Trump 2.0 and the U.S.-South Korea Alliance

Future of the U.S.-South Korea Alliance

Indeed, debate in Seoul over the future of the U.S.-South Korea alliance has intensified since Trump returned to the White House. Yet after the political upheaval sparked by then-President Yoon Suk Yeol’s declaration of a “state of emergency” on Dec. 3 — leading to his impeachment, a snap election and a new administration — the domestic situation is stabilizing.

Meanwhile, despite Lee’s recent White House visit, unusually direct comments from American officials regarding U.S. forces in Korea have fueled speculation about the alliance’s trajectory. The Trump administration has signaled its intent to reposition U.S. forces in the Indo-Pacific to bolster deterrence against China. These moves raise broader strategic questions for U.S. forces in South Korea beyond simple troop numbers.

In May, The Wall Street Journal reported – citing multiple defense officials – that roughly 4,500 U.S. troops could be relocated from South Korea to Guam and elsewhere, a report the Pentagon later denied. That same month, Gen. Xavier Brunson, commander of U.S. forces in South Korea, remarked that South Korea functions as “an anchored aircraft carrier between Japan and China,” noting that U.S. forces there are now focused on regional operations as part of the wider Indo-Pacific strategy. While reinforcing the value of a U.S. military presence on the peninsula, he also emphasized the need for “strategic flexibility” to project power beyond it.

Alongside these U.S. statements, South Korea’s conservative bloc has voiced concern over the new progressive administration led by President Lee, who took office in June. Lee has made the transfer of wartime operational control (often referred to in military contexts as “OPCON”) a top priority. Paired with Washington’s push to reposition troops in the Indo-Pacific, South Korean conservatives fear an alignment of interests between the U.S., which may welcome a reduction in U.S. forces, and the more liberal Lee administration, which seeks greater autonomy.

Should that occur, conservatives fear it would weaken U.S. forces in Korea while strengthening U.S. forces in Japan. In this scenario, key U.S. units might withdraw from South Korea, with command shifted to Japan — potentially elevating the U.S. commander in Japan to four-star rank while downgrading the U.S. commander in Korea to three stars. Such a shift could alter the military balance in Northeast Asia, moving the strategic center of gravity away from the Korean Peninsula.

Indo-Pacific strategy 

Also under scrutiny is Washington’s reported proposal to “modernize” the alliance — a redefinition that envisions a broader regional role, especially in countering China. While President Lee has publicly pledged not to involve South Korea in the cross-strait relationship, the U.S. continues to press for Seoul’s deeper integration into Indo-Pacific strategy, seeking more robust participation beyond the peninsula.

As of now, the U.S. maintains about 28,500 troops in South Korea under a bilateral agreement reached in 2008 between then-President George W. Bush and then-President Lee Myung Bak. Around 20,000 are U.S. Army personnel, with the core being the Stryker Brigade Combat Team, deployed on nine-month rotations from the U.S. mainland. The SBCT, totaling around 4,400 troops, is widely believed to be the unit referenced in The Wall Street Journal’s report on a possible withdrawal.

In the past, the forward deployment of U.S. Army units near the DMZ served as both a deterrent and a “tripwire” to ensure immediate American involvement in any North Korean aggression. However, U.S. global force realignment eventually reached the Korean Peninsula. In 2004, the U.S. and South Korea agreed to consolidate most forces south of Seoul, culminating in Camp Humphreys—the world’s largest overseas U.S. Army base—which now hosts the bulk of U.S. ground forces in the country.

Yet to offset perceived gaps in South Korea’s own capabilities, the 210th Field Artillery Brigade, equipped with multiple launch rocket systems (known as MLRS), remains near the front line in Dongducheon. Likewise, the unique U.S.-ROK Combined Division remains at Camp Red Cloud in Uijeongbu. These units help preserve the tripwire function and reassure those in Seoul who are concerned about abandonment.

There are also substantial air assets in South Korea. The U.S. Air Force maintains F-16 squadrons at both Osan and Gunsan air bases. Osan also houses U-2 reconnaissance aircraft and the world’s only overseas A-10 deployment. The A-10s are set to be phased out beginning this year, with F-16s expected to fill the gap. Reports also suggest Gunsan could host F-35As, boosting deterrence capabilities against China.

Meanwhile, the future of U.S. Army AH-64 Apache helicopters remains uncertain, as these units have long faced limitations on training range access. With U.S. Defense Secretary Pete Hegseth pushing to modernize the U.S. Army, a drawdown of helicopters is expected—possibly replaced by unmanned systems like drones. If so, reductions in personnel may not necessarily mean a loss in combat effectiveness.

Although The Wall Street Journal report was quickly denied by U.S. officials, the episode echoed past efforts to apply pressure—a familiar pattern from the first Trump administration. At that time, amid contentious cost-sharing negotiations, the Pentagon drew media attention by showcasing the large-scale movement of Abrams tanks from the U.S. to Korea.

More recently, a segment of USFK’s Patriot missile units was sent to Qatar following U.S. airstrikes on Iranian nuclear sites. When Iranian missiles targeted the base in response, Gen. Caine publicly praised the role of U.S. troops from Korea deployed there. Going forward, Washington may continue signaling the withdrawal of key capabilities — such as Patriot systems — that South Korea considers essential to its own defense.

In Japan, much attention has focused on U.S. demands for increased defense spending and possible troop reductions. Yet the more pressing issue is how the U.S.-Japan alliance functions operationally. Regardless of troop numbers, observers must closely track changes in joint training and exercises—across land, sea, air and newer domains like cyber, space and special operations. The scale and frequency of these activities will be critical indicators of alliance strength.

Finally, as the U.S.-South Korea alliance evolves toward a modernized posture, strategic ripple effects are likely to impact across the region. Considering the challenges emanating from Washington, a detailed and forward-looking analysis will be critical to understanding how these changes affect the U.S.-Japan alliance and the broader Indo-Pacific security framework.

（Photo Credit: SOPA Images / Getty Images)

Kohtaro Ito

Senior Research Fellow at the Canon Institute for Global Studies

Kohtaro Ito is a senior research fellow at the Canon Institute for Global Studies and an associate professor at Hosei University and visiting associate professor at Ritsumeikan University. Geoeconomic Briefing is a series featuring researchers at the IOG focused on Japan’s challenges in that field. It also provides analyses of the state of the world and trade risks, as well as technological and industrial structures.

---

Disclaimer: The views expressed in this Geoeconomic Briefing do not necessarily reflect those of the API, the Institute of Geoeconomics (IOG), or any other organizations to which the author belongs.

API/IOG English Newsletter

Edited by Paul Nadeau, the newsletter will monthly keep up to date on geoeconomic agenda, IOG Intelligence report, geoeconomics briefings, IOG geoeconomic insights, new publications, events, research activities, media coverage, and more.

Click here to subscribe