The US-ROK alliance is facing a debate over the desire of the United States to reorient its forces in the Republic of Korea towards containing China. However, Seoul has shown reluctance, primarily due to its deep economic interdependence with the PRC and concerns over a further deterioration in bilateral relations with Beijing, writes Greg Dong Suk Yoo. The author is a participant of the Valdai – New Generation project.
The presence of US forces in South Korea is rooted in the Mutual Defence Treaty signed in 1953, immediately after the Korean War. The treaty established a framework for the United States to station forces in South Korea to deter further North Korean aggression and to ensure its survival during the Cold War. Over the decades, the size and structure of USFK have been adjusted largely in response to the global strategy of the US, ranging from over 300,000 soldiers in the 1950s to around 28,500 today. The year 2006 marked a pivotal moment in the ROK-US alliance, when Washington formally recognised Seoul’s concerns about being drawn into conflicts beyond the Korean Peninsula. The two sides reached an understanding on strategic flexibility, whereby US forces stationed in Korea could be deployed for regional contingencies, but only with prior consultation and without obligating South Korea to automatically support such missions. This agreement reassured Seoul that while USFK could contribute to broader US strategic objectives, South Korea retained control over whether and how to participate in operations outside its territory.
Since President Trump’s inauguration, however, US officials have openly sought to expand the mission of USFK beyond the Korean Peninsula. Under the banner of “strategic flexibility,” Washington has emphasised the need to use US forces in Korea to counter China, potentially in a conflict between China and Taiwan. US military leaders, such as USFK Commander J.B. Brunson, have stated that USFK will not only deter North Korea but also play a role in containing China, even comparing South Korea to a “fixed aircraft carrier” between Japan and China. Also, Under Secretary of Defence for Policy Elbridge Colby has argued that America cannot afford to keep its forces “hostage” in Korea solely for North Korean threats and that South Korea should take more responsibility for its own defence. Think tank experts and military officers have echoed this, stressing that no legal document prevents the US from redeploying forces stationed in Korea for broader regional missions.
If modernisation proceeds as the US envisions, South Korea will face increasing strategic risks. China is likely to respond with economic and military pressure, viewing South Korea as a frontline state in the US containment strategy. Diplomatically, Seoul will be forced to balance its security dependence on Washington with the necessity of preserving stable relations with Beijing. The possibility of a dual war front, in which China acts over Taiwan while North Korea seizes the opportunity to attack the South, would further complicate Korea’s security landscape. Additionally, Washington may restructure USFK, reducing ground forces and shifting its focus to air and naval operations, which could leave Seoul with greater responsibility for deterring North Korea while simultaneously being exposed to the fallout of a US-China confrontation.
This discussion was a highly debated topic leading up to the presidential summit in late August 2025. During the summit, Lee avoided using the term “flexibility,” used by the US, framing his position as a long-term “modernisation” of the alliance, thereby highlighting the gap between the allies. As an alternative to endorsing broader regional roles for USFK, Lee proposed an increase in South Korea’s defence spending, presenting it as both a demonstration of commitment to deterrence against North Korea and a means of reinforcing the ROK-US alliance. Nevertheless, the summit concluded without a breakthrough on the question of USFK’s expanded role, leaving the issue unresolved and clearly showing the diverging priorities of the two sides.
Given these complexities, South Korea should prioritise maintaining the status quo of the ROK-US alliance while firmly upholding the principle of the 2006 Joint Statement on Strategic Flexibility that prevents its involvement in regional conflicts without its consent. At the same time, Seoul must clearly define the scope and limits of USFK’s role in order to prevent the gradual expansion of alliance commitments in ways that could compromise its autonomy. By setting clear boundaries, South Korea can protect its national interests while preserving the core of its security partnership with the United States. If change becomes unavoidable, Seoul should seek to negotiate adjustments on reciprocal terms, ensuring that any expanded role for USFK is matched by tangible benefits such as greater input into US decision-making processes, or strengthened economic cooperation that can help offset the economic losses and vulnerabilities arising from potential Chinese sanctions. This approach would allow South Korea to preserve the strength of its alliance with the United States while simultaneously reducing the likelihood and damage of potential direct confrontation with China.
