How to Constructively and Safely Reduce and Realign US Forces on the Korean Peninsula

(Source: USFK; Photo by Staff Sgt. David Chapman)

Over the past several months, reports have surfaced about the possibility of US President Donald Trump withdrawing troops from South Korea. The issue caused predictable hand-wringing and opposition in Seoul and Washington, particularly in light of the president’s decision to withdraw thousands of troops from Germany. Force reductions in Korea, however, should not be feared and reflexively opposed. Ideally, reductions would occur in a stable environment and be a part of a larger diplomatic settlement with Pyongyang. Nevertheless, while DPRK capabilities should be factored into the equation, policy decisions regarding US force levels in Korea should not be a function of the North’s preferences or held hostage to a comprehensive resolution of outstanding differences with North Korea. Even without progress in talks with Pyongyang, there are constructive ways to approach force reductions. Furthermore, some reductions may help to reduce the US footprint while increasing South Korea’s autonomy and defense burden sharing and tightening alliance cooperation and interoperability.

An Evolving Tripwire

Opposition to the withdrawal of US forces on the Korean Peninsula is understandable. Such reductions, even in normal times, are costly and time-consuming and require a deliberate process and decisions about what forces to withdraw and where to move them if they are retained in the force structure. More importantly, monetary considerations should not be the key driver of decisions on troop withdrawals. The forces are a symbol of the US security commitment to South Korea; removing them could call into question the strength of that commitment and US resolve to remain a major power in the Indo-Pacific region. COVID-19 and pressures to reduce defense spending will make all these costs more prohibitive.

The US force structure in South Korea was never meant to be permanent nor static. Its underlying mission has been consistent (i.e., to deter North Korean aggression, and in case of deterrence failure, to help defend South Korea). But the actual composition and alignment of US forces have changed notably over time, largely in response to force modernization and qualitative improvements in both US and ROK armed forces. History shows that what matters most is not the topline US troop number but the capability of the forces and how well they fit within the larger alliance architecture. Thus, troop reductions or realignments are not inherently beneficial or dangerous. What matters is the military logic and context of these changes and how they are implemented.

While the optimal conditions for a significant or full troop withdrawal—a stable, peaceful and unified Korean Peninsula—have never been met, past US presidents have nonetheless reduced and reconfigured the US military presence in South Korea. Driven by shifting international conditions and budgetary pressures and influenced by the evolution of the North Korean threat and South Korea’s own enhanced economic and defense capabilities, these earlier decisions both caused and reflected changes in the US-ROK relationship. Historically, the so-called US tripwire has meant different things at different times. Initially, it was quite literal: minimal US ground troops on the frontline that lacked the capability to provide a robust forward defense but whose presence guaranteed immediate US involvement in another Korean conflict. These forward-deployed ground troops were militarily necessary to slow down advancing North Korean forces to allow time for contiguous US (CONUS)-based reinforcements to arrive; they were also a credible physical manifestation of the US commitment to South Korean security embodied in the US-ROK Mutual Defense Treaty (MDT). No president or Congress would stand idly by as tens of thousands of US military and civilian personnel were killed in the early stages of a conflict.

However, the days are long gone when the credibility of the US commitment and the military defense of South Korea depended on a frontline US ground force presence. Despite the fitful process of force realignments and the tensions and misunderstanding that have often accompanied them, the alliance has continually adapted to changes in the US force structure. The result is arguably one of the most militarily interoperable and cooperative bilateral relationships in the world. Washington has made substantial progress toward its early post-Cold War goal of moving from a leading to a supporting role in Korea, though more work needs to be done.[1] As I’ve written elsewhere, Seoul took over frontline defense in the early 1970s and today contributes the overwhelming majority of manpower, material and military assets to deterrence and defense on the Korean Peninsula. Moreover, over the last decade, as North Korea’s conventional capabilities have grown weaker, ROK forces, as a result of impressive improvements, have become much more capable of defending against a North Korean invasion.

Disaggregating the Tripwire

Unlike in most other areas, Congress has worked on a surprisingly bipartisan basis to erect barriers against President Trump’s inclination to remove US troops from South Korea. The National Defense Authorization Act (NDAA) for Fiscal Year 2019 set the troop floor at 22,000, and progressively more stringent measures were included in the 2020 NDAA and current draft of the 2021 version (which upped the floor to 28,500). In addition, Republicans and Democrats included reporting and consultative measures to limit the administration’s ability to take precipitous actions and have added a 90-day restriction on troop reductions after the secretary of defense certifies that such a reduction “is in the national security interest of the United States and will not significantly undermine the security of United States allies in the region.” That said, legislative guardrails obstructing Trump from drawing down forces below the seemingly sacrosanct floor of 28,500 troops amount to stopgap measures, and are not the result of a systematic assessment of realistic and potentially constructive force reduction options. A look at the make-up of those forces and existing alliance arrangements provides one potential option moving forward.

In broad strokes, the 28,500 personnel that constitute United States Forces Korea (USFK) are made up of about 20,000 in the Eighth US Army (which includes the Second Infantry Division, or 2ID); 8,000 in the Seventh Air Force; 300 sailors in US Naval Forces Korea; 100 in Marine Corps Forces Korea (MARFORK); and 100 in the Special Operations Command Korea (SOCKOR). US personnel in these units plan, train, command and operate alongside their South Korea counterparts, who bring to bear 600,000 standing troops: 464,000 Army, 70,000 Navy and 65,000 Air Force.

One of the most notable examples of the tight division of labor and mutual trust built over many years of force realignments and alliance deepening is the US-ROK combined division established in 2015. A key component of the combined division is 2ID’s 4,500-strong armored brigade combat team (ACBT), which was transformed from the last permanently forward-stationed US ground combat brigade into a rotational ACBT in 2014, serving in-country on a nine-month rotational basis; the bulk of the division’s remaining ground forces come from the 16th Mechanized Brigade of the ROK’s 8th Mechanized Infantry Division.

This is a tangible example of the alliance’s maturation. Seoul proposed a similar combined arrangement amidst Nixon’s 1971 withdrawal of the Seventh Infantry Division (7ID), but the Americans demurred. John Holdridge, a senior staff assistant to Nixon’s National Security Advisor Henry Kissinger, noted that Defense would likely “balk at having U.S. forces in such a minority position.” Kissinger himself later remarked that such cadre divisions filled out mostly by ROK forces “would of course raise problems as to how to control their initiatives.” No such reservations exist today.

The combined division demonstrates shared interests and complementary capabilities and requires extensive and direct integration and planning. However, “it’s less about a complementary balance sheet than it is about stumbling together through daily mistakes and mishaps,” writes ML Cavanaugh, a staff officer during the division’s inaugural year. It’s a difficult and fitful process but also a possible recipe for future force reductions. When thinking of realistic and constructive options, expanding on this arrangement is one worthwhile approach among others. For example, the US could potentially disaggregate elements of brigade size and above (ranging from 3,000-5,000 soldiers but sometimes smaller), maintaining the brigade headquarters and one battalion of roughly 1,000 troops with ROK units backfilling the rest. Such an arrangement would maintain credibility and foster strategic flexibility. The headquarters would provide a consistent relationship with the ROK, and the US flag would remain “planted,” which is symbolically important. Moreover, such headquarters are difficult to reestablish, and plugging in battalions from CONUS is much easier when the infrastructure is already in place.

Indeed, this is what 2IDs rotational ACBT currently does within the combined division. Building upon this architecture would shift the focus away from maintaining a seemingly arbitrary troop floor number to identifying the best ways to maximize alliance capabilities and advance cooperation. Expanding upon this arrangement might also allow a smoother transfer of wartime operational control from the US to South Korea by moving the ROK closer to leading joint operations with the US, one of the key conditions of the now partially delayed transition.


Survey data shows that most South Koreans strongly oppose the Trump administration’s exorbitant cost-sharing demands, but there is strong public support for the US-ROK alliance and US force presence, just as there is in the United States. Now is the right time to ask hard questions and explore creative solutions, before a more neglectful or transactional approach to the alliance undermines such support and brings about a more precipitous and ill-conceived change that serves neither Seoul’s nor Washington’s interests.

The option described above would enable Seoul to adopt greater responsibility and authority (something ROK presidents have long sought) while simultaneously taking on greater defense burdens in the alliance, which many in Washington, not just Trump, would like to see. But it is just one possibility. Military and defense experts must work through and assess the technical, organizational and operational details presented by other available options. The transformative process will also require sustained attention from Congress and the executive branch and more in-depth alliance consultation; it should be a priority regardless of the electoral outcome in November.

  1. [1]

    United States, Office of the Assistant Secretary of Defense (International Security Affairs), and United States Department of Defense, A Strategic Framework for the Asian Pacific Rim: Report to Congress: Looking Toward the 21st Century, (Washington, DC: Department of Defense, 1990).

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