| Introduction

The vision for nuclear strategy presented by Elbridge Colby, U.S. Under Secretary of War for Policy, at the U.S. Strategic Command Deterrence Symposium in August 2026 can be distilled into two central lines of thinking.¹⁾ The first is to reestablish U.S. nuclear strategy on the basis of "common sense," grounded in U.S. interests, tolerable risk, and actual feasibility of execution. The second is to strengthen Conventional-Nuclear Integration (CNI), which brings nuclear and conventional forces together into a single system of strategic options in support of this recalibration. This can be understood as an approach that, while retaining nuclear weapons as a last resort within U.S. defense strategy, seeks to avoid the extreme binary choice between full-scale nuclear war and retreat. It does so by equipping the president with a closely calibrated range of actual response options tailored to the level and nature of the threat.

Under Secretary Colby's line of thinking is not confined to nuclear strategy alone. Read alongside the emphasis the 2026 National Defense Strategy (NDS) places on "denial defense" in the Indo-Pacific, the second Trump administration appears to be recalibrating not only its nuclear forces themselves, but conventional forces and the roles of allies as well, into a more comprehensive set of strategic options. In the Indo-Pacific in particular, a force posture with enhanced survivability, dispersion, and mobility is emerging alongside an expanded conventional role for allies, making it worth examining how the logic of conventional-nuclear integration that Under Secretary Colby has emphasized is actually interacting with these shifts in military posture. Above all, these changes present the ROK-U.S. alliance with both opportunities and challenges at the same time. South Korea's substantial conventional military strength, for instance, could serve as an important foundation for advancing ROK-U.S. CNI and for sustaining and deepening the Nuclear Consultative Group (NCG); at the same time, however, there remains the possibility that South Korea's conventional role alone will expand while the authority over U.S. nuclear decision-making remains unchanged.

Against this backdrop, the paper proceeds in three parts. Section II draws on Under Secretary Colby's remarks to examine the trajectory of U.S. nuclear strategy under the second Trump administration. Section III situates this trajectory within the "denial defense" concept articulated in the 2026 National Defense Strategy (NDS) and the broader realignment of force posture underway in the Indo-Pacific. Building on this analysis, Section IV considers the resulting implications for the Korean Peninsula and the ROK-U.S. alliance and outlines corresponding policy responses.

| The Substance of Colby's Remarks: "Common Sense" and Conventional-Nuclear Integration

The "common sense" that Under Secretary Colby emphasized amounts, in the nuclear domain as elsewhere, to the proposition that cost, risk, and operational feasibility must be weighed against one another. In these remarks, Colby noted specifically that if a president were called upon to use nuclear weapons in defense of interests in regions distant from the United States, that choice would need to be genuinely "worth it" from an American standpoint. This can be read as an approach that locates the credibility of extended deterrence not in a strong declaratory policy of retaliation, but in the number and granularity of actual military options available to the president in a real crisis. As Colby reiterated, because nuclear use would constitute a "titanic historic catastrophe," a sufficiently broad set of options must be secured precisely in order to avoid reaching that point.

This approach, however, differs in character from the existing discourse on nuclear prudence. Where that discourse has generally centered on the immense risk and uncertainty attending nuclear use, and accordingly emphasized restraining and limiting the role of nuclear weapons, the prudence, or "common sense," that Colby articulates is oriented not toward the absence of nuclear options but toward their feasibility. The thrust of the approach, in other words, lies in widening the president's decision space: assembling, prior to any nuclear threshold, a sufficient array of conventional and non-nuclear options, and, should that threshold be crossed, a set of limited and controllable nuclear options as well.

This line of thinking bears a certain resemblance to the Flexible Response strategy of the Cold War. Colby himself characterizes the current approach not as a straightforward restoration of Flexible Response, but as "downstream" of it. The Massive Retaliation strategy of the 1950s carried a measure of credibility while Soviet nuclear forces remained comparatively weak, but as the Soviet Union acquired a secure second-strike capability, the threat to deter aggression at every level through massive nuclear retaliation grew progressively less plausible. In response, the United States and NATO strengthened both conventional and nuclear forces and developed Flexible Response, a strategy allowing a spectrum of instruments, from conventional response through limited nuclear options to strategic nuclear forces, to be selected according to the level of threat and aggression encountered. NATO formally adopted this strategy in 1967, and later, in the 1970s, National Security Decision Memorandum (NSDM)-242, signed by President Nixon during James Schlesinger's tenure as secretary of defense, similarly sought to expand presidential choice by supplementing large-scale nuclear strike plans with limited nuclear options. The strategic environment has since changed considerably, not least because the United States now confronts simultaneous nuclear threats from Russia, China, and North Korea alike. Even so, the underlying concern that Colby raises retains its force: that sufficient options must be in place to spare the president what he termed the extreme binary choice between "suicide or surrender"—full-scale nuclear war or retreat.

CNI serves as the principal instrument for addressing this concern. Although Colby did not invoke CNI by name in these remarks, he stressed that nuclear forces ought to be operated as part of a more "integrated, broader, sensible strategy." The underlying logic is one in which the United States sustains a highly survivable strategic nuclear force and an assured second-strike capability, while linking limited, theater-level nuclear options with conventional and non-nuclear capabilities within a single response architecture, so as to secure a range of options commensurate with the threat at hand. This same logic informs Colby's related emphasis, directed at European allies, that investment in conventional forces ranks among the most consequential contributions to nuclear deterrence. The wider the gaps in conventional defense, the greater the risk that an adversary could combine limited attacks with nuclear threats to force the United States into that same binary choice between nuclear use and retreat.

| Nuclear Strategy under the Second Trump Administration and Denial Defense in the Indo-Pacific

Read alongside the denial defense concept set out in the 2026 NDS, Colby's remarks suggest that nuclear strategy under the second Trump administration is best understood not merely as an effort to expand nuclear forces in their own right, but as an attempt to recalibrate nuclear forces, conventional forces, and allied roles into a single, integrated system of strategic options. A concrete force structure and posture for the Indo-Pacific have yet to be articulated in detail, but the trends visible thus far are reasonably consistent. In the nuclear domain, the United States retains ultimate authority over nuclear use and employment as an exclusively American prerogative, even as the emerging direction points toward reducing vulnerability to an adversary's first strike and expanding a set of survivable, flexible nuclear options capable of employment in a variety of ways as circumstances require. In the conventional domain, the emphasis has similarly shifted away from the permanent forward stationing of large-scale forces within the First Island Chain (FIC) and toward a posture that provides support from greater distance while preserving the capacity for rapid deployment, dispersion, and maneuver when needed.

What merits particular attention is that the "denial defense" articulated by the Trump administration is not confined to defensive instruments such as air and missile defense. Traditional deterrence by denial emphasizes reducing an adversary's probability of a successful attack through defense; the denial defense the United States now emphasizes additionally encompasses offensive firepower aimed at reducing the probability of operational success by striking directly at the core capabilities an adversary requires to achieve its military objectives, including naval and air power, long-range fires, command-and-control facilities, and A2/AD systems. Equally significant is that the United States intends to employ such offensive instruments not through large-scale, permanent peacetime deployment within the First Island Chain, but by holding them further to the rear and deploying, dispersing, and maneuvering them rapidly once a contingency arises. Denial defense, in this sense, occupies an intermediate position between pure defensive denial and deterrence by punishment, which imposes costs on an adversary. The approach reduces, in peacetime, the costs and escalation risks that forward, fixed deployment of offensive forces would otherwise entail, while preserving, in a contingency, the option of rapidly deploying those forces to neutralize an adversary's core military capabilities directly, thereby diminishing both the adversary's prospects for military success and its incentive to attack in the first place.

Within this structure, U.S. medium- and long-range missile systems warrant particular attention as the instrument that would actually underpin American denial defense within the First Island Chain. The most representative example is the U.S. Army's Mid-Range Capability (MRC), known as "Typhon," which Russia recently warned²⁾ it would respond to in kind should the system be deployed to South Korea. The MRC is a system that adapts the Tomahawk (with a range of roughly 1,600 km) and the SM-6 (with a range of roughly 240 km), both previously operated from sea-based platforms, for employment from a ground-based mobile launcher, and it is organized within the long-range fires battalions of the Multi-Domain Task Force (MDTF). This places core military facilities on the Chinese mainland and along its coast within range, while simultaneously placing Chinese naval and air forces seeking to operate within and beyond the First Island Chain within striking distance, thereby constraining the conduct of Chinese operations themselves.

A review of recent cases illustrates this pattern. The United States first deployed the MRC to northern Luzon in the Philippines in 2024,³⁾ and subsequently deployed it on a temporary basis to Iwakuni Air Base in Japan in 2025, in conjunction with the U.S.-Japan combined exercise Resolute Dragon 25.⁴⁾ This reflects the broader trend in which the United States seeks to employ such long-range fires not through large-scale, fixed deployment within the First Island Chain, but by rapidly inserting them when needed and dispersing and maneuvering them within the region. When these capabilities are linked with regional missile defense systems and with sensor, ISR, and targeting data, offensive long-range fires and defensive missile defense together come to constitute the central pillars of denial defense, jointly working to reduce an adversary's probability of military success.

What is critical here is that such denial defense is unlikely to be achievable through U.S. forces alone, and it is precisely this constraint that underlies the Trump administration's emphasis on allies' own conventional capabilities and on burden-sharing. If the United States maintains its nuclear forces and strategic support capabilities at greater distance and employs its medium- and long-range fires on a rotational, maneuver basis, the long-range strike, missile defense, ISR, and basing and logistics capabilities that allies sustain within the First Island Chain from peacetime onward assume correspondingly greater importance. The permanent stationing of U.S. medium-range missiles on allied territory within the First Island Chain, moreover, carries no small degree of practical constraint, given not only strong Chinese objections but also the domestic political burdens and entanglement concerns confronting host nations. For this reason, an alternative advanced within the United States has been to distribute regional conventional denial capability by supporting allies in independently acquiring and operating their own medium- and long-range precision strike systems.⁵⁾ Japan's acquisition of U.S.-made Tomahawk missiles to expand its own standoff strike capability represents a leading case of an ally directly assuming the conventional component of denial defense.

Finally, the fact that the MRC employs the Tomahawk carries further, intriguing implications for the future of U.S. regional nuclear posture. The United States previously operated the TLAM-N, a nuclear-armed, sea-launched cruise missile in the Tomahawk family, before deciding to retire it in the 2010 Nuclear Posture Review (NPR). The 2018 NPR subsequently reintroduced a sea-launched nuclear cruise missile (SLCM-N) to reinforce more flexible regional non-strategic nuclear options, and the United States continues to develop a new SLCM-N variant carrying a W80-series warhead.⁶⁾ To be sure, the SLCM-N's delivery vehicle has not yet been confirmed to be the Tomahawk, and the Block V-series Tomahawks currently employed by the MRC are themselves conventional systems. Even so, the fact that the MRC was developed to draw on the Navy's Vertical Launch System (VLS) and the existing Tomahawk inventory, in the interest of logistical and production efficiency, warrants continued attention. This is especially so given that, following the termination of the Intermediate-Range Nuclear Forces (INF) Treaty, the United States is no longer subject to treaty-based constraints on the development and deployment of ground-launched missiles in the 500-5,500 km range. Against this backdrop, it cannot be entirely ruled out that, over the longer term, as the United States seeks to further diversify its portfolio of survivable and mobile theater-level nuclear options, the discussion could extend beyond the SLCM-N to encompass a nuclear-armed ground-launched cruise missile or a dual-capable successor to the MRC. Were this to occur, nuclear and conventional forces might come to share a common missile and launcher infrastructure, a development that would enhance commonality and operational flexibility across the services and potentially widen the range of choice between conventional and nuclear missions as circumstances require.

The direction now emerging in Indo-Pacific military posture under the second Trump administration can accordingly be summarized as a structure in which the United States sustains a survivable and flexible nuclear force alongside strategic support capabilities, while constructing denial defense within the First Island Chain through the combination of rapidly deployable U.S. medium- and long-range fires, a regional missile defense network, and allies' own conventional forces. This architecture connects directly with the logic of "options" that Colby emphasizes: the aim is to build a dense layer of conventional denial capability prior to any nuclear use, thereby reducing the very need for it, while ensuring that, should deterrence nonetheless fail, survivable and flexible nuclear options preserve a decision space between full-scale nuclear war and inaction. Nuclear strategy and denial defense under the second Trump administration should therefore be understood not as separate initiatives, but as a single strategic architecture that seeks to expand the range of options available at each stage of a crisis by combining nuclear and conventional forces with the roles of allies.

| Implications for the Korean Peninsula and Directions for Response

These shifts in U.S. nuclear and conventional posture present the ROK-U.S. alliance with new opportunities and new challenges in equal measure. As the United States seeks to expand the range of options available at each stage of a crisis by combining nuclear and conventional forces of enhanced survivability and mobility with allies' own conventional capabilities, the strategic value of South Korea, given its substantial conventional forces and high level of defense capacity, stands to grow correspondingly. Colby himself spoke favorably in these remarks of South Korea's conscription system, its large standing military, its high defense spending, and its willingness to assume a more leading role in the conventional defense of the peninsula. South Korea's long-range precision strike, missile defense, ISR, and C4I capabilities could, beyond their function in peninsula defense alone, become core assets within a CNI architecture linked to U.S. nuclear forces, providing a foundation for further concretizing ROK-U.S. CNI and for developing the Nuclear Consultative Group (NCG) into a substantive body for jointly examining nuclear and conventional response options calibrated to the level of threat.

At the same time, three challenges are equally apparent. The first is the possibility that CNI evolves into an asymmetric division of roles, one that expands South Korea's conventional burden more than it expands South Korea's participation in U.S. nuclear planning. The United States may seek to retain final authority over nuclear use and targeting decisions while drawing on allied conventional capabilities to widen the president's range of options; South Korea, by contrast, harbors substantial expectations that CNI will expand information-sharing, consultation, and participation in planning regarding U.S. nuclear strategy. Reconciling this divergence in expectations will be an important task as CNI continues to be institutionalized.

Second, the nuclear prudence Colby emphasizes carries a double-edged character for the Korean Peninsula. Preparing a range of nuclear and conventional options, rather than linking every North Korean nuclear threat to immediate large-scale nuclear retaliation, is advantageous for crisis management and escalation control. Should the trend toward locating deterrence credibility in actually usable options, rather than in strong declaratory statements, intensify, however, the weight accorded to strong declaratory commitments, such as the pledge that any North Korean nuclear attack would result in the "end of the Kim regime," could diminish correspondingly. Indeed, this phrase was absent from the joint statements of both the fifth NCG meeting in December 2025 and the sixth in June 2026. This alone is insufficient to conclude that U.S. declaratory policy has shifted, but the concern is that North Korea could misread such prudence as a weakening of U.S. resolve and test the alliance through limited nuclear use or nuclear coercion.

Third, as U.S. denial defense becomes more concrete, questions surrounding the strategic flexibility of the ROK-U.S. alliance and the scope of South Korea's regional role are likely to grow more consequential. From the U.S. perspective, the incentive to employ South Korea's long-range precision strike, missile defense, ISR, and basing and logistics capabilities in broader Indo-Pacific operations, including denial defense against China, rather than in peninsula defense alone, could grow. From South Korea's perspective, however, should the NCG and CNI come to be linked with operational planning against China beyond their original purpose of deterring North Korea's nuclear threat, the risks of military tension with China, base targeting, and entanglement in conflicts beyond the peninsula could grow in tandem. This bears as well on questions concerning the strategic flexibility of U.S. Forces Korea and of bases within South Korea, since, as noted above, the effective employment of long-range precision fires and missile defense depends significantly on access to allied bases and territory. There is at present no basis to conclude that the United States has formally requested the deployment to South Korea of a China-oriented missile system such as the MRC, but it is difficult to rule out that consultations over temporary or rotational deployments, additional missile defense stationing, or base access and support could arise in the future. Accommodating such requests could heighten Chinese and North Korean objections, the risk of base targeting, domestic political burdens, and the risk of entanglement in conflicts beyond the region; refusing them categorically, conversely, could see U.S. long-range fires and regional operational networks develop primarily around Japan and the Philippines, correspondingly diminishing South Korea's role and voice. South Korea may accordingly need to exercise a form of selective strategic flexibility, prioritizing peninsula defense while determining the scope and conditions of its regional contributions on a case-by-case basis.

Given these opportunities and challenges, South Korea's response to the shifting nuclear strategy of the second Trump administration should not be confined to demanding an expanded range of U.S. nuclear options, nor should it focus solely on expanding South Korea's conventional role. Rather, it should pursue, in parallel, the expansion of the United States' actually executable nuclear and conventional options and the expansion of South Korea's participation within the alliance.

First, the United States' strong declaratory commitment to extended deterrence should be developed alongside actually operable nuclear and conventional options. Given the multiple nuclear threats and escalation risks the United States now confronts, the effort to maintain prudence regarding nuclear use while expanding the range of options genuinely available to the president is likely to continue. Extended deterrence, however, operates not only through the existence of military options but through how allies and adversaries perceive U.S. resolve. South Korea should therefore actively convey, through the NCG and other consultative channels, the effect that shifts in U.S. nuclear strategy may have on allied anxiety and on North Korea's calculations, ensuring that prudence regarding nuclear use is not misread as a weakening of U.S. resolve. The strong declaratory message that North Korea stands to gain no strategic benefit from nuclear use, and would face unbearable consequences, must accordingly be clearly maintained, even as the NCG works to narrow the gap between declaration and execution by concretizing the nuclear and conventional options, and the deployment and consultation procedures, that would actually be available to the United States in a crisis.

Second, while maintaining peninsula defense as the alliance's primary mission, South Korea should develop linkages with the regional denial defense architecture the United States is building. Given the Trump administration's emphasis on expanding allies' own defense capabilities and roles, it is both unavoidable and consistent with South Korea's comparative military strengths and direct threat environment that South Korea assume greater responsibility for deterring North Korea and for the conventional defense of the peninsula. South Korea should accordingly make clear its principle of prioritizing peninsula defense while continuing to strengthen its own long-range precision strike, missile defense, ISR, and C4I capabilities. This specialization of roles, however, should not translate into disconnection from the regional denial defense architecture the United States is constructing. As U.S. medium- and long-range fires, allied strike capabilities, ISR and targeting data, command and control, and logistics systems become increasingly linked into a single distributed network, the interoperability and operational experience accumulated from peacetime onward will assume correspondingly greater importance. South Korea should therefore use combined exercises to develop linkages between U.S. long-range fires and South Korean precision strike, ISR, and command-and-control systems, and participate as appropriate in trilateral and other regional multilateral exercises. At the same time, South Korea should establish a case-by-case consultative principle regarding the temporary or rotational deployment of U.S. forces and questions of base access and support, weighing comprehensively their contribution to peninsula defense, their effect on deterrence against North Korea, and the risk of entanglement in conflicts beyond the region. South Korea's task is not to choose categorically between accepting or rejecting every U.S. request for an expanded regional role, but to secure the operational and technical connectivity it needs while preserving the centrality of peninsula defense.

Third, South Korea should closely track shifts in U.S. regional nuclear and conventional force posture and consider "flexible redeployment," rather than the permanent redeployment of tactical nuclear weapons, as a realistic option. As discussed above, U.S. denial defense in the Indo-Pacific is developing in a direction that privileges survivability, dispersion, mobility, and rapid deployment capability over the fixed, concentrated deployment of specific forces within the First Island Chain. This shift in posture may bear importantly on the employment of nuclear forces as well. In particular, the permanent, concentrated stationing of high-value nuclear forces at fixed bases within range of a potential adversary's missiles sits uneasily alongside the emphasis the United States has recently placed on survivability and dispersion in its force employment more broadly. Rather than confining the debate to the question, repeatedly raised within South Korea, of whether U.S. tactical nuclear weapons should be permanently redeployed to the peninsula, it is therefore necessary to examine more broadly how the evolving manner of U.S. nuclear force employment can be connected to extended deterrence on the Korean Peninsula.

One alternative worth considering in this light is a "flexible redeployment" of tactical nuclear weapons, under which nuclear forces would be operated outside the Korean Peninsula in peacetime, with the consultative procedures and readiness measures needed for their rapid deployment to the peninsula or nearby areas prepared in advance for a heightened crisis. Such an arrangement would reduce the political and military costs and vulnerabilities that permanent deployment would otherwise entail, while still signaling to North Korea that the U.S. nuclear commitment could translate into actual action if required, and providing the U.S. president with a more realistic set of response options. To this end, the NCG could work to concretize the options and consultative procedures for the deployment and reinforcement of nuclear forces at each stage of a crisis, examine the rotational deployment of dual-capable aircraft (DCA) to the peninsula or the wider region and related exercises, and consider the bases and support procedures needed to receive and sustain nuclear forces if required. Over the longer term, the SLCM-N examined in Section III, along with the development of more mobile theater-level nuclear systems, should likewise be considered as part of this discussion of flexible redeployment.

Fourth, the NCG should be developed into a body for the joint management of nuclear crises across both peacetime and wartime. The NCG already treats nuclear and strategic planning, together with CNI, as core areas of its work, and the two countries have established principles and procedures to support nuclear deterrence policy and posture through the NCG guidelines. Going forward, they need to examine, in greater and more concrete detail from peacetime onward, joint assessment of North Korea's intent and any indications of imminent nuclear use, phased response following any limited nuclear use, the combination of nuclear and conventional instruments, the employment of strategic assets, the prevention of further nuclear use and escalation control, and the conditions for war termination. While final authority over nuclear use decisions rests with the U.S. president, South Korea's views must be structurally reflected in the threat assessments, judgments regarding the operational situation on the peninsula, and consideration of response options that precede that decision.

Finally, the expansion of South Korea's conventional role should be linked to expanded South Korean participation in U.S. nuclear and strategic planning. This is the most important principle for addressing the possibility, raised above, of an asymmetric division of roles under CNI. As the weight of South Korea's ISR, missile defense, long-range precision strike, and C4I capabilities within ROK-U.S. CNI grows, South Korea's participation in information-sharing on nuclear crises, joint threat assessment and planning, and discussions of escalation control and war termination should expand in kind. Should the United States seek to expand its pre-nuclear-use options by drawing on allied conventional capabilities, a correspondingly higher level of information-sharing and prior consultation will be needed regarding precisely which crisis stage and operational objectives South Korea's contributions are being combined with U.S. nuclear and conventional options to serve. What matters, ultimately, is ensuring that CNI develops not as a unilateral division of roles that simply enlarges South Korea's conventional burden, but as a reciprocal structure in which South Korea's contribution and its participation within the alliance expand together.

1. Colby, Elbridge. (2026). "Remarks by Under Secretary of War for Policy Elbridge Colby at the 2026 United States Strategic Command Deterrence Symposium." U.S. Department of War, August 5.
2. 서울신문. (2026). "러시아 '한국, 두고 보라' 경고… 美 '타이폰' 뭐길래?", 7월 20일.
3. U.S. Army Pacific. (2024). "US Army's Mid-Range Capability Makes Its First Deployment in the Philippines for Salaknib 24," April 15.
4. Reuters. (2025). "US Typhon Missile System to Temporarily Deploy to Japan as Part of Exercise," August 28.
5. Hornung, Jeffrey W. (2022). "Ground-Based Intermediate-Range Missiles in the Indo-Pacific: Assessing the Positions of U.S. Allies," RAND Corporation.; Edelman, Eric et al. (2022). "Rings of Fire: A Conventional Missile Strategy for a Post-INF Treaty World," Center for Strategic and Budgetary Assessments, August.
6. CRS. (2025). "Nuclear-armed Sea-Launched Cruise Missile (SLCM-N)." September 19.

※ The opinions expressed in 'Sejong Focus' are those of the author and do not represent the official views of Sejong Institute.