Introduction

The United States is moving toward deployment of a nuclear-armed sea-launched cruise missile (SLCM-N), a low-yield, theater-range nuclear capability intended for deployment on U.S. attack submarines. First proposed in the 2018 Nuclear Posture Review, SLCM-N was designed to restore a forward-deployed, sea-based nuclear option in Europe and the Indo-Pacific following the retirement of comparable capabilities at the end of the Cold War. Since its proposal, the system has generated sustained debate over whether it would meaningfully strengthen deterrence and allied assurance or increase escalation and misperception risks without providing decisive military benefits.

That debate has unfolded largely in the context of present-day threats and historical analogies. Yet SLCM-N is a long-horizon capability. Even on optimistic timelines, it is unlikely to enter service until the mid-2030s. By that time, the United States is likely to face even more intense strategic competition with both Russia and China alongside continued advances in technologies that complicate military planning and readiness. The strategic environment in which SLCM-N would be deployed is therefore likely to differ meaningfully from the one in which it is being debated.

Despite years of analysis, the disagreement over the effects of deploying SLCM-N remains unresolved. This report argues that the persistence of this disagreement is not primarily the result of missing data or unsettled technical questions. Rather, it reflects deeper differences in how experts and policymakers expect future crises to unfold—how adversaries interpret U.S. intent, how escalation occurs amid increased uncertainty, and how technological change affects deterrence dynamics. These assumptions are often implicit, rarely tested against alternative futures, and difficult to adjudicate using traditional policy analysis.

This report approaches the SLCM-N debate from a different starting point. Rather than asking whether the system is inherently stabilizing or destabilizing, it examines how SLCM-N’s effects vary across plausible future strategic environments. This approach avoids static judgments about a dynamic system and instead evaluates the conditions under which the capability could mitigate risk, exacerbate instability, or prove strategically marginal. To do so, the project combines expert interviews, scenario planning, and forecasting to surface the assumptions shaping expert judgment and to assess how SLCM-N performs across four plausible versions of the mid-2030s. These futures vary along two dimensions: (1) how U.S. nuclear posture is perceived by allies and adversaries and (2) the pace of technological change. This approach allows us to clarify where assessments of SLCM-N are robust, where they are contingent, and which future developments matter most for policy decisions today.

Today’s nuclear security environment is defined by two structural shifts: the consolidation of tripolar nuclear competition and rapid advances in military-relevant technologies, which are reshaping deterrence, escalation, and force survivability. The United States now competes simultaneously with Russia and China, creating what U.S. Strategic Command has labeled a nuclear “three-body problem,” in which interactions among three nuclear-armed peers are more complex and less predictable than the bipolar dynamics of the Cold War.¹ At the same time, emerging technologies—from artificial-intelligence-enabled decision support to cyber capabilities and hypersonic glide vehicles—are changing how crises unfold and how quickly leaders must make decisions.

These external pressures are compounded by domestic political polarization. Disagreement over basic facts and long-term priorities has narrowed the space for shared threat assessment. Together, these trends make it harder to rely on a single set of expectations about how deterrence will work in the future.

Under these conditions, traditional planning approaches—grounded primarily in historical analogy or institutional experience—are increasingly insufficient, and relying on them risks locking in capabilities that perform poorly if key assumptions prove wrong.²

Anticipating divergent futures matters because it reduces that risk. Tools like scenario planning and forecasting do not attempt to predict a single outcome; instead, they surface underlying assumptions and test whether judgments hold when key conditions change. Applied to nuclear policy, these tools help distinguish between capabilities that remain useful across a range of plausible futures and those whose value depends on narrow or optimistic assumptions—an essential distinction when decisions are costly, long-lasting, and difficult to reverse.

The debate over SLCM-N centers on whether the United States should field a low-yield, nuclear-armed sea-launched cruise missile to address perceived gaps in deterrence. Proponents argue that SLCM-N would strengthen extended deterrence, provide credible response options below the strategic level, and hedge against adversaries’ growing nonstrategic nuclear arsenals. Critics counter that the system would add limited military value while increasing risks of misperception, inadvertent escalation, and blurring of the line between conventional and nuclear operations at sea.

At its core, this disagreement reflects different expectations about how future conflict is likely to unfold. Some experts anticipate intensifying military confrontation with China and Russia, including scenarios involving limited nuclear use, and therefore prioritize additional low-yield options. Others expect prolonged strategic competition below the threshold of major conflict, in which adding such capabilities could increase escalation risks rather than reduce them. These differences are less about factual disagreement than about how experts assess escalation, signaling, and risk amid uncertainty—assumptions that are often implicit and difficult to test directly.

Political polarization has further entrenched these divides. Since its introduction, SLCM-N has experienced repeated reversals between administrations and Congress, with executive branch decisions to advance or cancel the program frequently countered by congressional efforts to restore funding. Over time, SLCM-N has become a proxy for broader disagreements over deterrence, escalation risk, and the role of low-yield nuclear weapons, reinforcing debate driven more by competing worldviews than by shared strategic assessment.

Importantly, expert views on SLCM-N do not fall neatly into pro- and anti- camps. Instead, assessments span a spectrum—from strong confidence in the system’s deterrence value, to judgments that its effects would be modest or highly contingent, to firm opposition. As this report shows, this dispersion reflects divergent mental models of deterrence—the assumptions, priorities, and beliefs experts use to interpret the security environment and evaluate policy choices—rather than disagreement over the system’s basic technical characteristics.

The debate is further complicated by timing. Although SLCM-N is now funded and under development, it would not be deployable until the mid-2030s. Yet most assessments continue to evaluate the system through the lens of today’s crises or a single anticipated future rather than examining how its effects might vary if the strategic environment evolves in different plausible ways.

Finally, SLCM-N cannot be considered in isolation. It is being developed alongside a broader nuclear modernization effort as the United States replaces or upgrades all three legs of its nuclear triad (i.e., land-based intercontinental ballistic missiles, submarine-launched ballistic missiles, and strategic bombers) while introducing new capabilities. These systems will enter service on different timelines and interact in ways that may not be apparent in advance, making judgments about any single capability highly sensitive to assumptions about the future force as a whole.

For these reasons, SLCM-N is a useful test case for futures-based analysis. Its long deployment horizon, its integration into a changing modernization portfolio, and the persistence of expert disagreement make clear that the key question is not whether SLCM-N is inherently “good” or “bad” but which assumptions about the future are driving those judgments.