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Implications of U.S. Forces Korea’s Strategic Flexibility for Security on the Korean Peninsula |
| October 31, 2025 |
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Jeong-kyu LeeVisiting Research Fellow, Sejong Institute | jklee87@mofa.or.kr
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As the second Trump administration took office earlier this year, it startled allies and partners by unleashing sweeping tariff measures. The administration’s “America First (MAGA)” agenda functions as a double-edged sword for both economic and security policy. On the security side, Washington is signaling that it no longer intends to play its long-standing role as the world’s police force, insisting that allies in Europe and Asia should not rely on the United States for their security and must instead address their own defense challenges. Within this context, the administration has raised the prospect of adjusting the missions of U.S. forces stationed abroad as part of what it describes as “alliance modernization.” The United States views China as its pacing threat and has placed deterring China at the top of its national security priorities. To advance this strategy, Washington seeks to adjust the role of U.S. Forces Korea (USFK) so that they are utilized not only to deter North Korea but also to respond to wider security contingencies across the Indo-Pacific region. Discussions between the two countries regarding tariff issues and the size and operational framework of Korea’s investment fund in the United States reached a broad understanding during the October 29 ROK–U.S. summit. In the security domain, Washington has not yet formally presented its demands, but they are expected to include expanding USFK’s strategic flexibility, increasing South Korea’s defense budget, and raising Seoul’s contributions to defense cost-sharing.
In the early 21st century, the United States initiated the Global Defense Posture Review (GDPR) to realign U.S. military forces worldwide in response to the evolving post–Cold War security environment. In this process, Washington proposed that the role of USFK should not remain confined solely to the defense of the Republic of Korea against North Korean threats but should instead be repositioned within a broader regional security framework. Consequently, at the 2006 ROK–U.S. Strategic Dialogue between Foreign Minister Ban Ki-moon and Secretary of State Condoleezza Rice, the two governments adopted a joint statement on the “strategic flexibility” of USFK.1)
This agreement recognized that while USFK would remain stationed on the Korean Peninsula, they could, when necessary, be moved or deployed to areas outside the region. At the same time, the South Korean government added the condition that any such extraregional activities by USFK must not run counter to Korea’s national interests. 2)
Although the strategic flexibility of USFK has never been explicitly exercised, the issue has reemerged with the advent of the second Trump administration. Trump’s “America First (MAGA)” policy is unfolding not as an unconditional security commitment to traditional allies but as an attempt to recalibrate alliances in line with U.S. geopolitical interests. This shift has generated concerns about structural tensions within the ROK–U.S. alliance and the potential weakening of deterrence against North Korea. The implications of USFK’s strategic flexibility for security on the Korean Peninsula extend far beyond an adjustment in basing policy; they constitute a critical subject of analysis involving the future direction of the alliance, U.S. strategies for deterring China, and broader transformations in the security landscape of Northeast Asia. -
The Concept of Strategic Flexibility
“Strategic flexibility” refers to the concept in which U.S. forces are not fixed to a specific region but can be rapidly moved and deployed in accordance with operational objectives, enabling them to respond more dynamically to global threats.3)
This shift represented a move away from the fixed forward-defense posture of the Cold War era and formed part of a broader effort to strengthen the U.S. military’s dispersed and networked operational capabilities.
The Global Defense Posture Review (GDPR), announced in 2004 under the direction of Secretary of Defense Donald Rumsfeld, significantly reshaped the structure of U.S. overseas deployments and redefined the U.S. approach to allies through the concepts of a core “hub” and forward “spoke” bases.4) South Korea was reconfigured as a key hub in Northeast Asia, serving as a forward platform linked to the U.S. Indo-Pacific strategy.
Strategic flexibility does not simply refer to freedom of troop movement; it is tied to a broader transformation in modern U.S. operational doctrine that prioritizes dispersion, mobility, and unpredictability. In particular, the Air Force’s Agile Combat Employment (ACE), the Marine Corps’ Expeditionary Advanced Base Operations (EABO), and the Department of Defense’s Dynamic Force Employment (DFE) function as practical embodiments of this flexibility.
Within this framework, the strategic flexibility of USFK corresponds to the overarching tier above ACE, EABO, and DFE. If DFE provides the strategic-level direction, ACE and EABO serve as the operational and tactical concepts that put it into practice. In other words, while DFE determines when and where forces will be deployed, ACE and EABO define how they are deployed and sustained. Through the integration of these concepts, U.S. Forces Korea is shifting the Korean Peninsula from a location for forward-stationed, fixed forces to a hub for theater-wide mobility. This evolution goes beyond a simple redistribution of troops and represents a substantive transformation in the operational doctrine and deterrence strategy of the ROK–U.S. alliance.
Key Elements of the 2006 U.S.–ROK Agreement
After the Cold War, the United States began reassessing the structure of its global military deployments, and in 2004 the Bush administration announced the Global Defense Posture Review (GDPR), which sought to shift U.S. forces overseas from a forward-fixed posture to a mobility-centered one. USFK were included in this restructuring as a strategically mobile force. Accordingly, USFK needed to transition from a fixed, peninsula-focused deterrent posture to a force capable of responding to multiple regional threats, including China’s rise, North Korea’s asymmetric capabilities, and the spread of terrorism. The objective was to redefine USFK’s mission so that it would contribute to stability across Northeast Asia as a whole rather than remain exclusively oriented toward the defense of the Korean Peninsula.
At the ROK–U.S. foreign ministers’ meeting held in Seoul in January 2006, the two governments agreed on the following principles.5)- U.S. Forces Korea would maintain their primary mission of defending the Korean Peninsula.
- South Korea would consult with the United States to ensure that any extraregional operations by USFK do not adversely affect its national security.
- The United States could expand its contributions to security across the Indo-Pacific by employing USFK more strategically.
Reemergence of Strategic Flexibility under the Trump 2.0 Administration
In its first term, the Trump administration adopted a realist view of alliances, treating them as valuable only insofar as they reduced costs for the United States.6) This policy has continued into the second Trump administration, launched in 2025, where it has further contributed to distrust and instability within the ROK–U.S. alliance. In this context, “strategic flexibility” has resurfaced as a mechanism through which Washington seeks to use the Korean Peninsula as a platform for countering China beyond the North Korean threat.
The Indo-Pacific strategy, carried over since the first Trump administration, remains a central element of U.S. foreign and security policy aimed at checking China’s rise.7) If formally finalized as part of this strategy, the strategic flexibility of USFK would provide the basis for deploying USFK to areas such as the South China Sea or the Taiwan Strait when necessary. While this concept would expand the regional functions of the alliance, it would also expose South Korea more directly to the forefront of U.S.–China strategic competition.
Differences Between the Existing Agreement and Recent Discussions
The strategic flexibility of USFK was formally agreed upon by Seoul and Washington in 2006. Yet the issue has resurfaced under the current push for “alliance modernization,” once again becoming a sensitive point within the ROK–U.S. alliance. The key question is how today’s debate differs from the earlier understanding.
The 2006 version was largely conceptual and principle-based: it acknowledged that USFK could, in principle, be deployed beyond the Korean Peninsula while committing to respect South Korea’s sovereignty and public sentiment. The version now emerging, however, is distinct in several ways. It reflects an expanded mission profile, a reconfiguration of alliance roles and structures, the application of a modernization narrative to justify these changes, and a greater likelihood that they will be implemented in practice.
First, the scope of missions under discussion has widened. The United States is increasingly seeking to use South Korea not only to deter North Korea but also as a strategic platform for responding to extraregional challenges involving China, Russia, and other actors. Second, alliance roles are being recalibrated. South Korea is expected to assume more responsibility for countering North Korea, while U.S. forces are expected to remain less fixed on the peninsula and more mobile. USFK Commander Brunsen noted that “South Korea needs to be stronger against North Korea, and then we can have the flexibility to do other things.” Third, unlike the earlier conceptual understanding, the current debate is taking place under the formal banner of alliance modernization, making strategic flexibility more explicit and more visible, and extending it into discussions about structure, missions, and force posture. Fourth, whereas the 2006 understanding emphasized the need for Seoul’s consent and the will of the Korean public, today’s discussion places stronger expectations on South Korea to contribute to modernization and assume a larger role, accompanied by increased demands in areas such as cost-sharing and defense industrial cooperation.
In effect, the focus has shifted from a peninsula-centered defense concept to a posture that encompasses regional and extraregional contingencies, with South Korea facing heavier responsibilities and the debate moving into a more structural and operational phase. Moreover, while earlier debates sought to ensure that South Korea would not be drawn into external conflicts against its will, the current trend places greater emphasis on giving U.S. forces the ability to operate flexibly with fewer constraints. -
Discussion of the strategic flexibility of USFK exerts layered effects across the entire security architecture of the Korean Peninsula. Its impact extends beyond a simple adjustment of USFK’s missions or force posture and is reflected simultaneously in four dimensions: the credibility of deterrence, the political cohesion of the alliance, South Korea’s autonomous defense capabilities, and the broader spread of security instability in the region.
Military Dimension: Restructuring the Deterrence Framework and Rising Uncertainty
Strategic flexibility implies that USFK would no longer function as a fixed force dedicated solely to the defense of the South Korea against North Korean threats, but rather as a mobile asset capable of operating across the broader Indo-Pacific theater. This shift aligns with emerging U.S. operational concepts such as Dynamic Force Employment (DFE), Agile Combat Employment (ACE), and Expeditionary Advanced Base Operations (EABO). These developments alter the military deterrence posture on the peninsula in two contrasting ways.
On one hand, a dispersed and mobile force structure enhances the survivability of U.S. assets during the initial stages of a conflict and strengthens their integration with adjacent theaters, such as the Taiwan Strait, Okinawa, and Guam, thereby reinforcing extended deterrence at the theater level. In other words, once USFK becomes integrated into the wider Indo-Pacific military architecture rather than focused solely on deterring North Korea, the United States gains greater strategic flexibility and responsiveness, which can reinforce the psychological dimension of deterrence.
On the other hand, this shift risks weakening the symbolic value of physical deterrence that has long rested on a permanent, in-place U.S. presence. Historically, the stationing of U.S. forces in Korea signaled clearly to North Korea that the United States stood directly on the front line of Korean defense. As forces become more rotational, more dispersed, and more frequently deployed beyond the peninsula, North Korea may underestimate U.S. willingness to intervene or misjudge the immediacy of U.S. support for South Korea. This creates structural conditions that can contribute to deterrence failure. In this sense, strategic flexibility enhances the qualitative sophistication of deterrence while increasing symbolic uncertainty, making the security environment on the Korean Peninsula more complex and less predictable.
Political and Diplomatic Dimension: Recalibrating Alliance Cohesion and Burden-Sharing
Strategic flexibility also reshapes the political character of the ROK–U.S. alliance. If the United States begins to view USFK as a dynamic force pool for the entire Indo-Pacific rather than a force dedicated solely to the defense of the Korean Peninsula, South Korea can no longer remain in the position of a passive security beneficiary. Instead, it will be expected to exercise greater autonomy while making greater contributions to sustain the alliance.
In this process, an implicit exchange structure emerges between the two countries: an expansion of strategic flexibility on the U.S. side in return for increased defense contributions from South Korea. Based on the enhanced mobility and utility of USFK, the United States may call for higher defense cost-sharing, joint development of advanced weapons systems, and deeper cooperation in new domains such as cyber, space, and artificial intelligence. In turn, South Korea is likely to respond by seeking stronger sovereign control and more substantive participation in decision-making, potentially through a reinforced model of extended deterrence such as an expanded Extended Deterrence Strategy and Consultation Group (EDSCG).
While improvements in alliance consultation mechanisms can help build mutual confidence, the dynamic can also become a source of strain. If U.S. strategic flexibility is perceived domestically in South Korea as generating strategic dependency rather than mutual benefit, it can intensify political divisions and contribute to alliance fatigue. This means the issue is not solely a matter of military strategy but also a test of political credibility and transparent policy communication.
Strategic Dimension: Coexistence of Extraregional Entanglement Risks and Potential Defense Gaps The essence of strategic flexibility lies in mobility, but the problem is that such mobility does not necessarily operate around the Korean Peninsula. If USFK are deployed to a contingency in the Taiwan Strait or the South China Sea, the immediate defense posture on the peninsula may be temporarily reduced. This could create a window of opportunity for North Korea and increase the likelihood of local provocations or missile threats.
Conversely, if U.S. extraregional operations are conducted using South Korea’s bases, ports, or information networks, South Korea faces the possibility of becoming directly or indirectly implicated in those conflicts. If South Korean territory functions in practice as a launching platform for U.S. operations outside the peninsula, it becomes difficult for Seoul to maintain legal and political neutrality, and the country bears additional security burdens stemming from potential reactions by China and North Korea.
In effect, strategic flexibility creates a dual structure in which the risks of defense gaps on the peninsula and involuntary entanglement in extraregional conflicts exist simultaneously. Addressing this dilemma requires strengthening the capabilities of the South Korean military in parallel with any expansion of USFK’s mobility, particularly in areas such as countering long-range artillery, missile defense, and improvements in reconnaissance and command-and-control systems.
Societal and Psychological Dimension: Emerging Variables in Public Security Perceptions and Domestic Politics The expansion of strategic flexibility for USFK also reshapes public perceptions of security within South Korea. In the past, the mere presence of USFK on the peninsula was regarded as a form of deterrence in itself. Today, the question has shifted to when and where U.S. forces are positioned. A reduction in visible, fixed presence may be interpreted by some as a sign of withdrawal or weakening alliance commitments, potentially heightening public anxiety and fueling political debate.
This dynamic becomes more sensitive if the Trump administration’s characteristic transactional approach to alliances reemerges. Under such circumstances, strategic flexibility may be viewed not as an effort to make the alliance more effective but as a signal that the alliance is only conditionally maintained. This perception could reignite ideological divides in domestic politics between advocates of stronger self-reliant defense and proponents of alliance-centered security.
At the local level, temporary relocations of USFK units or the application of new operational concepts may trigger renewed concerns over environmental impact, noise, risk perception, and compensation responsibilities for host communities. These reactions underscore the reality that strategic flexibility cannot be sustained unless the concept secures broader social acceptance beyond the military domain.
Comprehensive Assessment
The current form of strategic flexibility has evolved beyond the political understanding reached in 2006 and has become an operational framework that is reshaping force employment, military posture, and the diplomatic architecture of the alliance. As a result, security on the Korean Peninsula is shaped by four interrelated dynamics: a reconfiguration of military deterrence, a recalibration of alliance politics, the internalization of extraregional risks, and persistent uncertainty over domestic acceptance.
This situation poses a shared strategic challenge for both South Korea and the United States: how to manage the advantages and risks of strategic flexibility in a balanced manner. For South Korea, strategic flexibility should not be viewed simply as an adjustment to U.S. force posture but as part of a broader process of reconstructing the combined deterrence framework of the alliance for the twenty-first century. Ensuring that strategic flexibility becomes a space where opportunities and risks coexist, rather than a source of vulnerability, requires strengthening three elements in an integrated way: political safeguards, military preparedness, and effective public communication. -
Modern Reaffirmation of the 2006 Agreement and Codification of Operational Guidelines
At the level of presidential and foreign and defense ministerial meetings, South Korea and the United States should reaffirm the 2006 agreement in updated terms and formalize, through legal and operational guidelines, the specific forms of support that South Korea will provide (or will not provide) in the event of an extraregional contingency. This includes defining the scope of information-sharing, logistics, maintenance and medical support, base access, and approval procedures within the combined command structure. This process would update and redesign the political safeguards established in 2006 to reflect the realities of 2025.
Maintaining the Visibility of Deterrence as a Safeguard Against the Costs of Strategic Flexibility
If rotational and dispersed deployments are unavoidable, two measures are essential to reduce perceptions of vulnerability and the risk of miscalculation. First, the visible and regularized deployment of symbolic assets such as strategic bombers, nuclear-powered submarines, and multidomain joint fires should be institutionalized. Second, exercises that operationalize extended deterrence, including nuclear and missile defense drills as well as cyber and space operations, should be established as standing combined training activities. These exercises should be designed to link multiple theaters, particularly between the Korean Peninsula and the first island chain, to institutionalize a form of compensatory deterrence.
ACE/EABO-Aligned Korean Model of Distributed Basing and Civil–Military Integrated Logistics
A distributed-operations checklist should be developed covering Air Force, Marine Corps, and Navy bases as well as civilian airports, commercial ports, and island-based infrastructure. Fuel, maintenance, munitions, and communications should be standardized across military and civilian sectors. In this process, the South Korean military should expand its roles in base defense, air and missile defense, and electronic warfare so that the perception of “distributed = vulnerable” is replaced with the understanding that “distributed = survivability + mobility.”
Pre-Defined Support Levels for Extraregional Scenarios
A contingency matrix should be developed for situations such as a Taiwan Strait crisis, a South China Sea contingency, or simultaneous multi-theater crises involving the Korean Peninsula. Based on this matrix, South Korea and the United States should pre-define a set of direct and indirect support levels for South Korea, structured along a 0~4 scale. For example, level 0 could involve non-military or diplomatic support; level 1 could include ISR + cyber defense; level 2 could provide rear-area hub functions + medical evacuation; and level 3 could involve maritime interdiction + long-range indirect fire support. Establishing these levels in advance would prevent politically contentious after-the-fact debates by replacing them with clear, pre-agreed rules.
Force-Posture Modernization Priorities: Transition from Ground and Artillery Forces to Long-Range Precision / Integrated Air Defense / Command-and-Control
Assuming a possible reduction in U.S. ground forces, South Korea should prioritize investments in long-range precision-strike capabilities designed to counter long-range artillery and hypersonic threats, layered integrated air and missile defense, and joint C2 + data-network integration to offset any perceived gaps. Unmanned–manned teaming systems (drones, USVs, counter-mine assets) and the protection of civilian digital infrastructure should also be incorporated as key pillars of posture modernization.
Domestic Consensus-Building Measures
Strategic flexibility carries both national-security and societal implications. Public debate over the threshold for extraregional support and the permissible scope of base use should be made transparent through National Assembly reporting, public hearings, and official white papers. Local-government compensation and safety protocols should be prepared in advance. This would update the 2006 agreement’s reference to public sentiment and translate it into a model of democratic oversight suited to the conditions of 2025. -
If the 2006 agreement provided operational flexibility anchored in political safeguards, the 2025 version has evolved into theater-level mobility shaped by DFE, ACE, and EABO. This shift has widened both the perceived risk of defense gaps on the peninsula and the gray zone of possible extraregional entanglement. Addressing this is not a matter of simply supporting or opposing strategic flexibility; it requires progress along six parallel lines: ① reaffirming updated political safeguards, ② normalizing visible deterrence, ③ integrating civil–military functions for distributed basing, ④ establishing pre-agreed contribution options for specific scenarios, ⑤ upgrading the quality of South Korea’s force structure, and ⑥ institutionalizing mechanisms for social consensus. Only a simultaneous approach can capture the benefits of strategic flexibility (deterrence, survivability, mobility) while managing its costs (gaps, entanglement, domestic division).
The strategic flexibility of USFK carries a double-edged impact on the peninsula. In the short term, it can strengthen alliance deterrence and elevate South Korea’s international standing. Over the longer term, however, it could heighten instability as U.S.–China rivalry deepens or as North Korea reacts more aggressively.
South Korea should therefore pursue three strategic tasks.
- Institutionalizing deterrence: creating a standing framework for extended-deterrence consultations and a mechanism for coordinating the rotational deployment of U.S. strategic assets.
- Securing autonomous oversight: codifying a formal consultation process for any extraregional use of USFK.
- Strengthening strategic-balancing diplomacy: using South Korea’s middle-power capacity to serve as a buffer in the midst of U.S.–China competition.
Strategic flexibility is more than an adjustment in the basing of U.S. forces. It is a central variable shaping the future trajectory of the alliance and the security order of Northeast Asia. Under the second Trump administration, U.S. alliance policy is being reconfigured through a form of pragmatic realism, producing a complex set of consequences for Korean Peninsula security.
At a fundamental level, USFK are a U.S. military asset, and the U.S. president, as commander-in-chief, holds ultimate authority over their employment. If Washington decides to expand USFK’s role and apply strategic flexibility in responding to extraregional contingencies, even an ally has no formal means to block that decision. Article III of the ROK-U.S. Mutual Defense Treaty, which defines the operational scope of the alliance as the “Pacific area,” also provides legal grounds for the United States to employ USFK flexibly.
At the same time, USFK were originally deployed under the Mutual Defense Treaty for the purpose of defending the Republic of Korea against North Korean threats. Their presence is therefore based on agreement with the host nation, and any new mission involving extraregional activity should follow prior consultation with South Korea in accordance with the spirit of the alliance.
Although no public details have yet been released regarding how the second Trump administration intends to implement strategic flexibility, South Korea should work with the United States to maximize its benefits while strengthening sovereign oversight through enhanced consultation mechanisms, balanced diplomacy toward China and North Korea, and the institutionalization of extended deterrence. South Korea must also carefully consider how it will build its own strategic autonomy going forward.
| Introduction
1) U.S. Department of State, Joint Statement of the ROK-U.S. Strategic Dialogue, January 2006.
2) 외교부, 「한미 외교장관 공동성명」, 2006. 2006년 1월 19일 한미 외교장관 회담 공동성명 “Strategic Consultation for Allied Partnership” "The ROK, as an ally, fully understands the rationale for the transformation of the U.S. global military strategy, and respects the necessity for strategic flexibility of the U.S. forces in the ROK. In the implementation of strategic flexibility, the U.S. respects the ROK position that it shall not be involved in a regional conflict in Northeast Asia against the will of the Korean people.“ “대한민국은 동맹으로서 미국의 글로벌 군사 전략 전환의 논리를 충분히 이해하며, 주한미군의 전략적 유연성의 필요성을 존중한다. 전략적 유연성이 실행될 때, 미국은 한국 국민의 의사에 반하는 동북아 지역 분쟁에 한국이 개입하지 않는다는 한국의 입장을 존중한다”
| The Concept of Strategic Flexibility and the U.S.–ROK Agreement
3) Donald Rumsfeld, Global Posture Review Report, U.S. DoD, 2004.
4) Michael J. Green, By More Than Providence: Grand Strategy and American Power in the Asia Pacific Since 1783, Columbia Univ. Press, 2017, p. 412.
5) 윤덕민, 「주한미군 전략적 유연성과 한국의 대응」, 『국제정치논총』 제46집 2호, 2006, 35쪽.
6) Walter Russell Mead, “The Jacksonian Revolt,” Foreign Affairs, Jan/Feb 2017.
7) White House, Indo-Pacific Strategy of the United States, 2019.
| Implications for Security on the Korean Peninsula
| Policy Recommendation
| Conclusion
※ The contents published on 'Sejong Focus' are personal opinions of the author and do not represent the official views of Sejong Institue
