

Updated June 23, 2020
U.S.-South Korea Alliance: Issues for Congress
Overview of Alliance
Agreement (CMA). The CMA established land, sea, and air
South Korea (officially the Republic of Korea, or ROK) is
buffer zones in the heavily armed Demilitarized Zone
considered one of the United States’ most important
(DMZ) that separates the two Koreas and around the
strategic and economic partners in Asia. The U.S.-ROK
maritime border, called the Northern Limit Line.
Mutual Defense Treaty, signed in 1953 at the end of the
Implementation of the CMA required U.S. military officials
Korean War, commits the United States to help South
to modify practices in the DMZ, including removing land
Korea defend itself, particularly from North Korea
mines and guard posts. While observers point to a marked
(officially the Democratic People’s Republic of Korea, or
reduction of tension in the DMZ until 2020, some critics
DPRK). South Korean troops have fought in U.S.-led
maintain that the CMA-mandated changes reduced alliance
conflicts, including in Vietnam, Iraq, and Afghanistan. The
readiness with little sacrifice in return from North Korea. In
United States includes South Korea under its “nuclear
June 2020, North Korea called the 2018 agreement “dead”
umbrella,” otherwise known as extended deterrence.
and said it would return troops to areas they had vacated
under the agreement.
The U.S. military has maintained a large troop presence in
South Korea since the end of the Korean War. Currently,
Figure 1. U.S. Forces Korea (USFK) Bases
approximately 28,500 U.S. troops are based in the ROK,
predominately Army personnel. Camp Humphreys, which
houses most of the troops, is the largest U.S. overseas
military base in the world.
The U.S.-ROK alliance is strained on several fronts.
President Trump’s periodic references to bringing U.S.
troops home from the Peninsula and his criticism of the
value of alliances more broadly have raised questions in
Seoul about U.S. security commitments. A cost-sharing
agreement that divides the price of hosting U.S. troops in
South Korea expired at the end of 2019, and negotiations
appear to be deadlocked halfway into 2020. South Korean
President Moon Jae-in wants to complete the long-delayed
process to transfer operational control of ROK forces in
wartime to a South Korean general, which could lead to
disagreements about the timing and conditions for the
Source: Jane’s Sentinel Security Assessment—China and Northeast
transition. In addition, differences in approach to dealing
Asia, posted April 15, 2010.
with North Korea and China add pressure on the alliance.
The Alliance and DPRK Policy
Burden-Sharing Negotiations
Negotiations in 2019-2020 revealed sharp differences
The threat from North Korea has defined the alliance since
between Washington and Seoul on how much South Korea
its formation. For years, the two militaries conducted
should contribute to offset the costs to the United States of
regular bilateral exercises, enhancing their capability as a
stationing troops on the peninsula. These negotiations on
joint force in the event of the resumption of hostilities with
cost-sharing arrangements—known as the “Special
the North. Under President Moon and President Trump,
Measures Agreement” or SMA—generally occur every five
both governments have pursued diplomacy with Pyongyang
years. The current talks aim to renew the one-year accord
and the alliance has adjusted its activities. For example, at a
signed in February 2019 that raised South Korea’s previous
summit with North Korean Leader Kim Jong-un in
annual contribution by approximately 8%. Press reports
Singapore in June 2018, President Trump called for a
indicated that the Trump Administration asked South Korea
cancellation of large-scale military exercises, declaring
to increase its contribution by roughly 400%. South Korea
them “very expensive” and “provocative.” With the
balked at the U.S. figure and the agreement expired on
resumption of smaller, less public exercises in 2019, U.S.
December 31, 2019. In spring 2020, South Korea offered a
military officials assert that the alliance has maintained
13% increase, which the U.S. side refused.
readiness, but some analysts warn that the reduction in joint
exercises could damage alliance cohesion.
Although the United States appears to have backed off its
initial figure, South Korea may be waiting to see if the U.S.
In 2018, South Korea and North Korea signed a tension-
election in November will yield a more flexible U.S. stance.
reduction agreement known as the Comprehensive Military
In the meantime, the impasse resulted in U.S. Forces Korea
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U.S.-South Korea Alliance: Issues for Congress
(USFK) furloughing about 4,000 South Korean workers. In
presence in the region, leaving Japan alone as the major
June, the two sides reached an agreement to pay the
host of U.S. forces in the region. China and Russia would
workers through the end of 2020 without resolving other
likely welcome a reduction in U.S. troops, having long
cost-sharing concerns.
criticized U.S. alliances as outdated and counter-productive.
In the past, South Korea generally paid for 40-50% (over
South Korean Defense Budget and
$800 million annually) of the total non-personnel costs of
Defense Industry
maintaining the U.S. troop presence in South Korea. South
Under Moon, South Korea has increased its defense budget
Korean officials also point to Seoul’s contributions to the
significantly. The government initially requested a $44
alliance beyond the SMA agreement. Military expenditures
billion budget for FY2020, a 9.3% increase from 2019. This
account for 2.6% of its GDP, the largest percentage among
followed a 7.6% increase in FY2017 and an 8.2% increase
all U.S. allies. The ROK government is paying $9.7 billion,
in FY2018, the highest among Organization for
or about 90% of the total cost of constructing Camp
Cooperation and Development (OECD) member countries.
Humphreys, and is a top buyer of U.S. defense systems.
The increases aim to boost acquisitions, facilitate OPCON
transfer, and respond to South Korea’s demographic
The SMA, which must be approved by the ROK National
challenges, which constrain its conscription forces.
Assembly, receives considerable attention from the South
However, in response to the economic impact of the
Korean press and ROK citizens. Although opinion polls
Coronavirus Disease 2019 (COVID-19), the defense budget
have indicated enduring support for the U.S. alliance among
was cut to $39.5 billion, which will likely affect
South Koreans, recent surveys have revealed limitations: in
acquisitions.
a November 2019 poll, nearly 70% of South Koreans
opposed paying more to the United States even if it resulted
South Korea is among the top customers for U.S. Foreign
in a reduction in U.S. troops. A drop in public support for
Military Sales (FMS). From 2008 to 2016, ROK FMS
the U.S. troop presence could undercut the political
contracts totaled $15.7 billion and approximately 75% of
viability of the alliance.
South Korea’s total foreign defense purchases came in the
form of FMS and commercial sales from U.S. companies.
Operational Control (OPCON)
South Korea plans to acquire U.S. systems that include
Another focus for the alliance is an agreement to transfer
Lockheed Martin F-35 Joint Strike Fighters, Boeing’s P-8
wartime operational control to a binational command led by
Poseidon aircraft, and Northrup Grumman’s RQ-4 “Global
a South Korean general with a U.S. deputy. President Moon
Hawk” unmanned aerial vehicles.
has indicated his desire to accelerate the transfer. Under the
existing arrangement, South Korean soldiers would be
South Korea also has developed indigenous defense
under a binational command led by a U.S. general in the
manufacturing capabilities by allocating resources to
event of war on the peninsula. In peacetime, the ROK
defense research and development. According to South
military is responsible for national security. The plan to
Korea’s 2018 Defense White Paper, by 2023 the “force
transfer OPCON, begun in 2007 and twice delayed,
enhancement” budget will account for more than 36% of
recognizes the ROK’s advances in economic and military
total defense spending, up from about 31% in 2019. The
strength since the Korean War and is seen by many Koreans
spending increase is tied to Seoul’s strategic objectives,
as important for ROK sovereignty.
including a plan to integrate missile defense and other
capabilities more closely with U.S. systems.
The two sides established conditions and benchmarks to
demonstrate that the ROK can assume wartime OPCON,
Congressional Involvement
including improved ROK capabilities to lead combined
Support for the alliance in the United States has been
forces and counter the DPRK nuclear and missile threat,
widely bipartisan, and many in Congress appear keen to
and a security environment on the peninsula conducive to a
limit the President’s ability to make major changes to force
transfer. Significant progress has been made on some
structure in the region. Section 1254 of the FY2020
objectives, but movement has been slower on others.
National Defense Authorization Act (NDAA) P.L. 116-92
prohibits the use of funds to reduce U.S. forces deployed to
Regional Issues
South Korea below 28,500 until 90 days after the Secretary
The U.S.-ROK alliance is part of the post-World War II
of Defense certifies to Congress (1) that such a reduction is
“hub and spoke” system of U.S. security relationships in the
in the U.S. national interest and will not significantly
Asia-Pacific. Although labeled a “lynchpin” of U.S.
undermine the security the U.S. allies in the region and (2)
presence in the region by U.S. officials, the U.S.-ROK
that U.S. allies have been “appropriately consulted” on the
alliance has focused most heavily on the defense of the
proposed reduction. The NDAA requires a report on South
peninsula itself from North Korean threats. The United
Korea’s (and Japan’s) contributions to U.S. alliances, and a
States has urged greater coordination with other U.S.
sense of Congress on the continued U.S. commitment to its
partners, particularly Japan, in confronting DPRK threats
Asian alliances and trilateral cooperation among the three
and countering China’s military rise. Trilateral cooperation
countries.
has been stymied by troubled Japan-South Korea relations.
Emma Chanlett-Avery, Specialist in Asian Affairs
A large-scale reduction or full withdrawal of the troops
stationed in South Korea would reshape the U.S. military
IF11388
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U.S.-South Korea Alliance: Issues for Congress
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