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........

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