U.S. calls to 'modernize' alliance with South Korea may put Seoul-Washington ties at a crossroads

U.S. calls to modernize its military alliance with South Korea may increase Seoul's defense responsibilities amid rising tensions with China, prompting concerns about potential entanglement in regional conflicts.

U.S. President Donald Trump speaks to reporters inside the Oval Office of the White House in Washington on May 23. [AP/YONHAP]

Calls by U.S. officials to “modernize” the alliance with South Korea and increase Seoul’s security burden are fanning longstanding debates over the nature of the American military presence on the peninsula and the possibility of South Korea being drawn into a U.S. confrontation with China.

To add to the South Korean conundrum, U.S. President Donald Trump declared at a cabinet meeting in Washington on Tuesday that Seoul pays “very little” for the American military presence on its soil and should spend more on its own defense.

His comments came as other senior officials signaled a broader reassessment of U.S. military posture in the Indo-Pacific, reflecting a shift in Washington’s focus toward strategic competition with Beijing.

Acting U.S. Ambassador to South Korea Joseph Yun said on June 24 that Seoul and Washington need to engage in a broader discussion on how to “modernize the alliance,” echoing Secretary of State Marco Rubio’s earlier statement upon the election of South Korean President Lee Jae Myung that the two countries’ security partnership needs to “meet the demands of today’s strategic environment and address new economic challenges.”

The Trump administration’s push for greater “strategic flexibility” — to allow U.S. Forces Korea (USFK) to be deployed in regional contingencies outside the peninsula — raises the risk that Seoul’s alliance with Washington could be used in ways that the South Korean government opposes, but also offers an opportunity to discuss future cooperation.

Flexibility or entanglement?

Although talk of expanding USFK’s regional role predates the Trump administration, the idea appears to be gaining traction in his second term.

In May, the Wall Street Journal reported that the Pentagon was considering relocating roughly 4,500 of the 28,500 U.S. troops stationed in South Korea to Guam and other locations in the Indo-Pacific.

U.S. defense officials denied the report, but comments by USFK Commander Gen. Xavier Brunson suggest the U.S. increasingly views its footprint in South Korea as serving a broader regional purpose.

South Korean National Security Adviser Wi Sung-lac, left, shakes hands with U.S. Secretary of State and National Security Adviser Marco Rubio in Washington on July 7. [PRESIDENTIAL OFFICE]

“South Korea is the closest allied presence to Beijing,” Brunson said during a symposium in May, calling the country a “fixed aircraft carrier” positioned between Japan and China.

Brunson’s remarks — and growing calls in Washington for greater “strategic flexibility” of U.S. forces stationed in South Korea — have unsettled policymakers in Seoul, particularly as Lee attempts to steer a more “pragmatic” foreign policy that includes improving ties with China.

“The comments by U.S. officials can be interpreted as encompassing not only deterrence against the North Korean threat but also checking the expansion of China’s military,” said Daniel Yu, a research fellow at the Korea Institute for Defense Analyses, in an interview with the Korea JoongAng Daily.

As Washington recalibrates its alliances to confront Beijing, some analysts say Seoul could soon face new pressures to support U.S. military operations beyond the Korean Peninsula — including in a potential Taiwan contingency.

“South Korea may see changes to the U.S. troop deployments on the peninsula, as well as demands that Seoul accept that the revised force presence be ‘strategically flexible’ to move off the peninsula in case of a Taiwan fight,” said Victor Cha, a senior official at the Center for Strategic and International Studies.

He also warned that such changes could stir Seoul’s fears of being drawn into a conflict that is not of its own making.

U.S. President Donald Trump, left, and Secretary of Defense Pete Hegseth attend a cabinet meeting at the White House in Washington on July 8. [UPI/YONHAP]

Those anxieties deepened in March, after a leaked version of the Pentagon’s Interim National Defense Strategic Guidance — reportedly circulated by Defense Secretary Pete Hegseth — appeared to prioritize defending Taiwan over other regional threats.

The document raised concerns in Seoul, which views defense cooperation with Washington as being primarily focused on warding off an invasion by Pyongyang and curbing its illicit weapons programs.

The risks of choosing sides

President Lee has made clear his desire to avoid being caught between Washington and Beijing. “Seoul must prioritize its national interests and avoid becoming too deeply involved in the China-Taiwan conflict,” he said earlier this year.

But that approach may become increasingly difficult, analysts say, as Washington shows less tolerance for ambiguity from its allies.

In a recent Atlantic Council report, Markus Garlauskas, director of the Indo-Pacific Security Initiative at the Scowcroft Center, argued that South Korea must come to terms with the “two-way” nature of its 1953 mutual defense treaty with the United States.

“If the United States were to come to the defense of Taiwan and China responded with strikes against U.S. bases, particularly on Guam, then South Korea must recognize that the United States could not accept Seoul remaining neutral and USFK being mere ringside spectators,” he wrote.

To reduce the risk of miscommunication, Yu said Seoul should push for clear protocols around the scope of USFK’s strategic flexibility.

“South Korea should make it clear to the United States that USFK deployments outside the peninsula should be subject to prior consultation and joint decision-making,” he said. “This principle could serve as a key mechanism for respecting South Korea’s sovereignty while maintaining trust within the alliance.”

More than just the money

Trump’s dual salvo against Seoul — slapping tariffs on trade and criticizing its defense spending — underscores his broader desire to reframe the alliance on terms more favorable to U.S. economic and strategic interests.

In addition to asserting that the new tariff rate is “far less than what is needed to eliminate the trade deficit disparity,” Trump argued that the U.S. military presence brings “tremendous money” to South Korea but represents “a tremendous loss” for the United States.

White House Press Secretary Karoline Leavitt shows a signed letter on tariffs from U.S. President Donald Trump to South Korean President Lee Jae Myung during a press briefing at the White House in Washington on July 7. [REUTERS/YONHAP]

He also took aim at the allies’ Special Measures Agreement, renewed last year, under which South Korea agreed to contribute 1.52 trillion won ($1.11 billion) toward the U.S. military presence on the peninsula, which Brunson told the U.S. Senate reduces American costs by almost 18 percent.

The Pentagon, meanwhile, has said that South Korea and other U.S. allies should adhere to a new “global standard” of spending 5 percent of their GDP on defense to help counter what it describes as the “pacing threat” posed by China.

Yu said Seoul could address U.S. concerns by modernizing its contributions on the ground.

“By upgrading base infrastructure within South Korea or promoting the digital transformation of the combined command and control system, South Korea can demonstrate its ability to provide tangible support in terms of operational mobility and responsiveness as demanded by the U.S.,” he said.

Some observers argue that the evolving strategic landscape could give South Korea the leverage to revisit longstanding alliance issues — chief among them, the transfer of wartime operational control, or Opcon, from U.S. forces to the South Korean military.

“The Trump administration’s interest in USFK ‘strategic flexibility’ could open a door for Lee to fulfill the Democratic Party’s longstanding pledge of transferring Opcon from the United States to the South Korean military, especially if the Trump administration encourages South Korea to take on greater responsibility for its own defense,” said Andrew Yeo, chair of Korea Studies at the Brookings Institution.

Alliance at a crossroads

While American officials frame the push to modernize the alliance as a strategic necessity, the implications for Seoul are far from straightforward. Any major realignment of the alliance to deter China could stoke domestic South Korean concerns and raise broader questions about regional security.

With sensitive negotiations likely ahead, Garlauskas said the success of the alliance may depend on how well the two governments resolve contentious issues surrounding South Korea’s financial, political and military commitments to the alliance “in a mutually equitable way.”

“If they can do this, then they can set the stage for a new, stronger and mutually beneficial alliance that will be better able to keep the fragile regional peace threatened by both nuclear-armed North Korea and China,” he said.

“However, if the coming months reveal deep rifts between Seoul and Washington on the U.S. presence in South Korea, Pyongyang and Beijing could see an opportunity to fracture an alliance that they view as brittle.”

BY MICHAEL LEE [lee.junhyuk@joongang.co.kr]