Strategic Flexibility of USFK and the Future of the ROK-US Alliance

Bee Yun Jo

bjo87@sejong.org​

Research Fellow

Sejong Institute

1. Introduction

Washington’s growing prioritization of China as the main strategic threat, combined with its desire to economize overseas military commitments, has revived the debates surrounding possible reduction and/or strategic flexibility of US Forces Korea (USFK). While previous discussions are often framed on either maintaining ‘status-quo’(preventing any drastic changes of USFK) or ‘some adaptation,’ this article argues that Seoul needs to break away from such reactive or restrained response to a more forward-looking and proactive stance. While Seoul’s geostrategic position limits itself from proposing concepts like ‘One Theater’ and ‘OCEAN’ by Japan, this article finds US’s reposturing in the region including USFK is a structural and less-reversible phenomenon. To ensure deterrence and alliance cohesion are not undermined in the midst of upcoming changes, Seoul needs to prepare for a future in which USFK may become lighter, more mobile, and more regionally oriented. In this backdrop, the article is structured as the following. First, this article provides a concise overview and reinterpretation of the US strategic flexibility debate and analyzes the recently released FY2026 Pacific Deterrence Initiative(PDI) to explore how a second Trump administration might reshape strategic flexibility by transforming the US military presence in the Indo-Pacific, including USFK. Second, acknowledging the structural nature of the anticipated USFK adjustments, the article examines the potential scope of changes- identifying key areas and components- and outlines Seoul’s strategic priorities alongside the range of changes it could reasonably accommodate. Third, the article concludes with policy recommendations, including the proposal of a regional initiative led by Seoul, titled “CNIIP (Conventional-Nuclear Integration in the Indo-Pacific).”

2. Understanding US’s Strategic Flexibility: Means for Congested Security Environment

US’s strategic flexibility is a form of internal balancing, with aim to maximize the use of limited resources. In the 2000s, when the concept of strategic flexibility surfaced most prominently, the Bush administration was pursuing Global Posture Review(GPR) to redeploy its overseas US forces including USFK to the wars in the Middle East after the 9/11. US was also adapting to the need to enhance flexibility in dealing with non-state and multi-domain threats.

Since the 2010s, what is driving the flexibility debate appears to be the increasing diversity of threats that US face. The growing two-peer problem by Russia and China, in addition to threats from North Korea and Iran all demand US to integrate and maximize use of dispersed forces overseas. Particularly since the first Trump administration, the US has continued to prioritize threats from China and Russia, and sought especially to counter China’s growing Anti-Access/Area Denial(A2/AD) capabilities. The first Trump administration, for instance, began to emphasize lethality of US forces via concepts like Dynamic Force Employment(DFE)-the agile and flexible deployment of strategic assets to where they are most needed. The inherent goal is to dilute and exhaust China’s expanding and diversified capabilities by dispersing US forces across the Indo-Pacific, while increasing uncertainties for adversaries through rapid and agile deployment(rotational) in times of crisis.

At the heart of concepts like Army’s “Multi-Domain Operations(MDO),” Navy’s “Distributed Maritime Operations(DMO),” and Marine Corps’ “Expeditionary Advanced Basing Operations” lie a common strategic objective to operate limited military resources more agilely, deter multiple adversaries across multiple areas and domains. The US Joint Chiefs of Staff, for instance, noted that flexible employment of forces can place adversaries in unfavorable positions. President Trump’s 2018 National Defense Strategy(NDS)’s emphasis on the importance of integrating US’s interagency efforts and achieving joint lethality, President Biden’s emphasis on integrated deterrence utilizing all US government agencies, allies, and partners, as to US’s efforts to integrate the command and control of all military branches under the concept of Joint All-Domain Command and Control (JADC2) are all interrelated to ensuring and enabling strategic flexibility of US forces.

In addition, US’s strategic flexibility should be also perceived as US’s means for external balancing. By enhancing flexibility of US’s overseas forces, it is also US’s efforts to integrate more allies and partners in efforts to deter its priority threats-China. In short, strategic flexibility is US’s means to respond to the ever-more congested security environment, including measures to lighten, disperse, and rapidly deploy its forces as well as enhancing operational integration of forces.

3. FY2026 PDI and Trump 2.0’s Reposturing in the Indo-Pacific

Since 2021, US’s Secretary of Defense has been mandated to develop PDI to better illustrate US’s efforts to enhance deterrence and posture in the Indo-Pacific. While PDI is not a separate budget item, it delineates the priorities of the department. In June 2025, President Trump’s FY2026 defense budget was released but PDI remains to be the only publicly available portion. Comparing FY2026 PDI to previous years, three foreseeable trends in US’s reposturing in the Indo-Pacific can be seen.

First is clarity in its strategic priority on China. Echoing Interim National Defense Strategic Guidance’s emphasis on US’s strategic priority in deterring China, FY2026 PDI also characterizes China’s military build-up to be “one of the most dangerous strategic environments in our Nation’s history.” Such depiction of China varies from FY2025 PDI’s “pacing challenge,” FY2024 PDI’s “preeminent pacing challenge,” and FY2022 PDI’s “security threat.”

Second, for the first time, FY2026 PDI highlights the need to enhance “deterrence by denial” in the Western Pacific and enhance defense capabilities of allies and partners to deter(burden) China’s “increasingly capable air and missile threats.” Such emphases also echo President Trump’s Golden Dome as well as FY2026 defense budget, which is known to have allocated the budget along the order of securing US borders($5 billion), America’s Golden Dome ($25 billion), nuclear enterprise (~$60 billion), and deterring China ($12.4 billion). It also connects to President Trump’s demand for Seoul and other Asian allies to allocate 5% of their GDP to defense spending on June 19, 2025. All in all, despite 6% increase in the total FY2026 PDI budget, significant cuts are made to areas such as forward-deployed forces, logistics, and allied/partner capability-building, signaling a more selective approach in resourcing in the Indo-Pacific, <Table 1>.

Third, there is downsizing of ground forces and realignment along first and second-island chain. Most explicitly, Army-related budget under the “Modernized and Strengthened Presence” category of PDI(FY2026) has decreased by approximately 68% from the previous year, <Table 2>. In addition, out of the ④ Infrastructure Improvements category ($2.6 billion), about 88% is allocated to Navy/maritime-related facilities, 7% to the Air Force, and 5% to ground forces. A significant portion is assigned to Guam and surrounding areas in and beyond the Second Island Chain, as well as Palau, Australia and Japan. Another notable point is that the FY2026 PDI newly states its budget support for transitioning US Forces Japan(USFJ) into a Joint Force Headquarters(JFH), highlighting how JFH will enhance regional operational readiness, strategic posture, and theater-level logistics capabilities.

In sum, FY2026 PDI suggests that US ground forces in the Indo-Pacific region will become lighter, with a continued shift toward rotational deployment and rapid force projection centered on air and maritime power. Second, to enable strategic flexibility, there will be continued push for enhanced integration of intelligence and command-and-control systems, as well as a missile defense–focused posture. Third, there will be increased alignment of hubs along the first and second island chain, outside of the Korean Peninsula.

4. Forseeable Adjustments of USFK

In the backdrop of US’s reposturing in the Indo-Pacific, there are three areas of possible adjustments of USFK.

1) Ground Force Reductions

As the Wall Street Journal reported, there is possibility of withdrawing 4,500 U.S. troops from Korea. It projected that the rotationally deployed Stryker Brigade(under 2nd Infantry Division), currently rotating every 9 months since 2022, may be redeployed to Guam or other Indo-Pacific bases. The FY2026 National Defense Authorization Act (NDAA) versions passed by the Senate and House Armed Services Committees differ regarding the USFK’s troop level. The Senate NDAA permits adjustments if the Secretary of Defense can “certify” to Congress that they align with US’s national interests. The House NDAA, by contrast, includes language supporting the maintenance of the current USFK force level of 28,500. Final coordination is pending by year-end, but the broader trend of downsizing ground forces in the Indo-Pacific as outlined in the FY2026 could make partial adjustments to the 2nd Infantry Division likely.

2) Air Force Reconfiguration

The two fighter wings under the 7th Air Force are already undergoing overall reconfiguration and realignment. First, the 25th Fighter Squadron under the 51st Fighter Wing (Osan) is set to retire its 24 A-10 Thunderbolt aircraft by September 2025. Afterward, half of the personnel will be reassigned either to new aircraft or to other A-10 units in the US. Second, the F-16s previously split between Osan and Kunsan have been consolidated at Osan since July 2024. F-16s from the 8th Fighter Wing (Kunsan) have been transferred to the 36th Squadron under the 51st Fighter Wing at Osan, forming a “super squadron” currently operating 31 F-16s. By October 2025, the remaining F-16s at Kunsan will also be relocated, forming two super squadrons of 62 F-16s total. About 1,000 personnel from Kunsan will also be temporarily assigned to Osan. Third, with the F-16 relocation, the US is reportedly considering permanent deployment of one F-35A squadron (approx. 20 aircraft) and rotational deployment of another at Kunsan.

In sum, the adjustment of air assets are centered around: A-10 retirement, F-16 relocation to Osan, and permanent and rotational F-35A deployment at Kunsan. This restructuring echoes increased flexibility via air force and expanded rotational basing in the FY2026 PDI.

3) Potential Large-Scale Reductions and Adjustments

Dan Caldwell, who served as senior adviser to Secretary of Defense Pete Hegseth, recently argued that the US should withdraw most ground forces from Korea and retain only two Air Force squadrons, reducing the troop level to around 10,000. He recommended shifting the center of gravity for US presence to the Second Island Chain centered on Guam. If flexibility within Korea remains limited, a substantial withdrawal of ground forces and even air squadrons could be pursued, along with greater reliance on rotational deployments. As noted earlier, the US is already pursuing modernization of key bases along the First and Second Island Chains. Following large-scale redeployment abroad, the US may increase the proportion of forces rotated into Korea.

Yet, such large scale adjustments may not be about reductions only. With emphasis on deterrence by denial, additional THAAD deployments, demand for increased integration of missile defense systems could be also expected. Furthermore, as discussed in Track 1.5 workshops by NBR (April 25, 2025, Honolulu), there has also been discussions of restoring or modernizing tactical nuclear storage facilities in the Indo-Pacific, leaving open the possibility of temporary or rotational deployment of US’s tactical nuclear weapons.

4) Implications for Seoul

What does this all mean to Seoul? Five points could be made here. First, there will be increased divergence on threat perceptions and priorities between US and ROK. As widely known, North Korea remains Seoul’s top priority. This has pushed Seoul to often view strategic flexibility as signals to reduced US’s strategic focus on North Korea and potential risks of entrapment in US-China competition. Second, ROK forces will have to burden early ground warfare. Especially with wartime OPCON transition, South Korea’s defense burden could increase rapidly. Third, there is risks of downgrading of USFK. There is increasing speculations that USFK transformation will be simultaneously implemented with wartime OPCON transfer and appointing a three-star general as USFK/future CFC’s deputy commander while a four-star general leads USFJ and also relocate UN Command to Japan. Fourth, there is a potential for public backlash. President Trump’s pressure on tariffs, cost-sharing, and defense spending increases may fuel negative public sentiment toward the alliance. Unilateral adjustments and increased rotations and traffic from USFK could provoke friction with local communities. Last but most importantly, there are risks of weakening of deterrence against North Korea. As threat perceptions and priorities diverge, disagreements within the alliance may intensify. Trump’s “alliance-bashing” rhetoric may further damage the relationship. USFK adjustments done in the backdrop of such environment will directly affect the credibility of the alliance’s deterrence and extended deterrence, possibly encouraging opportunistic or provocative behavior from North Korea and other regional actors.

5. Policy Suggestions

Given this structural shift, I argue that Seoul must craft a multi-pronged, forward-leaning strategy centered on conditional accommodation, alliance management, and regional visioning. For Seoul, its strategic priorities under the second Trump administration should be to prevent radical adjustments to USFK, engage in careful alliance management and message coordination, and gradually enhance its own defense capabilities.

1) Accepting a Reasonable Level of Adjustment

Acknowledging the possibility of partial drawdowns, Seoul should engage in constructive dialogue to maintain a stable force baseline. This includes, proactively suggesting a new minimum of ground presence of ~15,000 troops, retention of two F-16 super squadrons at Osan and one F-35A squadron at Kunsan, and maintaining four-star command structure.

2) Shaping Strategic Flexibility for ROK-US’s Mutual Interests

While accepting partial expansion of rotational deployment and flexibility of USFK ground and air forces, Seoul should negotiate the limits of USFK’s flexibility, particularly regarding Taiwan contingencies. Dr. Kim Jungsup at Sejong Institute, for instance, argues that while accepting flexibility, Seoul should demand US to dispatch its forces to other third base first so that no direct deployments are made between USFK and Taiwan at times of contingency.

3) Seamless ROK-US Communication and Active Message Coordination

The above adjustments require seamless bilateral communication and active message coordination. Any changes to USFK must proceed through close bilateral consultation. Joint understanding of threats and strategic goals should be actively coordinated.

4) Deepen ROK-US Conventional-Nuclear Integration (CNI)

Since the 2023 Washington Declaration and the launch of the Nuclear Consultative Group (NCG), the ROK and US have worked to strengthen integration of US nuclear forces and allied conventional capabilities. Seoul has sought to specify its conventional role within US nuclear strategy, thereby locking in US security commitments including nuclear means and improving the ROK’s own capabilities. Within the CNI framework, Seoul should use the momentum to demand the continuity of extended deterrence commitments. For example, Seoul should seek to reaffirm declaratory policy that if North Korea uses nuclear weapons, the US will use “all range of capabilities” and bring about the “end of the Kim regime.” Seoul should also seek to maintain institutionalized mechanisms such as the Extended Deterrence Strategy and Consultation Group (EDSCG) and the NCG, as well as ensure regular combined drills and exercises. In terms of capability, Seoul should also use USFK adjustments to secure deployment of new strategic assets (e.g., F-35A, missile defense networks). USFK’s F-35A can be a dual-capable aircraft (DCA) and potentially serve as a tactical nuclear delivery platform, enabling discussions on US tactical nuclear deployment and Korea’s participation in DCA missions. It should be also noted that the F-16 super squadrons are already serving as an effective deterrent against North Korea. On May 1, 2025, North Korea’s Rodong Sinmun denounced the creation of the first super squadron as a “dangerous military act for a preemptive strike.” Missile defense will be also a critical tool within the CNI and extended deterrence frameworks. It complements preemptive strike credibility and ensures second-strike (denial) capability alongside F-16s and F-35As stationed on the Korean Peninsula.

5) Shaping Strategic Flexibility Through Seoul’s Regional Proposals

Seoul should counterbalance potential abandonment risks by proactively proposing regional security concepts. What I propose here is the “CNIIP” (Conventional-Nuclear Integration in the Indo-Pacific)-to institutionalize nuclear-conventional coordination among the US, ROK, and like-minded partners. Including workstreams such as intra-alliance cooperation on information-sharing, joint training and exercises in the Indo-Pacific, Seoul can propose efforts to engage in US’s regional initiatives, lock-in US commitments, and continue to enhance bilateral CNI within. Strategic messaging of such initiative will be valuable to any adjustments made to the USFK and US’s military presence in the Indo-Pacific. For other regional allies, CNIIP will be also meaningful as it can enhances information-sharing and understanding of US nuclear operations in the region, which are otherwise unilaterally determined. It will also enable allies to expand and define their conventional roles, boosting national capabilities. Should US also newly deploy strategic assets in the region, CNIIP can be new means to for coordination and consultation. From US’s perspective, CNIIP serves to encourage allied participation in deterring China, secure conventional assets and burden-sharing of the allies, and send a clear message of commitment to both allies and adversaries.

For details, I propose a common strategic goal of CNIIP to be “peace and stability in the Indo-Pacific.” To implement CNIIP, I suggest CNIIP through alliance-based division of labor. According to the KIDA’s quadrilateral POL-MIL conducted in September 2024, the ability to secure robust deterrence and early termination of war on the Korean Peninsula are essential to deterring and controlling escalation in a Taiwan contingency. Seoul should frame its contribution to Indo-Pacific security under CNIIP through: (1) Maintaining deterrence against North Korea, (2) Ensuring early victory in crisis on the Korean Peninsula.