Appendix

• ALCM: air-launched cruise missile
• Assurance: measures and commitments intended to signal to allies that the U.S. will defend them credibly and reliably
• ASW: anti-submarine warfare
• CONOPS: concept of operations
• Extended deterrence: U.S. commitment to defend allies—including through the threat of nuclear retaliation—against coercion or attack
• ICBM: intercontinental-range ballistic missile
• IOC: initial operational capability
• Low-yield: describes a nuclear warhead with a smaller explosive yield compared with strategic warheads; typically intended for regional or limited-use scenarios
• LRSO: long-range standoff weapon
• NC3: nuclear command, control, and communications
• NDAA: National Defense Authorization Act
• NPR: Nuclear Posture Review
• SLBM: submarine-launched ballistic missile
• SLCM-N: nuclear-armed sea-launched cruise missile
• SSBN: nuclear-powered ballistic missile submarine
• SSN: nuclear-powered attack submarine
• Theater-range/short-range: describes a nuclear or conventional weapon designed for regional use rather than intercontinental distances—typically capable of striking targets within a specific theater (e.g., Europe or the Indo-Pacific)
• TLAM-N: Tomahawk land attack missile–nuclear

Questions asked during the interview were adapted as needed based on interviewee responses and positions.

Fundamental Assumptions, Conditions, and Context Dependencies

• How does the nuclear-armed sea-launched cruise missile (SLCM-N) affect strategic stability? Would you characterize its effects as positive, neutral, or negative?
• Would the deployment of SLCM-N detract from an existing state of stable deterrence? And if so, how?
• Could SLCM-N lower the threshold for nuclear use? And if so, how?
• How much do you think the argument in favor of SLCM-N hinges on perceived “deterrence gaps”?
  • Is there a gap to begin with?
  • What is it?
  • How do we know?
• Why—and under what conditions—do you believe that SLCM-N would deter adversaries effectively?
• How does the justification for SLCM-N today compare to the justification that led to the deployment of the Tomahawk land attack missile–nuclear (TLAM-N) during the Cold War?
• What strategic theory underpins the belief that the presence of SLCM-N enhances deterrence without increasing escalation risks?
  • Does restoring deterrence hinge primarily on doctrine, policy, or strategy?
  • How would we know whether deterrence is “restorable”?
• Does effective deterrence require a plan for the use of SLCM-N?
  • Do you conceive of SLCM-N primarily as a first-use option, a response option, or something else?
• What is the value of SLCM-N relative to existing U.S. capabilities?
• How survivable is SLCM-N?
• Why might the United States need the flexibility that SLCM-N is said to provide?
• To what extent does the effectiveness of SLCM-N rely on adversary uncertainty about its location (stealth) and readiness status?
• How prompt is SLCM-N?
• Of peacetime, crisis, and conflict, in which context—if any—is SLCM-N most likely to have value?
  • To uphold peace?
  • To manage a crisis?
  • To prevail in a conflict?
• How might the strategic value of SLCM-N vary by regional theater (e.g., Europe versus Indo-Pacific versus Middle East)?
  • Is SLCM-N more beneficial in one theater than another?
• How does SLCM-N’s credibility as a deterrent depend on a specific adversary’s nuclear doctrine or escalation thresholds? What about those of allies?
  • What effect would SLCM-N deployment have on U.S. commitments in different theaters or geopolitical contexts?
• What about the cost? Is it worth it?
• If you think SLCM-N is necessary for deterrence, is it sufficient?
• What potential costs or trade-offs does SLCM-N introduce?

Drivers and Key Uncertainties

• What specific factors make it hardest to predict how SLCM-N readiness or deployment would affect U.S. and global security?
• What political, technological, military, or alliance-related unknowns are most likely to influence the future relevance of SLCM-N?
• What two or three factors could unexpectedly shift the strategic calculus around SLCM-N?
• Is there anything that could emerge as high-priority enough to replace the perceived desirability of SLCM-N?

Uncertainty About Adversary Responses

• How—and under what conditions—could SLCM-N escalate a conventional conflict to a nuclear one?
• How could SLCM-N trigger reciprocal deployments by other nuclear powers, potentially leading to arms racing at the strategic level?
• Could SLCM-N’s ambiguity and dual-capable nature create misperceptions, and if so, how?
  • Would conventional and nuclear SLCMs be distinguishable?
  • What difference does the ability to distinguish between them make for adversary response?
• Could this ambiguity be beneficial or stabilizing in some scenarios?
• Does China or Russia have the intelligence, surveillance, and reconnaissance (ISR) capabilities necessary to distinguish SLCM-N deployments?
• Under what circumstances does the potential for entanglement give China greater incentive to automate response systems?
• How could U.S. attempts at nuclear signaling involving SLCM-N be misunderstood?
• Would China perceive SLCM-N as eroding its deterrent?
• How would China interpret a low-yield nuclear strike? As a tactical signal? As a prelude to strategic escalation?
• Under what circumstances might China or Russia assume the worst?
• Does the United States intend to use SLCM-N as a first-strike option or as a response to adversarial aggression?
  • What would the U.S. definition of “deterrence failure” be?
• What is the difference between a country’s nuclear doctrine and its declaratory policy?
  • To what extent does declaratory policy matter?
• How would we know whether adversaries view SLCM-N as credible?

Uncertainty About Allies

1. If allies see SLCM-N as a credible nuclear guarantee, would it strengthen alliances or merely maintain them?
   1. Would it reduce incentives for nuclear proliferation?
2. If the stated goal of SLCM-N is to reduce a deterrence gap and reassure allies, what should the United States do in cases such as South Korea, where some argue U.S. assurances have reached their limits?
3. What if South Korea pursues nuclear weapons anyway?
4. How does SLCM-N factor into Europe/NATO dynamics?
5. How should Franco-British signals to “Europeanize” nuclear forces be understood in relation to SLCM-N?

Imagining Future Worlds

• Is there a future in which SLCM-N has the opposite effect on strategic stability from what you described earlier? What factors or events would be most likely to determine that outcome?
• In a future where SLCM-N deployment becomes widespread, what major events or developments led to that world?
• Imagine a future in which SLCM-N is canceled or withdrawn. What key events caused that outcome?
• Could SLCM-N help facilitate arms control negotiations with Russia (for example, as the SS-20 and Pershing II missiles led to the Intermediate-Range Nuclear Forces [INF] Treaty)?
• In a world where the United States deploys SLCM-N while seeking to mitigate destabilizing effects, what information exchanges, notifications, or diplomatic mechanisms would help?
• Is there a future in which SLCM-N has little to no utility?
• What developments could render SLCM-N ineffective or obsolete—for example, advances in adversary anti-submarine warfare that undermine stealth?
• Is there a future in which SLCM-N is less destabilizing than expected? Are there additional arms control concepts—such as total warhead regimes—that become more relevant in these futures?

Wrap-Up

• Is there anything important about SLCM-N that most analysts or policymakers overlook?
• How confident are you in your views on SLCM-N compared with other nuclear policy issues?
• What is something you wish others understood better about SLCM-N?

1. Will China go back on its No First Use policy before the end of 2035? The event is considered to have occurred if China is credibly reported to have gone back on its No First Use nuclear weapons policy by either (a) officially stating that it no longer abides by a policy of No First Use or (b) conducting a deliberate, non-retaliatory nuclear attack. (For the purposes of this question, inadvertent or accidental launches do not count as violations of the policy.)
2. Will China initiate a large-scale military operation to seize or control Taiwan’s territory before the end of 2035? The event is considered to have occurred if Beijing launches an armed operation with the clear intent to seize/occupy Taiwanese territory, as assessed by a consensus of regional security experts. (Limited clashes, blockades, gray-zone actions, cyber-operations, or coercive exercises without attempts to seize territory do not count.)
3. By the end of 2035, will the Chinese government issue an official statement explicitly threatening the use of nuclear weapons if the United States or its allies intervene militarily to defend Taiwan? The event is considered to have occurred if an official Chinese government document or a statement by senior leadership explicitly threatens nuclear use in response to U.S. or allied military intervention on Taiwan's behalf.
4. If the United States cancels SLCM-N before the end of 2035, will Chinese officials or official Chinese policy documents publicly characterize this decision as influencing Beijing’s calculus on Taiwan? The event is considered to have occurred if China’s government statements or senior leaders explicitly link U.S. cancellation of SLCM-N to Beijing’s Taiwan strategy, deterrence calculus, or military planning. If SLCM-N is not canceled before December 31, 2035, the question will be annulled and will not be scored, so you should forecast the likelihood the event occurs assuming SLCM-N has been canceled.
5. Will China have more than 1,000 strategic nuclear warheads by the end of 2035 based on publicly available sources? The event is considered to have occurred if the Federation of American Scientists’ Status of World Nuclear Forces database reports that China possesses more than 1,000 strategic nuclear warheads (as defined by the federation) as of December 31, 2035.
6. By the end of 2035, will Russia revise its publicly available military doctrine or strategic deterrence policy to explicitly reference U.S. sea-based nuclear weapons (such as SLCM-N) as a justification for changes in its nuclear or conventional force posture? The event is considered to have occurred if Russian military doctrine or strategic deterrence policy explicitly mentions U.S. sea-based nuclear systems (e.g., SLCM-N) as justification for force posture adjustments. If SLCM-N never enters service and Russian military doctrine and/or strategic deterrence policy is not revised, the question will resolve as “No.”
7. By the end of 2035, will the United States withdraw from NATO? The event is considered to have occurred if the U.S. government formally withdraws from NATO.
8. By the end of 2035, will the United States withdraw from the Treaty on the Non-Proliferation of Nuclear Weapons (NPT)? The event is considered to have occurred if the U.S. government formally submits a notice of withdrawal from the NPT.
9. Will South Korea or Japan obtain a nuclear weapon before the end of 2035? The event is considered to have occurred if, by December 31, 2035, either (a) the government of South Korea or Japan claims to be in possession of at least one nuclear weapon and/or has successfully tested a nuclear weapon and/or (b) credible reports from other governments, intelligence agencies, or international bodies such as the International Atomic Energy Agency (IAEA) indicate that the named country is in possession of at least one nuclear weapon and/or has tested a nuclear weapon. For the purposes of this question, radiological weapons do not constitute nuclear weapons.
10. Will South Korea or Japan withdraw from the NPT before the end of 2035? The event is considered to have occurred if either government submits an official withdrawal notice to the United Nations in accordance with NPT Article X.
11. Will South Korea or Japan obtain a nuclear weapon before the end of 2035? For the purposes of this question, the event is considered to have occurred if, by December 31, 2035, either (a) the government of South Korea or Japan claims to be in possession of at least one nuclear weapon and/or has successfully tested a nuclear weapon or (b) credible reports from other governments, intelligence agencies, or international bodies (such as the IAEA) indicate that the named country is in possession of at least one nuclear weapon and/or has tested a nuclear weapon. Radiological weapons do not constitute nuclear weapons for the purposes of this question.
12. Will the United States conduct a nuclear explosive test before the end of 2035? A nuclear explosive test is defined as a test producing a self-sustaining nuclear chain reaction, exceeding subcritical experiments under the Comprehensive Nuclear-Test-Ban Treaty (CTBT). The event is considered to have occurred if a nuclear explosive test is evidenced by U.S. government declarations, credible public reporting, or independent assessments.
13. In what year will SLCM-N first enter service? This question is resolved in the year the U.S. Navy declares initial operational capability (IOC) or conducts the first operational deployment of the SLCM-N. Respondents are asked to assign probabilities across the following timelines such that the four percentages add up to 100 percent: (a) sooner than 2034, (b) in 2034, (c) later than 2034, or (d) never.
14. By 2035, will a majority of surveyed experts assess that the underwater domain is effectively transparent to persistent adversary surveillance (i.e., that adversaries can reliably track U.S. submarines)? The event is considered to have occurred if, among experts participating in this exercise who respond to a follow-up survey in 2035, at least 50 percent agree that the underwater domain is effectively transparent to persistent adversary surveillance.
15. In 2035, will a majority of surveyed experts agree that U.S. homeland missile defense can intercept a limited nuclear strike (e.g., 10 solid-fueled intercontinental-range ballistic missiles [ICBMs] or SLCM-Ns)? The event is considered to have occurred if, among experts participating in this exercise who respond to a follow-up survey in 2035, at least 50 percent agree that U.S. homeland missile defense can reliably intercept such a strike with greater than 80 percent probability.