File
|
Modernizing the ROK–U.S. Alliance and Washington’s Strategic Perspective |
| September 25, 2025 |
-
Jungsup KimPrincipal Fellow, Sejong Institute | jungsupkim@sejong.org
-
The modernization of the ROK–U.S. alliance stands as one of the most critical challenges in the Lee Jae-myung administration’s relations with Washington, alongside ongoing tariff negotiations. At the bilateral summit held in Washington on August 25, the two governments reached a broad agreement on a 15 percent tariff rate and $350 billion in South Korean investment in the United States. However, disagreements remain over the structure and scope of this investment, and debates on tariff-related issues continue. By contrast, the issue of alliance modernization has so far drawn relatively limited public attention. Although President Trump positively assessed South Korea’s commitment to increasing its defense budget, detailed discussions have yet to take shape. Nevertheless, the modernization of the alliance is as sensitive and consequential as the tariff and investment agenda, as its core issues such as the strategic flexibility of U.S. Forces Korea (USFK), potential troop reductions, and adjustments in mission and force composition directly affect South Korea’s national security. Since the summit addressed only the general framework of the issue, these matters are expected to be further elaborated through follow-up consultations between the two countries’ foreign and defense authorities. The Trump administration’s vision for alliance modernization can be summarized in two main directions. First, South Korea should assume greater primary responsibility for its own defense against North Korean threats. Second, the role of the USFK and the alliance should be realigned to serve more effectively in deterring China. In this context, Washington has emphasized South Korea’s defense budget increase and raised the possibility of changes in the mission, troop levels, and overall structure of the USFK.
Since the signing of the ROK–U.S. Mutual Defense Treaty in 1953, the alliance and the USFK have consistently focused on deterring the North Korean threat. During the Cold War, countering communist North Korea was synonymous with containing Soviet expansion, and after the collapse of the Soviet Union, the USFK together with U.S. Forces Japan remained a core pillar of American power projection and stability in the Asia-Pacific region. Recently, however, Washington has begun stressing that South Korea should no longer remain a unilateral security beneficiary but rather share in the responsibility of supporting the United States in times of crisis, arguing that this is consistent with the principles and intent of the mutual defense treaty. From a historical perspective, however, the United States’ core interest in its East Asian alliances has never been mere protection but rather securing base access and strategic positioning. The forward deployment of U.S. forces on the Korean Peninsula has served both to stabilize the region and sustain American hegemony. Yet with China’s rapid rise and the growing strategic importance of the Taiwan Strait and the South China Sea, the North Korean threat has been relegated to a secondary concern, while U.S. military resources are increasingly being reoriented toward deterring China. This evolution has begun to expose fractures in the alliance’s shared threat perception and may weaken the strategic consensus underpinning the alliance’s purpose and role.
If the modernization of the alliance proceeds as envisioned by the United States, it could bring significant changes to the size and role of USFK as well as to the overall ROK–U.S. command structure. Within South Korea, various projections and scenarios have emerged regarding the scope and trajectory of such changes. In the case of a potential reduction of U.S. forces, proposals range from modest adjustments involving several thousand troops to large-scale withdrawals numbering in the tens of thousands. Related discussions have also intensified over the transfer of wartime operational control (OPCON), the credibility of extended deterrence, and even the possibility of South Korea’s independent nuclear armament. However, a notable limitation in current debates is the tendency to present hypothetical scenarios and policy issues in a purely descriptive manner, without sufficient attention to the diverse strategic perspectives in Washington regarding the future of the alliance and the USFK.
While the broader trend of U.S. strategic priorities shifting toward countering China’s economic and military rise is clear, there is no single consensus in Washington on the precise role and position that the ROK–U.S. alliance and the USFK should play within this overall framework. Differing assessments of USFK troop levels and posture adjustments, the reliability of extended deterrence, and the timing of OPCON transfer all stem from these varying schools of strategic thought. In particular, within the debate over China containment, there are contrasting views on the scope and focus of U.S. military posture in the Indo-Pacific: one traditional perspective identifies the Taiwan Strait as a vital theater for U.S. interests, while another advocates reorganizing U.S. force posture around the Second Island Chain as the core of regional strategy. These differing visions of China policy inevitably have profound implications for the ROK–U.S. alliance.
This paper categorizes the range of strategic perspectives on the alliance and the USFK within Washington into three broad frameworks: the alliance adjustment view rooted in a China-first approach; the alliance reduction view associated with strategic retrenchment; and the alliance expansion view emphasizing the enduring strategic value of the Korean Peninsula. Since the rise of Donald Trump, U.S. foreign policy debates have increasingly revolved around three schools of thought: prioritizers, who advocate focusing on China; restrainers, who argue for reduced overseas commitments; and primacists, who seek to sustain U.S. global dominance. Though not perfectly aligned, these three frameworks roughly correspond to alliance adjustment, reduction, and expansion, respectively, when applied to the ROK–U.S. context. By analyzing these competing strategic perspectives in depth, this study aims to provide a more systematic understanding of the modernization of the ROK–U.S. alliance and to contribute to the formulation of South Korea’s policy responses in navigating the evolving strategic environment. -
A leading rationale driving the modernization of the alliance is a strategy of prioritization advocated by U.S. Deputy Secretary of Defense for Policy Elbridge Colby, which places countering China at the core of U.S. national security strategy. Colby has consistently argued that America’s strategic focus and resources should be concentrated on deterring China.1)
The Interim National Defense Guidance, released by the U.S. Department of Defense in April 2025, also reflects this strategy of prioritization, identifying China deterrence as its foremost objective while assigning responsibility for addressing other regional threats such as Russia, North Korea, and Iran to U.S. allies. In this context, Washington seeks greater financial and military burden-sharing from its allies, while concentrating its own strategic investments and posture adjustments on countering China, particularly in preparation for potential contingencies in the Taiwan Strait.
The strategy of prioritization challenges the liberal hegemonic diplomacy that has guided mainstream U.S. foreign policy since the end of the Cold War, advocating a reduction of U.S. engagement in the Middle East and Europe. It is based on the recognition that the United States no longer has the resources to sustain two simultaneous theaters of operation and on the assessment that neither the Middle East nor Europe constitutes a vital national interest for America.2) In February 2025, Secretary of Defense Pete Hegseth stated explicitly in a NATO address that “harsh strategic realities prevent the United States from focusing its primary attention on European security,” marking a sharp departure from the traditional position that emphasized America’s historical and cultural ties to Europe and its dominance over the Eurasian continent. Furthermore, the strategy of prioritization led by Colby is skeptical of linking strategies between the Indo-Pacific and NATO. While the Biden administration highlighted the importance of Europe’s military presence in the Indo-Pacific and the deepening of cooperation between the AP4 (South Korea, Japan, Australia, and New Zealand) and NATO, the Trump administration’s Pentagon now argues that the United States should not disperse its resources across both regions. Instead, it maintains that Washington should assign Europe primary responsibility for its own defense and concentrate America’s strategic focus on the Indo-Pacific. This reflects a division-of-roles approach in which Europe manages the Russian threat while the United States and its Indo-Pacific allies focus on countering China.
The strategy of prioritization criticizes traditional hegemonic diplomacy while maintaining a hardline stance that rejects strategic retrenchment in competition with China. It views the Taiwan Strait as a critical strategic locus that will determine the regional balance of power between the United States and China in East Asia and the Western Pacific.3) The logic rests on a pessimistic assessment that if Taiwan were forcibly absorbed by China, America’s strategic position in the region would rapidly erode, potentially leading to its eventual expulsion from East Asia and the Western Pacific. Accordingly, the United States must establish a strong anti-China alliance network in the Indo-Pacific, identifying Australia, Japan, and the Philippines as key partners. The ROK–U.S. alliance is also treated within this Indo-Pacific strategic framework. From this perspective, alliances remain important U.S. assets, but their function is reduced to an instrumental role, valuable only insofar as they contribute to containing China. Discussions about the possible redeployment of USFK to Guam or other locations, as well as debates over granting the USFK ‘strategic flexibility’ to operate beyond the Korean Peninsula rather than remaining a fixed force solely for deterring North Korea, are rooted in the same logic. Within this strategic vision, the USFK could face partial downsizing, with emphasis placed on reconfiguring force composition and ensuring operational flexibility for missions beyond the peninsula. Ultimately, maintaining the status quo of the USFK and the alliance would likely be rejected under the strategy of prioritization.
However, it remains uncertain whether the defense strategy outlined by the Department of Defense will actually be realized. When the concept of alliance modernization was first introduced, expectations were high for a reduction of USFK, greater strategic flexibility, and possible adjustments to the ROK–U.S. command structure. Recently, however, a different trend has begun to emerge. According to individuals familiar with the draft of the forthcoming National Defense Strategy (NDS), the top priority has shifted from countering China to homeland defense. Strengthening border security along the southern frontier, addressing illegal immigration and drug trafficking, and enhancing missile and space threat responses under the ‘Golden Dome’ system have been highlighted as primary objectives. In practice, the Trump administration has already mobilized thousands of National Guard troops to reinforce the southern border and support immigration enforcement, while deploying multiple fighter aircraft and naval assets to the Caribbean region.
If homeland defense becomes the central pillar of U.S. defense strategy, it could affect both Washington’s plan to concentrate its military resources on countering China and the driving logic behind the modernization of the ROK–U.S. alliance. In broad terms, the United States remains aligned with the ‘prioritizers,’ yet there are growing signs that the focus is shifting from countering China toward defending the U.S. homeland and the Western Hemisphere. Even if the forthcoming National Defense Strategy emphasizes the importance of countering China, translating that emphasis into concrete policy is another matter. Within the Trump administration, there are notable divisions over China policy: while the Department of Defense and the Joint Chiefs of Staff continue to view China as the top strategic threat, President Trump appears more interested in achieving favorable outcomes in trade negotiations.4) Therefore, it is uncertain whether the Department of Defense will actually seek to align the ROK–U.S. alliance with its broader strategy toward China. -
The most radical proposal concerning the adjustment of U.S. overseas force posture has been put forward by Dan Caldwell, a former senior adviser to Secretary of Defense Pete Hegseth, and Jennifer Kavanagh, a senior fellow at the Washington-based think tank Defense Priorities. In their joint report released on July 9, 2025, they argued that the United States should fundamentally restructure its global military posture to better align with its national interests. Regarding USFK, they proposed a drastic reduction of troop levels from the current 28,000 to approximately 10,000.5)
Specifically, they recommended the withdrawal of all ground combat units not directly tied to base defense, including most of the 2nd Infantry Division, rotational brigade combat teams, and army aviation brigades, as well as two fighter squadrons and personnel from maintenance and other support units. The remaining maintenance and support staff would be retained only under the assumption that U.S. bases in Korea could serve as logistical and maintenance hubs in the event of regional contingencies. Ultimately, their position calls for reducing the U.S. military presence on the Korean Peninsula to a level approaching full withdrawal.
Such a radical proposal stems from a deeply skeptical view of employing USFK for operations beyond the Korean Peninsula. Proponents argue that the USFK’s main component, the ground forces, is inherently ill-suited for out-of-area missions, and that even the U.S. Air Force units based at Osan and Kunsan have limited operational utility due to their extended range to the Taiwan Strait. Although the U.S. Army has established Multi-Domain Task Forces (MDTFs) equipped with long-range precision strike, electronic warfare, and cyber capabilities to prepare for potential conflict with China, these analysts dismiss such efforts as little more than an institutional survival strategy. In their view, the Army’s role in any potential U.S.-China confrontation across the Indo-Pacific maritime domain would remain extremely limited. Accordingly, they argue that partial troop reductions or minor adjustments in force composition would be insufficient, making a comprehensive restructuring of the U.S. force posture in the region inevitable.
More fundamentally, the retrenchers envision an Indo-Pacific strategy distinct from that of the prioritizers. While the latter regard the Taiwan Strait as a strategic linchpin from which the United States can never retreat, retrenchers seek a stable balance of power between Washington and Beijing rather than the preservation of American hegemony in East Asia and the Western Pacific. They argue that even if Taiwan were absorbed by China, it would not cause fatal damage to the U.S. strategic position in the Indo-Pacific, and therefore Taiwan does not constitute a vital national interest 6) This view rests on the assumption that several regional powers such as Japan, Australia, ASEAN members, and South Korea would resist subordination to a Chinese-led order, and that East Asia’s maritime geography limits Beijing’s ability to achieve total regional dominance. 7) Accordingly, retrenchers contend that while “Taiwan certainly matters to the United States, it does not matter enough to justify a war with China.”
The retrenchers criticize the current forward deployment of U.S. forces, arguing that bases located too close to China’s coastline are vulnerable and militarily unsuitable even in the event of conflict. They therefore call for a redistribution of U.S. forces in both South Korea and Japan toward the east and the second island chain. Specifically, they propose relocating two air wings from Kadena Air Base to Misawa or Yokota, and transferring several thousand Marines to Guam, effectively reducing the number of U.S. troops stationed in Okinawa by roughly 14,000. While they seek to lower the strategic importance of the Taiwan Strait and the South China Sea, they emphasize maintaining control over the Philippine Sea, which they regard as essential for ensuring U.S. access to Asian markets and sustaining a minimum balance of power with China.
A comprehensive realignment of U.S. forces in Korea could weaken the ROK–U.S. alliance and undermine the credibility of extended deterrence. Such a shift could fuel domestic calls for an independent nuclear capability in South Korea and strain the global nonproliferation regime. Although the retrenchers recognize these risks, they consider them unavoidable trade-offs. In their view, limited nuclear armament by allied states is an inevitable consequence of reducing U.S. overseas deployments and could even promote greater self-reliance and defense autonomy among allies. This reasoning reflects a belief that an absolute guarantee of extended deterrence is not necessarily desirable, as it could entangle the United States in nuclear escalation over conflicts that do not involve its vital interests.
While this comprehensive realignment proposal departs from both traditional U.S. grand strategy and the current Trump administration’s policy orientation, it cannot be entirely dismissed. Given growing public fatigue with foreign interventions and the rising tide of isolationist sentiment in the United States, such views could still emerge as a significant undercurrent in future debates over U.S. defense strategy. -
While both the China prioritizers and the retrenchers differ in degree, they share a common inclination to propose a reduction or adjustment of U.S. forces stationed in South Korea. This stems from the belief that America’s strategic priorities should shift from the Korean Peninsula to the Taiwan Strait, or that the overall U.S. military posture in the Indo-Pacific should be restructured around the second island chain. However, even within the broader Indo-Pacific strategy aimed at countering China, there remains a perspective that emphasizes the strategic and operational importance of the Korean Peninsula. This view regards the response to the North Korean threat and the containment of China not as separate missions but as interrelated challenges, and it recognizes the military assets on the Korean Peninsula as potentially playing a vital role in any future Taiwan Strait contingency. 8)
From this perspective, U.S. bases on the Korean Peninsula should not be reduced but instead developed into operational and logistical hubs that would allow the United States to respond rapidly to contingencies across the region. In this context, the future posture of USFK is not limited to preservation and could be reinforced. During his visit to Seoul on September 14, Indo-Pacific Commander Samuel Paparo reportedly told Minister of National Defense Ahn Kyu-baek and Minister of Foreign Affairs Cho Hyun that “there are no plans to reduce U.S. forces in Korea.” Although the basis for his remarks was not specified, his statement suggests that Washington continues to recognize the strategic importance of USFK even amid intensifying U.S.–China rivalry. A former commander of U.S. Army Pacific, speaking anonymously in a recent interview with Korean experts, likewise argued that USFK’s role and capabilities should be strengthened. He emphasized the need for USFK to evolve into a ‘dynamic force’ combining permanent and rotational deployments, reorganized around capability areas such as electronic warfare, cyber operations, and unmanned systems. Under such a structure, USFK could effectively function as a de facto Far East Command responsible for broader regional operations across Northeast Asia.
The strategic value of the Korean Peninsula derives from its high operational utility during regional conflicts. Power projection from the second island chain or the U.S. mainland would entail enormous costs and operational risks. The distance between the Taiwan Strait and the U.S. Navy base in San Diego exceeds 10,000 kilometers, while even Guam is nearly 3,000 kilometers away. Without assured air and maritime superiority across the Pacific, the secure deployment of forces into contested areas would be extremely difficult. In contrast, as USFK Commander Xavier Brunson has described, the Korean Peninsula serves as “an aircraft carrier located between Japan and mainland China,” enabling decisive contributions to regional operations. The peninsula can also function as a hub for ammunition and materiel stockpiling, transportation, maintenance, and logistics support.
This perspective also highlights the linkage between the North Korean threat and a potential conflict in the Taiwan Strait. A crisis in the Taiwan Strait could prompt provocations from Pyongyang, while a conflict on the Korean Peninsula could invite Chinese military involvement. Even short of a full-scale attack on South Korea, North Korea might, as it did by deploying troops to support Russia, seek some form of coordination with China during a Taiwan contingency. Stability on the Korean Peninsula is therefore inseparable from stability in the Taiwan Strait, and enhancing U.S. defense capabilities in Korea directly contributes to Taiwan’s security. However, this logic assumes that South Korea would align with the United States in the event of a Taiwan contingency; if Seoul keeps its distance out of concern for entanglement, the peninsula’s strategic value to Washington would diminish.
For this reason, those who emphasize the strategic importance of the Korean Peninsula argue that the ROK–U.S. Mutual Defense Treaty should be understood as a framework for mutual assistance rather than one-sided protection, and they call for South Korea’s more proactive participation in the alliance.9) They also view ‘strategic flexibility’ as a concept that has already been realized in practice. Just as U.S. infantry, aviation, and air defense units stationed in Korea have been repeatedly deployed to conflicts in the Middle East, they believe that, in the event of a U.S.–China conflict, Washington would likely employ U.S. Forces Korea as it deems necessary, regardless of any peacetime agreements between the two governments. -
It is difficult to predict how the modernization of the ROK–U.S. alliance will actually take shape on the Korean Peninsula. Whether U.S. Forces Korea will be reduced, and if so, to what extent, remains uncertain. Conversely, the possibility of maintaining or even enhancing the current posture cannot be ruled out. As examined earlier, multiple strategic perspectives coexist within Washington regarding alliance modernization, and these competing lines of thought will continue to interact before ultimately converging toward a single direction. In particular, the modernization of the alliance is not an issue confined to the Korean Peninsula; it is closely linked to the broader realignment of U.S. global military posture, including in the Indo-Pacific. According to reports, the National Defense Strategy (NDS) currently being finalized places greater emphasis on homeland and Western Hemisphere defense than on countering China, contrary to earlier expectations. This underscores the inherent uncertainty in the overall strategic orientation of U.S. defense policy. Moreover, many experts in Washington note that the role of Deputy Secretary of Defense Elbridge Colby, often labeled a hardliner on China, has been overstated, while Secretary of State Marco Rubio appears to wield greater influence. Given Rubio’s emphasis on the importance of alliances, the scale of change stemming from alliance modernization may be more limited than anticipated.
A more fundamental source of policy uncertainty lies in the weakened coordination function of the National Security Council (NSC), which has diminished the ability to reconcile differing views within the Trump administration. The Department of Defense, the State Department, and the White House are reportedly divided, raising the likelihood of policy incoherence shaped by bureaucratic and factional power struggles. Trump’s personal style of leadership further complicates any stable policy outlook. His foreign policy departs sharply from traditional U.S. liberal internationalism, prioritizing tariff negotiations and domestic manufacturing revival under an ‘America First’ agenda while showing limited interest in global leadership or alliance management. Yet Trump’s foreign policy is not purely isolationist in the manner many ‘MAGA’ supporters envision. His authorization of strikes on Iran, continued engagement in NATO, and occasional moderation toward China on security issues demonstrate a more pragmatic approach driven by situational judgment. Ultimately, in the absence of a coherent decision-making process, policy direction is likely to be determined by whoever most effectively captures Trump’s attention at a given moment, underscoring the unprecedented influence of presidential intuition and personal networks.
Despite these uncertainties, understanding the underlying strategic logic behind alliance modernization remains essential. Only by grasping the rationale and context of potential shifts can Seoul formulate appropriate responses. Simply listing scenarios such as troop adjustments, command structure changes, or extended deterrence issues without systematic analysis will not lead to an effective policy strategy. At present, preparing for change on the basis of the ‘alliance adjustment’ framework appears most prudent. In other words, while maintaining clear limits on the alliance’s role in U.S. efforts to contain China, South Korea should take greater responsibility for its own defense. This requires securing key indigenous military capabilities, advancing the transfer of wartime operational control (OPCON), and developing nuclear fuel enrichment and reprocessing capabilities.
Given the complex power dynamics within the Trump administration, it is possible that the alliance will experience less change than expected. However, the gradual transition toward greater South Korean responsibility for national defense is an irreversible trend, and Seoul must shift from a reactive stance to proactively shaping a new alliance framework that reflects its own strategic interests. The ‘alliance expansion’ perspective, while beneficial for cohesion, carries the risk of entanglement in U.S.–China conflicts, potentially jeopardizing South Korea’s security and straining relations with China. Conversely, ‘alliance retrenchment’ remains an unlikely short-term outcome, as it would imply a U.S. strategic retreat acknowledging China’s sphere of influence and risk excessive dependence on Japan. Still, given the evolving military balance between the U.S. and China and the resurgence of isolationist sentiment within the U.S., such views may gain traction over time and thus warrant close monitoring. Ultimately, the modernization of the ROK–U.S. alliance arises from shifts in Washington’s broader China strategy. Despite the Trump administration’s unpredictability, alliance evolution is inevitable amid changing international dynamics. Therefore, South Korea must not passively follow U.S. decisions but instead deepen its understanding of American strategic thinking and craft an active, forward-looking strategy for alliance redesign.
| Introduction
| Alliance Adjustment Perspective of Prioritizers
1) Elbridge A Colby, The Strategy of Denial: American Defense in an Age of Great Power Conflict, New Haven & London: Yale University Press, 2021.
2) Jennifer Lind and Daryl G. Press, “Strategies of Prioritization: American Foreign Policy After Primacy”, Foreign Affairs, July/August 2025.
3) Gideon Rachman, “Why Taiwan Matters to the World”, Financial Times, April 10, 2023. Regarding a comprehensive assessment of Taiwan’s geopolitical, commercial, and ideological value, Luke P. Bellocchi, “ The Strategic Importance of Taiwan to the United States and Its Allies: Part One”, The US Army War College Quarterly: Parameters, vol. 53, no. 2, 2023.
4) James Palmer, “The Pentagon Fixates on War Over Taiwan”, Foreign Policy, May 6, 2025.2023.
| Alliance Retrenchment Perspective of the Pivot Home Advocates
5) Jennifer Kavanagh and Dan Caldwell, “Aligning global military posture with U.S. interests,” Defense Priority, July 9, 2025. https://www.defensepriorities.org/explainers/aligning-global-military-posture-with-us-interests/
6) Charles L. Glaser, “Washington Is Avoiding the Tough Questions on Taiwan and China”, Foreign Affairs, April 28, 2021.
7) Jennifer Kavanagh and Stephen Wertheim, “The Taiwan Fixation: American Strategy Shouldn’t Hinge on an Unwinnable War”, Foreign Affairs, March/April, 2025.
| Alliance Expansion Perspective based on the Strategic Value of the Korean Peninsula
8) Brian Kerg, “South Korea is the ideal anchor for the first island chain”, Atlantic Council, July 10, 2025.
9) Markus Garlauskas, “The ‘ironclad’ US-South Korea alliance is outdated: A new age requires a ‘titanium’ alliance”, Atlantic Council, June 24, 2025.
| Prospects and Policy Responses
※ The contents published on 'Sejong Focus' are personal opinions of the author and do not represent the official views of Sejong Institue
