Give the Low-Yield SLBM its Day in Court

Two weeks ago, a draft of the Trump administration’s Nuclear Posture Review was leaked to the Huffington Post . The draft reportedly calls for lowering the explosive yield on a portion of U.S. submarine-launched ballistic missiles, or SLBMs, and it immediately triggered concerns that the United States will lower the nuclear threshold. If this draft-level proposal is approved, we are unlikely to see the official U.S. rationale until the Review is released. In the meantime, a few points are worth injecting into the nascent debate.

Lowering the yield on some SLBMs would be a modest force structure change, not a major departure from the United States’ approach to nuclear deterrence. A low-yield SLBM would diversify the means of delivery for proportionate response options to limited nuclear attack. Similar to the much-maligned nuclear air-launched cruise missile , the purpose of a low-yield SLBM would be to raise the nuclear threshold of potential adversaries rather than lower it for the United States.

As Air Force vice chief Gen. Selva explained last summer, the United States has long retained lower-yield weapons for deterring limited nuclear attack. One pathway to a nuclear war is an adversary miscalculating that a limited attack would end a conflict because the United States’ only means for responding are profoundly destructive and disproportionate high-yield nuclear weapons that it would not use. Low-yield weapons play an important role in preventing this miscalculation. Deterring limited nuclear attack is not some Cold War throwback; it remains a serious contemporary national-security challenge. Potential adversaries are exploring strategies for threatening and using nuclear weapons in conventional conflicts with the United States. There is evidence that Russia sees calibrated nuclear use as a potential option for convincing the United States to capitulate.

The United States can deliver lower-yield nuclear options with its bombers, which can carry gravity bombs and cruise missiles. Dual-capable aircraft also provide options for delivering gravity bombs. These systems are credible, but the United States must ensure that its nuclear forces can fulfill their missions under a variety of plausible technological and military conditions that could emerge. With that in mind, there are advantages to lowering the yields on a portion of SLBMs.

Ballistic missiles are the most effective delivery vehicle in the U.S. arsenal, due to their range, speed, and penetrability. A low-yield SLBM option would hedge against a future in which bombers, cruise missiles, and aircraft are unable to reliably penetrate vastly improved air defenses to deliver a response to a limited nuclear attack. It would also hedge against scenarios where air-breathing platforms are unable to reach the selected targets with sufficient speed to achieve U.S. strategic objectives. And it would hedge against scenarios where bombers (and thus the cruise missiles) and dual-capable aircraft might be unavailable because they are already conducting conventional operations, have been destroyed or disabled by an adversary (possibly during conventional missions), or are out of service due to unforeseen technical problems.

The current SLBMs would be available under these conditions but only with a much higher yield. As a result, their use might cause significantly more causalities and destruction than a lower-yield system and trigger additional nuclear attacks. For these reasons, two former senior defense officials, James Miller and John Harvey , have recently expressed support for the low-yield SLBM option.

Some analysts argue that low-yield SLBMs would create a discrimination problem in a war. If the United States launched one against a Russian target, Russia’s ballistic missile early warning system would detect the incoming missile, but it would be unable to determine whether the missile is carrying a lower-yield warhead. According to the argument, Russian leaders would conclude that their best option is to launch a massive strike before the incoming SLBM’s payload reached its target.

This concern should not be dismissed, but it is not prohibitive of deployment.

The imperative to launch under attack arises when an incoming strike would either destroy a significant portion of one’s nuclear weapons or command, control and leadership. One SLBM, even carrying multiple high-yield warheads, would not destroy a large portion of Russia’s large, diverse, and mobile nuclear arsenal . Nor is the United States likely to strike Moscow or military targets associated with command, control, and leadership in a limited attack. Additionally, a massive Russian response against the United States would guarantee uncontrolled nuclear escalation, the very outcome they are trying to avoid. Thus, riding out the strike before deciding how to respond would be in Russian leadership’s interests. A massive response against the United States would be suicidal, and they would still have ample capability to do that after the SLBM reaches its target.

Finally, as part of its peacetime deterrence strategy, the United States could explain to Russia that some SLBMs will carry lower-yield variants.  Just as importantly, U.S. officials could explain why they made this change. Eventually Russia’s strategic community would understand that U.S. low-yield SLBMs provide viable options for countering nuclear escalation.

This is not to argue that a decision to employ an SLBM in response to a nuclear attack would be automatic or low-risk. Senior advisors would need to address many strategic and operational questions before recommending this course of action. For example, what is the status of Russia’s upgrades to its ballistic missile early warning system, would it be able to distinguish that the United States only launched one SLBM rather than dozens or hundreds, and can it estimate the SLBMs destination based on the trajectory?

Importantly though, the bar for deployment is not whether the United States would use a modified SLBM in every imaginable scenario, but whether they might use it under some conditions and whether that possibility might reinforce deterrence. Few strategists would argue that the United States should limit its nuclear arsenal solely to air-breathing platforms, yet that it is precisely what it has done for its low-yield options. The crux of this issue is whether the emerging strategic environment warrants a force structure change.

If the Trump administration makes the case for a low-yield SLBM, the national-security professionals who conducted the NPR should engage in a thorough debate with members of Congress and the expert community. Building consensus on changes to U.S. nuclear forces is important, and because the United States has viable low-yield options today, it has the luxury of moving out deliberately on this modification.

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