A Closer Look at the Arguments against the Low-Yield SLBM

Low-yield nuclear weapons are center stage in the debate over the nascent 2020 defense authorization act. The House of Representatives and the Senate have competing positions on the low-yield submarine-launched ballistic missile, or L-SLBM. While the United States is moving to lower the yield on a small number of W-76 warheads, debate on this force structure change has continued since the release of the Nuclear Posture Review in early 2018. MIT political scientist Vipin Narang recently observed there are two camps of opposition to L-SLBM: those who disagree with having low-yield nuclear weapons in general, and those who support having them but believe the risks of L-SLBM outweigh the benefits. Each camp raises a different set of issues that deserve a closer look. 

General Opposition 

Opposition to low-yield nuclear weapons is about whether the United States should retain limited nuclear response options or instead rely solely on high-yield weapons. A recent article by Representative Lieu and Senator Markey reflects this perspective. They argue that limited options are “irresponsible and dangerous.” Their premise is that nobody knows whether a nuclear war would stay limited, which of course is true. 

Yet their argument presumes that the purpose of U.S. low-yield weapons is to enable the United States to use nuclear weapons first, when in fact the principal role of these systems in U.S. strategy is to deter an adversary from using nuclear weapons first against allies and U.S. forces fighting abroad. Thus, Representative Lieu and Senator Markey sidestep the central nuclear deterrence challenge facing the United States: what happens if the country the United States is fighting uses a small number of low-yield weapons? Should the United States adopt a policy of responding with high-yield nuclear weapons to any nuclear attack, even “one atomic weapon of any size”? Representative Lieu and Senator Markey do not explicitly take this position, but that is the logical culmination of their argument that “preparing for limited nuclear war is folly.” 

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Representative Adam Smith, on the other hand, does follow this thought through: “We want our adversary to be clear on the point that we’re going to kick their ass if they take us on.” He said this in the context of arguing against proportionate responses, so presumably he believes that only high-yield nuclear weapons suffice for deterrence. 

This is a logical strategy, but is it wise? 

If Russia used a handful of low-yield nuclear weapons in Europe, which it possesses in large numbers, do we really want the United States to respond with a massive strike against Russia? After an adversary crossed the nuclear threshold, preventing further nuclear attacks would be a top priority. But a high-yield rather than a calibrated U.S. response, especially against Russia, would increase the likelihood of catastrophic nuclear escalation. 

This raises the question of whether it would be credible for the United States to rely solely on high-yield weapons to deter limited nuclear war. Would such a strategy convince potential adversaries and U.S. allies? To be clear, current U.S. policy does not guarantee that the United States would respond to a limited attack with low-yield weapons. Instead, the United States is ambiguous about precisely how it would respond while retaining some degree of flexibility in U.S. nuclear forces, which President after President has wanted. Eminent scholar Scott Sagan argues that this approach to deterrence makes nuclear war less likely and that the United States should develop “more lower-yield nuclear warheads” to make U.S. strategy more effective and ethical. 

Thus, the question for those who oppose limited response options is whether they would be comfortable with having only high-yield weapons if they were responsible for deciding how to respond after Russia used a few one-kiloton nuclear weapons in Europe during a conventional war. Phrased differently, if their only options are to continue fighting the conventional war or employ a high-yield nuclear weapon, how would they protect U.S. allies and deter further Russian nuclear use without triggering nuclear escalation? If they do not have a convincing answer, it would be unwise to base a deterrence strategy on their preferred nuclear posture. 

Opposition to L-SLBM

Opposition to the L-SLBM hinges on two arguments: that the system is uniquely destabilizing due to operational dangers and that it would not contribute to deterrence. The Congressional Research Service has published a good summary of this robust debate. Defense strategist Austin Long has published two strong analytical assessments of the operational dangers argument, but a few of the arguments against the deterrence value of L-SLBM deserve more scrutiny. 

First, many opponents argue that L-SLBM is redundant because the United States has a large arsenal of low-yield nuclear weapons. But a quantitative expansion in the number of U.S. low-yield weapons is not the rationale for L-SLBM. Rather, L-SLBM adds lower-yield options for U.S. strategic ballistic missiles, the most effective delivery vehicle in the U.S. nuclear arsenal. In doing so, it creates limited nuclear response options that are faster and more difficult to defend against than existing delivery vehicles for lower-yield weapons, bombers and dual-capable aircraft. 

The guaranteed ability to penetrate defenses is particularly important because there are challenges to the effectiveness of U.S. bombers and dual-capable aircraft. The U.S. ability to deliver low-yield weapons in the face of modern air and missile defenses will be far greater when the nuclear-capable B-21 and Long-Range Standoff weapon —the next generation air-launched cruise missile—begin service in the late 2020s and early 2030s. But the cruise missiles carried by the B-52H may become obsolete before then. L-SLBM is a great hedge against this risk because it does not require new platforms (e.g., submarines and missiles), is relatively inexpensive, and the United States can deploy it soon. Paradoxically, the sustained opposition to the Long-Range Standoff weapon from the disarmament community, which typically describes it as simultaneously redundant to other U.S. capabilities and profoundly destabilizing, increases the value of a near-term hedge. 

Second, opponents of L-SLBM argue that there is no evidence that Russian leadership is undeterred by existing U.S. capabilities. Certainly, there is not publicly available smoking-gun evidence in which President Putin and his senior advisors reveal that only L-SLBM would deter them from nuclear-backed aggression, but complete and unambiguous evidence of this nature is hardly ever available to inform U.S. national security decisions. 

So what is the evidence to support the case for this force structure change? 

There are compelling indications that Russian strategy envisions the use of both non-nuclear and nuclear weapons for the purpose of splitting the United States from its allies and convincing U.S. leadership to abandon its strategic objectives in a war. Under this framework, Russia would calibrate the initial use of force and hold in reserve the threat of larger nuclear attacks if the war continues. For some exemplary analyses, see here, here, and here. 

While nobody knows whether a Russian leader would act on this approach in an actual war, there is sufficient evidence that the United States must have a strategy for convincing Russian leaders that nuclear escalation would be the most dangerous choice and that restraint is a better alternative. Not having operationally effective limited response options would reinforce the case that Russian coercion would succeed. It makes it much easier for Russian leaders to convince themselves that they could set the conflict at a level where the United States cannot fight and that U.S. officials would be unwilling fight at higher level, where they have ample capability that is too dangerous to use in a limited war. 

On this basis, the United States makes judgements about whether existing and planned U.S. nuclear forces provide sufficient capacity at an acceptable level of risk. Current U.S. limited nuclear response options are consolidated on platforms that face near-term challenges from Russian defenses and whose replacements programs are set to arrive on tight timelines. During this transition phase, there are risks that Russian leadership might conclude that they can effectively defeat any limited response with a U.S. bomber or dual-capable aircraft. U.S. policymakers judged that L-SLBM is an effective means for mitigating this risk. These types of decisions are judgement calls made amid uncertainty and imperfect information. 

It is reasonable for critics of L-SLBM to reach a different judgement, yet there is a question they have not satisfactorily answered: Given the profound danger of any nuclear use, why should the United States forgo a modest technical adjustment that dramatically improves the overall resiliency of the nuclear forces supporting U.S. strategy for deterring limited nuclear war?