A call for new low‐​yield nuclear weapons in the Trump administration’s Nuclear Posture Review (NPR) has generated a good deal of controversy and debate among American experts, and for good reason. However, there has been little attention paid to the assumptions that undergird the arguments made in the NPR to justify such capabilities. Flawed assumptions lead to flawed policy prescriptions, and the NPR’s assumptions are shaky at best. Congress should not move forward on the administration’s wish list of low‐​yield nuclear weapons without rigorously questioning the faulty assumptions made in the 2018 NPR.

The first key assumption in the new NPR is that the international threat environment facing the United States has worsened considerably since the last review was released in 2010. Unlike the last NPR, which downplayed the role nuclear weapons played in U.S. strategy, the new NPR argues that the growing nuclear capabilities and sharp elbows of America’s adversaries create a compelling need for a tailored and flexible nuclear arsenal. Low‐​yield nuclear weapons are not intended for “nuclear war‐​fighting,” the NPR argues, but are meant to bolster deterrence by convincing adversaries that they will not gain a decisive advantage from their own nuclear weapons.

The chief problem with this assumption is that it views deterrence as a contest of capabilities while ignoring the role of interests. In other words, the new NPR implies that capability gaps in the U.S. nuclear arsenal encourage bad behavior from other countries while downplaying the role stakes play in an adversary’s cost‐​benefit calculation. Credible deterrence requires the United States to make an adversary believe that it will face higher costs than benefits if the target takes an action that the United States is trying to prevent. U.S. nuclear capabilities are one part of this equation, but if the target believes that it has vital interests at stake then it may act regardless of U.S. threats. Low‐​yield nuclear weapons will impact the cost‐​benefit calculation of U.S. adversaries, but they probably won’t deter the kinds of actions that have vexed Washington in recent years. For example, the United States was unable to deter China’s island‐​building activities in the South China Sea and Russia’s annexation of Crimea because in both instances the other countries had greater interests at stake than the United States.

Nuclear weapons are useful tools for deterring things like a nuclear attack against the United States or a Russian attack against NATO, but such actions can be deterred without more low‐​yield nukes. However, nuclear weapons—regardless of their yield—are poorly suited for preventing other nuclear states from pursuing interests that are much more important for them than the United States.

Another faulty assumption in the NPR is related to Russia’s nuclear strategy. The NPR states that Russia’s “escalate to deescalate” nuclear strategy is a serious threat that requires new U.S. nuclear capabilities to solve. Under the “escalate to deescalate” strategy, Moscow would use or threaten to use low‐​yield nuclear weapons in a conflict with NATO in order to end the conflict on favorable terms. Therefore, the United States must have new low‐​yield nuclear weapons of its own in order to prevent Russia from using the threat of limited nuclear escalation to coerce the United States or its allies.

The NPR’s assumptions about “escalate to deescalate” ignore recent developments in Russian military capabilities that suggest “escalate to deescalate” no longer reflects Russian nuclear thinking. Moscow’s economic and military weakness following the end of the Cold War led to greater reliance on nuclear weapons and lower thresholds for nuclear use in order to deter a much stronger NATO. While Russia still lags behind the United States and NATO in military technology, Moscow’s conventional military power and asymmetric capabilities—such as cyber and electronic warfare—have grown much strong over the past decade. Russia has not abandoned the possibility of using nuclear weapons first in a conflict, but U.S. fears over “escalate to deescalate” gloss over the changes that Russia has made to reduce its dependence on nuclear weapons in recent years.

The Trump administration’s NPR makes nuclear strategy based on important assumptions about the state of the world and the nuclear strategies of U.S. adversaries. These assumptions—and the policy solutions that flow from them—must be rigorously questioned in order to craft an effective U.S. nuclear strategy. The case for new low‐​yield nuclear weapons made in the NPR rests on shaky assumptions about what nuclear weapons are capable of deterring and the characteristics of Russia’s nuclear strategy.