Adaptive Warhead Limits for Further Progress on Strategic Arms Control

Adaptive Warhead Limits for Further Progress on Strategic Arms Control
Adaptive Warhead Limits for Further Progress on Strategic Arms Control

Center for Global Security Research, Lawrence Livermore National Laboratory

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Strategic arms control will soon reach a point where it must be adapted or abandoned as a core element of the U.S.-Russian security relationship. A new approach based on adaptive warhead limits offers a way for the United States to re-engage Russia on previous calls for further reductions in deployed strategic warheads. Adaptive warhead limits would allow the United States to account for Russian concerns without changing U.S. policy on ballistic missile defense (BMD). Notionally, the treaty would limit each side’s deployed nuclear stockpile to the larger of 1000 warheads or 10 times the number of BMD interceptors deployed by the other as part of its national missile defense system. In addition to providing a possible path to further nuclear warhead reductions, this initiative would strengthen strategic stability and provide increased transparency regarding missile defense programs.

Uncertain future of arms control

The New START Treaty requires Washington and Moscow to meet its prescribed limits on nuclear warheads and delivery platforms by February 2018, but there is no clear path to extending or replacing the treaty when it expires three years later. Concerns on each side about the other’s compliance with existing agreements—most notably the U.S. determination that Russia is violating the landmark Intermediate-Range Nuclear Forces (INF) Treaty—contribute to this pessimistic outlook, but constitute only part of the problem. Washington and Moscow appear to hold divergent views regarding the role of nuclear arms control and perhaps even the basic utility of these agreements in addressing the most pressing security threats.

Since the Cold War, the U.S. approach to arms control has emphasized the value of reducing numbers of nuclear weapons, while the Russian approach continues to emphasize the value of the weapons themselves. The Obama administration promoted a long-term objective of global nuclear disarmament and charted its path based on a determination that steps to reduce stockpiles and expand the scope of agreements will strengthen mutual security—and therefore U.S. security. This ideal conflicts with Russia’s determination that its strategic and nonstrategic nuclear forces are the surest way to resist U.S. and Western power, which it sees as threatening to override Russian regional and global interests. Moscow fears that continuing cuts to strategic force levels could eventually threaten strategic stability and that the United States might pursue additional cuts with that very goal in mind. Thus Russia has rejected U.S. proposals to negotiate an additional one-third reduction in accountable deployed strategic warheads and to discuss expanding nuclear arms accords to cover nonstrategic nuclear weapons and non-deployed warheads.

Russia has long been wary of any combination of U.S. military capabilities that might threaten its ability to effectively retaliate against a first strike on Russian nuclear forces or command and control. For their part, U.S. officials and analysts often characterize Russia’s stated concerns about U.S. BMD and conventional prompt global strike (CPGS) as either disingenuous rhetoric or implacable paranoia. They insist that U.S. intentions toward Russia be benign and dismiss Moscow’s narrative on the grounds that the scale and technical characteristics of current and envisioned U.S. systems do not offer the capability Russia fears. Russian officials counter that U.S. technologies might be expanded in the future to provide large-scale offensive capabilities. They highlight the United States’ unwillingness to accept limits on BMD development and deployment as evidence that the door is deliberately left open to the possibility of such expansions. Finally, they maintain that Russian security cannot be subject to whether U.S. intent is or will remain benign, and that in any case, Russia must retain (or reestablish) its ability to resist U.S. and allied dictates.          

Further progress on strategic arms control is thus unlikely unless the United States addresses Russian concerns about the impact of future agreements on strategic stability. Some argue that even acknowledging Russia’s concerns amounts to needlessly placating an unreasonable opponent at the expense of greater U.S. security. A more sober view suggests that U.S. interests would be well served by addressing the long-term potential for BMD and conventional strike programs to threaten the assured response capability upon which Russia relies for strategic deterrence. This is because to the extent Russia’s expressed concerns are sincere, U.S. BMD and conventional strike programs pose a potential threat to strategic stability even if the United States, in fact, possesses no real first-strike capability. In the aftermath of an unlikely but also devastating scenario in which Russia miscalculates U.S. intent and determines in a crisis to undertake a preemptive nuclear attack, it would not matter whether or not its concerns about U.S. BMD capability were justified.

Adaptive warhead limits offer a path forward

The United States could pursue adaptive warhead limits as a means to re-engage Russia on an additional one-third reduction in deployed strategic warheads. This new approach would seek to account for Russian concerns without changing U.S. policy on ballistic missile defense. Indeed, adaptive warhead limits can help reconcile seemingly disparate elements of a policy that rules out accepting any limits on BMD, yet also maintains that the United States has no intention to negate Russia’s assured response capability and sees no technical feasibility in defending against a massive nuclear attack.

Instead of limiting BMD, a treaty with adaptive warhead limits would tie one side’s allowed strategic nuclear deployment to the other side’s chosen level of BMD deployment. Notionally, the United States could propose to limit each side’s deployed strategic arsenal to 1000 accountable warheads or ten times the number of ballistic missile interceptors the other side deploys on its territory, whichever is larger.

Each side would have to accept an initial limit of 1000 warheads, which is just over a one-third reduction below New START Treaty limits. Warhead limits would be reciprocal and unaffected by BMD capability unless or until one side built up to 100 interceptors. If for example, the United States moved to expand its BMD system above the level of 100 interceptors, the treaty would then allow Russia to add (or upload) 10 additional nuclear warheads for each additional interceptor the United States added. If Russia chose not to expand its BMD force in response, the treaty would then allow Russia significantly more deployed nuclear warheads than it allowed the United States.

Like the current New START Treaty, the adaptive limit follow-on would constrain the potential for future CPGS deployments by counting all warheads deployed on strategic delivery systems, regardless of whether they are nuclear or conventional. Adaptive limits on strategic offensive delivery systems might be included as well, but are not considered in detail here.

At a minimum, the treaty would count ballistic missile interceptors deployed as part of each side’s national missile defense system. Meaning all U.S. deployed Ground-Based Interceptors (GBIs), and all interceptors deployed within Russia’s Moscow Anti-Ballistic Missile (ABM) system. Current U.S. plans call for increasing the number of GBIs at the system’s two sites (at Fort Greely, Alaska, and Vandenberg, California) from 30 up to 44. Even if a future administration decided to add an East Coast GBI site, as some advocate to mitigate growing missile threats from North Korea and Iran, a 10-to-1 ratio would still leave the United States well below the level at which Russia’s allowed warhead limit would be affected.

An alternative approach would be to broaden the treaty beyond national missile defense systems and stipulate that all land-based missiles with homeland BMD capability count towards determining the other side’s adaptive warhead limit. Sea-based interceptors would necessarily be excluded because their inherent ability to move into and out of locations suitable for homeland defense would otherwise challenge effective implementation of the agreement. Russia would likely reject this exclusion initially, but it would provide a starting point for negotiation. Since Russia openly claims its next-generation aerospace defense missile, the S-500, will have BMD capability, it would be included in this alternative version of the treaty. Similarly, any U.S. SM-3 Block IIA missiles would be counted if deployed in AEGIS Ashore launchers located within the United States or otherwise in areas suitable for intercepting Russian ICBMs launched against the U.S. homeland. Because the European Phased Adaptive Array (EPAA) sites in Poland and Romania are not suitable for this purpose, their deployment would not in any way impact Russia’s strategic warhead limit. Remaining the case unless the United States ever developed and deployed a new interceptor capable of much higher speeds than current plans envision. Acknowledging that any such developments would be captured in the treaty would help address Russian concerns about long-term prospects for U.S. technical achievement, as well as U.S. intent, without sacrificing U.S. and allied security. 

The envisioned treaty would necessarily include BMD transparency measures to facilitate the establishment of each side’s adaptive warhead limit. While current U.S. policy rules out limits on BMD, it does not constrain negotiated transparency measures. Thus, the United States and Russia could agree to a regular mutual exchange of information on BMD interceptor levels, and perhaps five- or ten-year plans for future BMD developments. This exchange could be accompanied by a legally binding agreement to provide advanced notice before expanding deployment plans. Finally, prior to the treaty’s entry into force, the agreement might require declarations of all current and mid- to late-stage developmental CPGS programs.

Strategic stability at lower numbers

A negotiated adaptive warhead limit would preserve and strengthen strategic stability while further reducing U.S. and Russian strategic nuclear arsenals. Adaptive limits would deliberately constrain each side’s potential for acquiring a first-strike capability. They would contribute to arms race stability by encouraging mutual restraint in both offensive and defensive deployments.

The basic idea is that BMD does not threaten strategic stability above some ratio of warheads to interceptors because at higher ratios one side’s offensive force could reliably overcome the adversary’s defenses. A 10-to-1 ratio is a notional starting point, but the final ratio would need to reflect some agreement on a plausible probability of kill associated with a single interceptor. Together with BMD deployment level, single interceptor kill probability is key to determining the threat these systems pose to strategic stability.

If one side takes actions that appear consistent with seeking large-scale BMD capability, the treaty would allow the other an advantage in strategic nuclear warheads that would be unprecedented in the history of strategic arms control. This structure might alleviate Russia’s long-term fears and provide a tangible manifestation of U.S. intention to preserve stability.

Adopting this approach would help assure Russia that technical advancement will not expose it to the threat of a U.S. first-strike, even if U.S. statements of benign intent might later change or prove insincere. It would do so without requiring the United States to negotiate on the basis that existing U.S. systems actually threaten Russia’s strategic deterrent. Regardless of whether or not Russian concerns about U.S. BMD programs are well-founded, the future of arms control as a net contributor to mutual security depends on U.S. willingness to address those concerns.

Aaron R. Miles is a physicist at Lawrence Livermore National Laboratory, currently on assignment as a senior policy advisor on nuclear deterrence in the Office of the Secretary of Defense. He holds a Ph.D. in physics from the University of Maryland and a graduate certificate in National Security Studies from Texas A&M University. He has researched and written on a range of science and security topics, including deterrence policy, nuclear weapons, fusion energy, and astrophysics. 

This includes two publications in the last few months on nuclear deterrence policy and strategy:  

Miles, Aaron R., "The Dynamics of Strategic Stability and Instability," Comparative Strategy 35(5), pgs. 423-437, December (2016).       

Manzo, Vincent A. and Miles, Aaron R., "The Logic of Integrating Conventional and Nuclear Planning," Arms Control Today 46(9), pgs. 8-14, November (2016).

This document was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor Lawrence Livermore National Security, LLC, nor any of their employees makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or Lawrence Livermore National Security, LLC. The views and opinions of the author expressed herein do not necessarily state or reflect those of the United States government or Lawrence Livermore National Security, LLC, and shall not be used for advertising or product endorsement purposes.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

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