American and South Korean officials often describe the Republic of Korea (ROK)-U.S. alliance as ironclad, with both allies working in lockstep on their combined defense posture. While nice sounding, such truisms will soon be tested.
From the secretary of defense to military commanders in the region, U.S. authorities increasingly characterize Indo-Pacific alliances and U.S. forces stationed on allied territory – like United States Forces Korea (USFK) – as critical to deterring and, if need be, contesting China. However, policymakers across South Korea’s highly partisan political spectrum do not welcome the expansion of USFK’s role beyond deterring and defending against North Korea, nor the increased expectations such a change would invariably place upon Seoul and the alliance.
Given this existing tension, the Trump administration must engage the next South Korean administration, due to come into office on June 4, on several issues, including how to recalibrate USFK’s force posture to cover peninsula deterrence alongside addressing shared regional priorities, evolve the alliance’s combined command structure for Seoul to take a lead role, and align (or at least deconflict) Seoul and Washington’s respective understandings of their mutual defense obligations.
Recent Signals Raise Concerns in Seoul
The Pentagon and regional commanders have sent clear signals about U.S. intentions. In both the “Interim National Defense Guidance” and Statement on the Development of the 2025 National Defense Strategy, Secretary of Defense Pete Hegseth made it abundantly clear that U.S. force posture and alliances in the region must prioritize deterring China as the sole pacing threat, and denying China’s seizure of Taiwan as its sole pacing scenario.
Furthermore, in recent public testimony, U.S. Indo-Pacific Command (INDOPACOM) Commander Adm. Samuel J. Paparo and Gen. Xavier T. Brunson, commander of the United Nations Command, Combined Forces Command (CFC), and USFK, revealed “an unprecedented level of synchronization and strategic integration” between INDOPACOM and USFK. More than previous commanders, Brunson highlighted the fact that U.S. forces on the peninsula posed dilemmas for and could impose costs on multiple adversaries, including China, Russia, and North Korea, with particular emphasis on the areas around the peninsula.
Additionally, Brunson emphasized that the ROK-U.S. Mutual Defense Treaty “commits both nations to provide mutual aid if either faces an external armed attack, but it does not specifically name an adversary.”
Mutual defense obligations in Article III of the treaty apply in the event of “an armed attack in the Pacific area on either of the Parties.” An “armed attack” could come from anywhere, and the “Pacific area” extends well beyond the Korean Peninsula, presumably including U.S. bases and facilities throughout the region. While Brunson thus gave a seemingly innocuous literal reading of the treaty – mirroring mutual defense obligations in the original NATO charter – neither South Korean policymakers nor the general South Korean public embrace it.
Japan has reportedly leaned into the shift in U.S. signals. Last month, Japanese Defense Minister Gen Nakatani laid out a “one theater” concept encompassing the East and South China Seas, the Korean Peninsula, and surrounding areas as a single “theater” for wartime operations. Seoul has responded differently. South Koreans have raised concerns about whether USFK will be reduced or repurposed for off-peninsula operations. The South Korean Defense Ministry denied that any such changes in USFK’s role are underway or that it has been asked by U.S. counterparts to clarify Seoul’s role and commitment in a potential military conflict in the Indo-Pacific region involving China.
Many in South Korea raised similar concerns in response to comments by Elbridge Colby, now the under secretary of defense for policy, who has been tasked by Hegseth to begin development of the 2025 National Defense Strategy. In the run-up to the 2024 U.S. presidential election, Colby – who was then out of government – argued that USFK needed an overhaul to make it more relevant to handling China-related contingencies rather than being held “hostage” to countering North Korean challenges.
South Korea’s progressive national security commentators – long inclined to preserve the nation’s autonomy in relation to U.S. regional strategy and avoid choosing between Washington and Beijing – have been critical of what they view as contradictory and unilateral U.S. signals, which demand more from Seoul while appearing to reduce the U.S. commitment and sidestep allied consultation. Others have called Japan’s one theater concept “unacceptable.”
To be sure, there are a range of nuanced perspectives within South Korea’s foreign policy and national security establishment, including in the progressive camp. Yet if progressive candidate Lee Jae-myung wins South Korea’s presidential election on June 3 – an outcome that appears likely – such views will more directly impact policy outcomes.
Past as Prelude?
Despite Brunson’s literal reading of the treaty, Seoul has long viewed its own and U.S. mutual defense obligations in the treaty – and USFK – as confined to the Korean Peninsula.
South Korean leaders – conservatives particularly – have often sought an upgrade of mutual defense obligations but in the opposite direction: namely, a more automatic and concrete U.S. commitment to the peninsula. Many conservatives want a U.S. imprimatur for robust retaliation against North Korean provocations. In his campaign platform, Kim Moon-soo, the presidential candidate for the conservative People Power Party, said he would pursue a “nuclear attack protection clause” in the mutual defense treaty, the latest iteration of this tendency.
South Korean progressives, while hardly abjuring the U.S. commitment, often push back against overt demonstrations of U.S. military power, lest it undermine engagement with Pyongyang, and bristle against expanding mutual obligations – or the USFK’s operation – beyond Korea, lest it offend China or entrap Seoul in a regional conflict (i.e., over Taiwan). Conservatives, while more willing to rhetorically lean into U.S. regional strategy and draw linkages between the peninsula and regional contingencies, are no less concerned about entrapment and tend to see the USFK’s expanded aperture as a step toward abandonment.
These discordant currents of South Korean thought played out in the early and mid-2000s. In the context of the U.S. Global Posture Review and Iraq war, the Bush administration – and Defense Secretary Donald Rumsfeld in particular – aimed to reduce and realign static, heavily concentrated Cold War-era force deployments, like those in South Korea. Rumsfeld sought greater mobility for U.S. forces on the peninsula and strategic flexibility off it to deploy those forces elsewhere, whether to Iraq or to address a regional contingency.
While passing key military missions to South Korea, the Pentagon aimed to evolve the alliance’s combined command structure so that the South Koreans would take the lead in their own defense, with the United States providing a supporting role. This included transitioning wartime operational control (OPCON) of the ROK military from the United States to South Korea. Both sides agreed to dissolve the bilateral combined command into separate, parallel (yet interlinked) national commands. The parallel construct would avoid placing U.S. forces under the OPCON of a ROK commander, which would not fly back in Washington given the politically controversial optics.
For its part, the progressive Roh Moo-hyun administration (2003-2008) eagerly sought a more equal alliance and greater self-reliance, and strenuously pushed OPCON transition to regain South Korean sovereignty. However, some Roh administration officials feared U.S. force realignments were a prelude to U.S. preemptive strikes against North Korea. The progressive camp felt that North Korean nuclear weapons were not really a threat to the South, and engagement would temper the threat anyway, further enabling Seoul to handle the bulk of the defense burden.
Meanwhile, while the Roh administration agreed in principle with strategic flexibility, it also insisted any off-peninsula U.S. deployment to a regional contingency would require consultation and South Korean agreement beforehand. In short, sending forces to the Middle East was acceptable, but involving USFK – and by extension South Korea – in a fight with China was not.
Officials in the Ministry of Foreign Affairs Treaties Bureau pointed to Article IV of the treaty, which states the ROK “grants, and the United States of America accepts, the right to dispose United States land, air and sea forces in and about the territory of the Republic of Korea as determined by mutual agreement,” as a legal obstacle to USFK’s flexibility elsewhere. Both sides eventually came to an “understanding” on strategic flexibility in 2006, yet it was actually an artful diplomatic agreement to disagree.
South Korean conservatives, viewing such changes as harbingers of a dissolution of the alliance, responded apoplectically and opposed Roh at every turn, erecting political obstacles to the changes described above. As it turned out, U.S. and South Koreans officials soon delayed or significantly altered each policy. With a shift to conservative political leadership in Seoul, the Bush administration stopped further troop reductions in 2008, stating that U.S. forces would remain at 28,500. In a context of growing North Korea aggression, concerns about China’s rise, and the U.S. pivot, mention of strategic flexibility in alliance statements first was revised – to signify U.S. forces flexibly deploying to the peninsula, not away from it – and then disappeared entirely.
Finally, U.S. and South Korean officials agreed to delay OPCON transition a couple of times, to ditch the parallel command concept for a return to the existing combined command structure under CFC, and to adopt more strenuous conditions before completing the transition. In short, this period raised various questions about alliance transformation, which neither side has been able to fully address. These questions are now becoming salient again.
Past Echoes in a Transformed Present
Present trends indicate elements of those earlier debates may soon reemerge. To be clear, unlike in that earlier period, current U.S. policy does not entail a reduction of U.S. forces. Indeed, the 28,500 U.S. service members on the peninsula are not a ceiling but a “manpower floor,” according to Brunson. Furthermore, Hegseth has directed the secretary of the army to increase the U.S. Army’s forward presence in the Indo-Pacific by expanding pre-positioned stocks, rotational deployments, and exercises with allies and partners to enhance strategic access, basing, and overflight. If anything, this implies the potential for increasing force deployments or at least enhancing capabilities, such as long-range fires or enhanced aerial assets, on the peninsula.
Such capabilities may assuage abandonment concerns, yet they also raise difficult cost-sharing questions and fears of entanglement since they range well beyond Pyongyang. U.S. forces and assets on the peninsula are increasingly folded into broader INDOPACOM planning, with deterring China foremost in mind, reawakening the strategic flexibility debate.
In the early 2000s, the debate about USFK’s strategic flexibility was ultimately more hypothetical and involved meeting demands well beyond Northeast Asia. Today’s discourse on the issue centers in the Indo-Pacific in the context of a potential Taiwan conflict. Also, compared to the early 2000s, adversaries and threats are more interconnected. Strategic demands on the United States to integrate its force posture and defense infrastructure in the region are more urgent. And the need to call on allied support is greater than ever before, as are U.S. allies’ capabilities to meet the call.
However, U.S. and South Korean officials have not discussed in much detail or depth how to navigate a Taiwan conflict and the added risk of a simultaneous conflict in Korea, despite former (and now disgraced) President Yoon Suk-yeol having been outspoken in linking peace and stability around Taiwan with the Korean Peninsula. Yoon’s most likely successor, Lee Jae-myung, has openly questioned the linkage, and members of Lee’s Democratic Party proposed legislation to bar ROK forces from being used in a Taiwan contingency and pushed back against USFK’s strategic flexibility. Ironically, while certain U.S. officials increasingly link the Korean Peninsula and Taiwan to argue that U.S. forces on the former should not be restricted from supporting operations around the latter, this ends up reiterating an either-or distinction between them. It is a false dichotomy.
Fortunately, both U.S. and South Korean analysis has begun to dig into the complex linkages between Korea and a Taiwan conflict. For one, simplistic conceptions about USFK’s strategic flexibility – which is to say, the deployment of some units from the peninsula to a Taiwan conflict and then back again – miss the point. USFK’s current posture and mission make it ill-suited to support off-peninsular contingencies. More problematic, U.S. forces normally apportioned in operational plans for a Korean conflict would be diverted in the event of a Taiwan contingency, thus creating a deterrence vacuum on and around the peninsula the U.S. and ROK military would have to fill.
Moreover, strategic flexibility is overly crisis-oriented, centered as it is around a plausible if still hypothetical Taiwan conflict. Rather than transforming USFK into a flexible expeditionary force that flows on and off the peninsula toward a Taiwan conflict – which is often how it is reported – more thought should be put into how U.S. forces and the alliance can be bolstered now, in peacetime, to help to fill the gap between Northeast Asia and Taiwan. This should translate into more serious discussions about third-party intervention scenarios on and around the peninsula and maintaining trilateral U.S.-Japan-ROK cooperation. If Washington and Seoul disagree on how to navigate these issues, they must talk about it.
To be sure, a not insignificant number of progressive foreign policy and national security experts are well aware of the challenge China poses. The next South Korean president must grapple with Beijing’s increased maritime assertiveness in the West/Yellow Sea, which tracks with its previous behavior in the East and South China seas. And the idea that North Korea’s nuclear weapons do not target Seoul – considering Kim Jong Un’s suite of short-range capabilities and tactical nukes, 2022 nuclear policy law, and the historic shift on inter-Korean relations – is far less compelling a notion than it once (if ever) was. Meanwhile, Kim’s firm embrace of Russian President Vladimir Putin and the upgraded North Korea-Russia ties only amplify the complexities Seoul faces.
Candidate Lee has adopted a pragmatic foreign policy stance. On the one hand, he has echoed past progressive leaders’ arguments about engaging Pyongyang, resetting relations with Beijing and Moscow, and striking a more balanced – even hedging – foreign policy. On the other hand, he has noted that the South Korea-U.S. alliance is “the foundation” of his country’s foreign policy and that Seoul “must engage in security cooperation with the U.S. and Japan.” Only time, and the advisers Lee appoints to key positions if elected, will tell how campaign statements translate into policy.
When it comes to OPCON transition – another way to shift a greater burden to Seoul – the Trump administration may have a ready partner in Lee. He pushed to complete the process when running against Yoon in the 2022 presidential election and has made it part of his current campaign platform. Experts in Seoul expect the OPCON issue to quickly reemerge.
The current bilaterally agreed upon Condition-based Wartime Operational Control Transition Plan (COTP) requires South Korea to acquire certain capabilities and demonstrate it can lead a combined fight, and for the environment around the peninsula to be conducive to OPCON transition. While the final condition is broad and political – and served to delay the process in the past – the political winds in Washington and Seoul may soon align to shift momentum forward.
Before his appointment, Colby said the current environment, considering competition with China and South Korea’s robust capabilities, was conducive to OPCON transfer. In his confirmation hearing, he more circumspectly noted that he “would need to review this delicate issue carefully” but added that Trump’s vision of foreign policy “involves empowering capable and willing allies like South Korea, and thus I support efforts to bolster South Korea’s role in the alliance.”
Seoul and Washington should examine whether a future combined command structure, with a South Korean commander and a U.S. deputy commander, is consonant with the political tenor of the times. Put simply, a U.S. deputy commander is not America First. U.S. and South Korean officials may need to return to a parallel command concept, as was the case in the mid-2000s, opting for ROK OPCON over its own military and avoiding placing U.S. forces under OPCON of a South Korean commander, an arrangement that would face a very skeptical audience in Washington.
If Washington desires greater flexibility for U.S. forces on the peninsula yet also aims to retain its presence to support South Korea while Seoul takes the lead role in deterring North Korea, a return to a parallel arrangement may be necessary. The process would also require Seoul to continue building up its strategic command, alongside ongoing efforts to deep conventional-nuclear integration within the alliance. In addition, South Korea would have to think hard about how taking a lead role on the peninsula requires more coordination with Japan’s defense authorities and U.S. Forces Japan’s emerging joint force headquarters.
To be fair, these efforts could face confounding countercurrents depending on whether Washington and Seoul reestablish positive engagement with Pyongyang. Trump indicated he may be interested in doing so, and candidate Lee has made it a priority, even saying that Washington “will have to improve its relationship with North Korea” to contain China. That said, North Korea has agency. Diplomacy is not entirely dependent upon Seoul and Washington’s actions and entreaties, despite what some South Korean progressives argue.
However, in all these efforts, U.S. officials would be advised to seek buy-in from their South Korean counterparts. As history and current sensitivities show, that will be immensely difficult. U.S. officials must insist on alliance transformation, but they can best achieve U.S. objectives by bringing South Korea into the process early and often. If not, the changes would become politicized in a manner that threatens the underlying health of the alliance, which could significantly undermine U.S. interests in the region.
