Table of Contents

Modern SLCMs are highly capable. They are accurate, difficult to detect and track, and have sufficient ranges—approaching 3,000 kilometers (1,900 miles)—to reach targets deep within an adversary’s interior.1 Both Russia and the United States deploy large forces of conventional SLCMs, creating escalation risks. Russia is concerned that U.S. weapons threaten its nuclear forces, particularly mobile ICBMs but also docked SSBNs, heavy bombers on the ground, and perhaps even silo-based ICBMs.2 Meanwhile, Russian nonnuclear SLCMs threaten critical U.S. military assets, including ground-based C3I capabilities that support nuclear operations, and potentially national and military leadership.

The development of SLCMs can also feed arms racing. The United States is acquiring (for now, at least) a nuclear-armed SLCM in response to its concerns about Russian NSNWs, which include SLCMs. If deployed, this new U.S. missile will likely further exacerbate Moscow’s concerns about the survivability of its nuclear forces, potentially catalyzing a Russian response.

SLBGMs have not yet been deployed by either state. But if various technical challenges are overcome, these missiles could be as accurate as SLCMs, while traveling much faster and over potentially greater distances.3 As such, they could exacerbate both escalation risks and arms racing. The United States is currently developing nonnuclear SLBGMs.4 There is no public evidence of an equivalent Russian effort—though it would come as no surprise to learn that Moscow has initiated one. Russian SLBGMs would most likely be nuclear-armed or dual-use.

These dangers will be heightened if either Russia or the United States overestimates the other’s current or future deployments of SLCMs or SLBGMs. This kind of threat inflation could result from the very real difficulties associated with estimating the current size of an adversary’s sea-based missile force—let alone its future size.

Estimating current SLCM deployments is challenging because modern vertical launching systems can accommodate various types of missiles, including air and missile defense interceptors and anti-ship and anti-submarine weapons, in addition to land-attack SLCMs. As a result, surface ships (and perhaps submarines) may carry fewer SLCMs than the maximum possible in order to facilitate the deployment of other kinds of weapons. The United States faces the additional challenge of estimating how many of Russia’s deployed SLCMs are nuclear-armed. Russia will face the same problem if the United States deploys nuclear-armed SLCMs of its own.

The challenges of estimating future deployments are even greater because states could change the number of SLCMs they deploy surreptitiously and relatively quickly. The probable deployment of nonnuclear SLBGMs, which will likely share a launching system with SLCMs and other missile types, will complicate matters further. There may also be uncertainty in each state’s estimates of how many ships the other is likely to deploy. To be sure, this uncertainty is limited by the difficulty of hiding shipbuilding activities and of changing construction plans. Even so, it could be significant, particularly four or five years into the future.

Of course, without access to both Russian and U.S. classified information, it is impossible to determine whether official deployment estimates are indeed prone to exaggeration. It is perhaps significant, however, that well-connected Russian experts have used estimates of the United States’ future force of “strategic conventional warheads”—including sea-based systems—that turned out to be inflated.5 If there is a similar bias in official Russian and U.S. estimates, then the resulting increase in danger is entirely unnecessary (unless it is the result of a deliberate policy of ambiguity in SLCM deployments, and there is no evidence that it is). Moscow and Washington, therefore, should have a shared interest in greater transparency.

Solution Concept

In theory, limiting SLCMs and SLBGMs in a follow-on to New START would be the optimal solution. However, the challenges to doing so would be insurmountable—with the sole exception of making nuclear-armed SLBGMs accountable.6 Given the military importance of nonnuclear SLCMs, in particular, neither the United States nor Russia would likely accept limits. Moreover, there would be daunting practical challenges. Most SLCMs are deployed on attack submarines and surface ships, and nonnuclear SLBGMs will likely be too. Neither Russia nor the United States has any interest in opening these vessels up to inspection.7

Ensuring transparency about current and planned future deployments of nuclear and nonnuclear SLCMs and nonnuclear SLBGMs is a more practical way forward.8 The proposed transparency arrangement outlined below is an expanded version of a U.S.-Soviet SLCM data exchange, which was negotiated as a side agreement to the Strategic Arms Reduction Treaty I (START I) and expired with that treaty in 2009.9

U.S.-Russian Data Exchange on Sea-Launched Cruise Missiles and Nonnuclear Sea-Launched Boost-Glide Missiles

On April 1 and October 1 of each year, Russia and the United States should exchange confidential declarations of the number of missiles in the following categories that each state deployed on, respectively, March 1 and September 1 of the same year:

  • Long-range nuclear-armed SLCMs
  • Long-range nonnuclear SLCMs
  • Long-range nonnuclear SLBGMs
  • Nuclear-armed SLCMs with ranges between 300 and 600 kilometers (between 190 and 370 miles)
  • Nonnuclear SLCMs with ranges between 300 and 600 kilometers
  • Nonnuclear SLBGMs with ranges between 300 and 600 kilometers

The declaration covering deployments for March 1 should also include the following:

  • The maximum number of missiles, in each category, that are anticipated to be deployed as of March 1 each year for the following five years
  • All types of currently deployed surface ships and submarines, any one of which has ever been equipped with at least one SLCM or SLBGM launcher

In implementing these provisions, the following definitions would apply:

  • “Boost-glide missile” means a weapon-delivery vehicle that sustains unpowered flight through the use of aerodynamic lift over most of its flight path. A reaction control system designed to change a vehicle’s attitude is not considered capable of powering flight.
  • “Cruise missile” means an unmanned, self-propelled weapon-delivery vehicle that sustains flight through the use of aerodynamic lift over most of its flight path.
  • “Deployed SLCM (or SLBGM)” means a cruise missile (or boost-glide missile) that is contained in any surface ship or submarine that is equipped with at least one SLCM (or SLBGM) launcher.
  • “Long-range SLCM (or SLBGM)” means a SLCM (or SLBGM) with a range in excess of 600 kilometers.
  • “SLCM (or SLBGM) launcher” means a device intended or used to contain, prepare for launch, and launch a SLCM (or SLBGM).

Definitions of missile ranges are discussed in appendix A.


This proposed transparency arrangement, which would lack legally binding verification provisions, would probably not be as effective as a treaty in building confidence in the current and future size of SLCM and SLGBM forces. It would, however, represent a significant improvement over the status quo (no regulation at all).

Importantly, Russia and the United States would not need to take the exchanged information purely on the basis of trust. If intelligence information available to either participant confirmed at least some parts of the other’s declaration, without contradicting other parts, it would boost the credibility of the declaration as a whole. Discussions could help to address any concerns—though they would not be easy. If one participant had intelligence information suggesting the other’s declaration was incorrect, it might not be able to explain the basis for its concerns without compromising the sources and methods used to collect the intelligence. Nonetheless, a constructive dialogue might still be possible. For instance, a participant might be able to build confidence in the planned size of its future navy by providing unclassified information about shipbuilding plans and potentially even allowing occasional visits to shipbuilding facilities.


Technical feasibility. From a technical perspective, the proposed transparency regime should be straightforward to negotiate and implement.

Political feasibility. Russia and the United States each have concerns about the other’s current and future forces of SLCMs and nonnuclear SLBGMs. They may each appreciate greater insight into the other’s force. Such transparency could also mitigate the risks that could be caused by potentially exaggerated deployment estimates. A secondary benefit for Moscow would be the possibility of using U.S. data to better estimate the number of Standard Missile-3 (SM-3) missile defense interceptors deployed on U.S. ships (those interceptors share launchers with SLCMs), thus helping to address Russian concerns about those interceptors. Although Russia has traditionally been skeptical of politically binding confidence-building measures, its participation in an earlier SLCM transparency arrangement sets a helpful precedent.

The primary political challenge to implementing this proposal would probably be the concern that exchanging classified data would undermine national security. The security risks, however, should be minimal. This proposal is narrowly tailored to mitigate one particular risk. It would not reveal a capability that was previously unknown to the other state or the precise mix of deployed weapon types—let alone the armaments on any particular ship. Therefore, unless either state seeks to use uncertainty about the scale of SLCM or SLBGM deployments to enhance deterrence, this data exchange should enhance the security of both states.


1 Defense Intelligence Ballistic Missile Analysis Committee, “Ballistic and Cruise Missile Threat,” July 2020, 36,

2 For a detailed review and critical assessment of Russian concerns, see Alexey Arbatov, Vladimir Dvorkin, and Petr Topychkanov, “Entanglement as a New Security Threat: A Russian Perspective” in Acton, ed., “Entanglement,” 24–31.

3 Indeed, first-generation U.S. SLBGMs will likely have shorter ranges than the most advanced contemporary SLCMs.

4 Kelley M. Sayler, “Hypersonic Weapons: Background and Issues for Congress,” R45811, Congressional Research Service, August 25, 2021, 4–6,

5 Alexey Arbatov, “Gambit or Endgame? The New State of Arms Control,” Carnegie Moscow Center, March 29, 2011, 21–22 and note 23,

6 Vaddi and Acton, “A ReSTART for U.S.-Russian Nuclear Arms Control,” 13–14.

7 The United States deploys some SLCMs on converted SSBNs, to which Russia is given limited inspection rights pursuant to New START.

8 The idea of a SLCM data exchange was revived in Jeffrey Lewis, “Russia and the United States Should Resume Data Exchanges on Nuclear-Armed Sea-Launched Cruise Missiles” in James M. Acton, ed., “Beyond Treaties: Immediate Steps to Reduce Nuclear Dangers,” Carnegie Endowment for International Peace, October 10, 2012, 4–5,

9 The United States and Russia also made a political commitment not to deploy more than 880 long-range nuclear-armed SLCMs. The text of the original data exchange has never been made public. A detailed description is available in U.S. Senate Committee on Foreign Relations, The START Treaty, S. HRG. 102-607, part 1, 102nd Cong, 2nd session, 1992, 436–437,