States lacks the forces to confidently deter nuclear attack, allies will not have confidence in extended deterrence over the long term. The ability to execute the nuclear employment strategy underpins deterrence. An important purpose of the Nuclear Posture Review is to articulate U.S. policy for fulfilling this enduring role.

Deterring Nuclear Attacks

To appreciate the scope of this challenge, it is useful to examine the two basic types of nuclear attacks the United States must deter: a massive nuclear attack on the U.S. homeland and more limited attacks on U.S. forces and U.S. allies.

A fundamental tenet of U.S. strategy is that deterrence of large-scale nuclear attack requires nuclear forces that can impose unacceptable costs on an attacker even after a disarming first-strike. The concept of a survivable second-strike capability is straightforward, but sustaining a survivable second-strike capability decades into an uncertain future is a complex undertaking. Any number of unwelcome developments could require the United States adjust its nuclear posture to ensure it retains a second-strike capability. The New START Treaty with Russia could expire in 2021 or 2026 without a follow-on agreement, leaving no binding constraints on the size of Russia’s strategic arsenal. Although the United States and Russia have ample time to find a path forward on sustaining strategic arms control, the United States must be positioned to retain survivable forces if Russia tries to escape nuclear parity in a post-New START world. Nor is it inconceivable that the United States will need to deter counterforce attacks from not one but two major powers. China’s nuclear arsenal does not pose a counterforce threat to U.S. nuclear forces today, but its continued growth and diversification might provide China with counterforce options in the future. Additionally, the potential for profound technological change could have significant implications for the survivability of nuclear forces. Improvements in counterforce targeting would support U.S. efforts to sustain extended deterrence toward North Korea despite its maturing nuclear posture; however, these technological advancements could create new challenges for U.S. forces as Russia and China incorporate them into their own strategic postures (and of course vice versa).

On the other end of the spectrum, U.S. nuclear forces must support a strategy for deterring limited nuclear escalation in conventional conflicts. Former Secretary of Defense Ash Carter aptly summarized this challenge last year, warning that the most likely use of a nuclear weapon could come in “smaller but still unprecedentedly terrible attacks, for example, by Russia or North Korea to try to coerce a conventionally superior opponent to back off or abandon an ally during a crisis.” Deterring a limited nuclear attack, especially against a U.S. ally, is a foreign policy challenge. It requires convincing a potential adversary that protecting an ally from nuclear coercion and attack is a vital interest for the United States, and U.S. nuclear forces play an essential supporting role. Just as survivability is crucial for deterring a large attack, versatility and flexibility in U.S. nuclear forces are important for deterring limited use. Otherwise, potential adversaries might convince themselves that the United States would concede after a limited attack because it lacks proportionate response options. If an adversary actually used nuclear weapons in a limited way and the United States sought to convince it to refrain from using nuclear weapons again (i.e., reestablishing nuclear deterrence), a diverse suite of delivery vehicles and yields increases the likelihood that U.S. nuclear forces can support, not constrain, the strategy national leadership selects for reestablishing nuclear deterrence.

Nuclear Modernization and Sustaining Deterrence

These deterrence challenges set the parameters for the Trump administration’s decision on nuclear modernization. This will be one of the most consequential decisions that come out of the Nuclear Posture Review.

The Trump administration inherited a solid foundation of analysis and policy choices. The Obama administration conducted major reviews of U.S. nuclear strategy and forces in 2009-10 and 2011-2013, and it reviewed the nuclear modernization plan during its final year. In every instance, it chose to maintain the current nuclear force structure and initiate the necessary investments to have follow-on systems ready when the current ones are retired.

In 2016, both sitting and former national security officials explained and endorsed these investments. To be sure, the modernization policy has its critics (see here and here for two very different for critiques). The decision to develop a replacement for the air-launched cruise missile generated the most scrutiny, yet six of seven expert witnesses in a high-profile Senate hearing supported it.

Secretary of Defense Mattis recently said that the United States will retain the triad, suggesting that the Trump administration is unlikely to scale-back the modernization plan. But whether the Trump administration will add to it remains an open question. Unconfirmed reporting based on anonymous sources suggests that developing new lower-yield nuclear weapons are under consideration. Non-government analysts have recommended that the United States consider developing lower-yield ballistic missile options, a nuclear-capable carrier variant of the F-35, and a nuclear-capable sea-launched cruise missile.

While analysis of the pros and cons of these proposals is beyond the scope of this essay, a few points on this topic are directly relevant to the role of U.S. nuclear weapons.

Modifying existing nuclear systems or adding new capabilities is not synonymous with increasing reliance on nuclear weapons, giving them new roles in U.S. strategy, or lowering the threshold for use. In all likelihood, a decision to pursue any of these changes would be based on the judgement that they are necessary to continue fulfilling the central role of deterring nuclear attack in the face of geopolitical and technological changes. As a matter of principle, the United States must be willing to consider altering its nuclear posture as strategic conditions evolve.

Four Questions for a Constructive Discussion on Modernization

Any decision to make major changes merits serious scrutiny and a constructive national discussion. Addressing four fundamental questions would be particularly important:

First, how would this new or modified capability contribute to fulfilling the central role of U.S. nuclear weapons? How would it strengthen deterrence of large and/or limited nuclear attack? Upon what evidence is this judgement based? Of course, U.S. officials must also continue answering these questions about the existing suite of nuclear forces in order to sustain and increase support for the current modernization plan.

Second, how might potential adversaries react to changes in U.S. nuclear forces? This is a complex question that requires nuanced analysis. It would be vitally important to avoid strawman extremes that suggest any U.S. deviation from the status quo is destabilizing or that Russian and Chinese strategic planning occurs with absolutely no consideration of U.S. capabilities. Policymakers must consider whether potential adversaries will react in a way that ultimately undermines U.S. and allied security. Analyses and judgements will be qualitative and speculative because they are anticipating how other countries will respond to a U.S. decision that has yet to be made about a capability that does not exist. But addressing the question in a structured analysis will enable a more informed decision. Opponents of the current modernization plan should address a related question: How would potential adversaries and allies react if the United States chose not to replace portions of its current force?

Third, how much would this posture change cost? Spending on U.S. nuclear forces will increase in the coming decades as the United States modernizes. While the current modernization plan is affordable as long as the United States prioritizes it in the defense budget, drastically increasing the bill would create additional funding challenges.

Fourth, what impact would this decision have on the bipartisan consensus supporting the current nuclear modernization program of record? Maintaining and modernizing U.S. nuclear forces is impossible without bipartisan political support. Any decision that risks derailing political support for modernization could, at the end of the day, weaken deterrence if the result is insufficient funding for the current plan. The U.S. nuclear posture is more survivable, flexible, and versatile with the current force structure than with a dyad or less-capable triad (e.g., a triad with half as many Minuteman III ICBMs or a bomber with no cruise missile).