Overview

The U.S.-South Korea (Republic of Korea, or ROK) alliance dates to the 1950-1953 Korean War, during which more than 36,000 Americans died in-theater helping South Korea repel an invasion by North Korea (Democratic People's Republic of Korea, or DPRK) intended to reunify the country under communist rule. An Armistice Agreement was signed in 1953, the same year the United States and South Korea signed the U.S.-ROK Mutual Defense Treaty, which commits the United States to defend South Korea, particularly from North Korea. The U.S. military has maintained a presence in South Korea since. More than 28,500 U.S. troops are based in the ROK, predominately U.S. Army personnel. Most U.S. troops are stationed at Camp Humphreys, the largest U.S. overseas military base in terms of land (Figure 1).

Amid evolving threats by the People's Republic of China (PRC, or China), Russia, and North Korea, the United States and the ROK appear ready for the ROK to take a more active role in the alliance and to broaden the alliance's mission. Through legislation, oversight, and other tools, Congress may direct and influence the executive branch's efforts to modernize the alliance, including on strategic flexibility, wartime operational control, burden sharing, extended deterrence, and defense industrial cooperation.

Figure 1. U.S. Military Facilities in South Korea

Source: Created by CRS using data from the U.S. Department of Defense and U.S. Marine Corps.

"Strategic Flexibility" of USFK

Over the decades, U.S. officials have wrestled with whether and to what extent USFK should address broader regional threats. In a 2006 joint statement, the U.S. and ROK "confirmed their understanding" that the ROK "respects the necessity for strategic flexibility of the U.S. forces in the ROK."

"Strategic flexibility" has garnered renewed attention as the second Trump Administration has indicated it will modernize the U.S.-ROK alliance to deter not only North Korea but also China, with the ROK leading in the defense of the Peninsula. In 2025, the allies committed to "enhance U.S. conventional deterrence posture against all regional threats to the Alliance, including the DPRK." Some observers say the Trump Administration appears to be positioning U.S. military assets for potential use in a China-Taiwan contingency. For example, the U.S. Air Force deployed MQ-9 Reaper drones at Kunsan Air Base in 2025.

Current ROK President Lee Jae Myung's efforts to improve ROK-PRC relations may affect South Korea's approach to "strategic flexibility." Some experts say the Lee administration appears reluctant to more forcefully support U.S. "strategic flexibility" because doing so may signal weaker deterrence vis-à-vis North Korea.

Wartime Operational Control (OPCON)

The United States and South Korea plan to transfer wartime operational control (OPCON) of the U.S.-ROK Combined Forces Command (CFC) to an ROK commander with a U.S. deputy, reflecting the ROK's military advancements since the Korean War. (Operational control, or OPCON, refers to the legal authority to direct and control military forces in the execution of missions or wartime operations.) No date has been set for the transfer. If wartime OPCON is transferred, an ROK commander would lead the CFC, answering to U.S. and ROK civilian authorities; neither side would relinquish command over its own troops. The OPCON transfer, announced in 2006, has been twice delayed, and now is on a "conditions-based" timeline. Many South Koreans view the transfer as a key tenet of ROK sovereignty. The Lee administration reportedly has proposed to complete the transition by 2030, when Lee's term ends. In April 2026 congressional testimony, USFK Commander Xavier Brunson said a roadmap has been developed to achieve the transfer by the second quarter of FY2029.

Defense Cost Sharing

Since 1991, South Korea has offset the cost of stationing U.S. forces on the Peninsula via "Special Measures Agreements" (SMAs). U.S.-ROK SMAs since the late-2000s generally have lasted five years. ROK payments—a combination of in-kind and cash contributions—fall into three categories: labor (salaries for South Koreans who work on U.S. bases); logistics; and construction (by ROK firms for U.S. facilities). A 2021 analysis from the Government Accountability Office (GAO) found that South Korea provided $3.3 billion cumulatively in cash and in-kind support between 2016 and 2019.

Although U.S.-ROK cost-sharing talks historically have been contentious, they became particularly divisive during the first Trump Administration, which reportedly asked South Korea to increase its contribution by roughly 400%. Amid the impasse, the previous SMA expired in December 2019, leading to the furlough of about 4,500 Koreans who worked on U.S. bases. After President Biden's January 2021 inauguration, the two sides concluded a new, five-year SMA. Under that 11^th SMA, South Korea agreed to pay $1 billion in 2021, a 13.9% increase from the 2019 SMA.

In a break from previous patterns, in November 2024, the allies concluded their 12^th SMA, more than a year before the 11^th SMA was set to expire in December 2025. This 12^th SMA covers the years 2026-2030 and increases South Korea's 2025 contribution by 8.3%, to roughly $1.19 billion in 2026. Under the SMA, subsequent annual increases are tied to South Korea's inflation rate and "shall not exceed" 5%. Like its immediate predecessor, the 12^th SMA includes a mechanism for ROK workers on U.S. bases to be paid for up to a year after the agreement expires, upon written request by the United States. The mechanism is designed to minimize the risk of South Korean base workers being furloughed as they were in 2020.

Extended Deterrence

Since the early 2000s, multiple North Korean nuclear weapon and missile tests have sharpened North Korea's threat to South Korea. In a sign of South Koreans' heightened sense of vulnerability, some South Koreans have questioned the reliability of U.S. security guarantees, and some have advocated that the United States redeploy tactical nuclear weapons to the country (the United States withdrew nuclear weapons from South Korea in 1991). Some public opinion polls suggest that a majority of South Koreans support developing a domestic nuclear weapons capability. President Lee has called a nuclear buildup in South Korea "a senseless move."

In an apparent bid to reassure South Koreans skeptical of U.S. extended deterrence—the ability and commitment to deter nuclear threats against allies, sometimes referred to as the "nuclear umbrella"—the two governments issued the "Washington Declaration" during then-ROK President Yoon's April 2023 State Visit to the White House. The declaration articulated a pledge to enhance bilateral planning, exercises, and other consultations related to nuclear deterrence. It also established a Nuclear Consultative Group (NCG), which met for the first time in June 2023. The NCG is intended to align and advance efforts to bolster deterrence against DPRK nuclear threats, with an emphasis on joint planning for ROK conventional support to U.S. nuclear operations and on enhancing the visibility of U.S. "strategic asset deployments" to the Peninsula. Many in Congress and the second Trump Administration have offered statements of reassurance about U.S. extended deterrence commitments to the ROK.

South Korea's Defense Industry

In 2025, South Korea had the world's 13^th-largest military budget, spending roughly $47.8 billion or 2.6% of its gross domestic product (GDP). President Lee has committed to boost ROK defense spending to 3.5% of GDP. South Korea has a mature and sophisticated defense industry and was the world's 9^th-largest supplier of major arms from 2021 to 2025, accounting for 3.0% of global arms exports.

South Korea is a major purchaser of U.S. defense articles and services through the Foreign Military Sales (FMS) and Direct Commercial Sales (DCS) programs. As of 2025, the United States had over $30 billion in active government-to-government sales cases with South Korea under the FMS System. Recent FMS sales to South Korea notified to Congress include the F-35 Joint Strike Fighter, Patriot Advanced Capability-3 missile systems, Global Hawk Unmanned Aerial Vehicle, and Aegis Combat Systems. In November 2025, South Korea pledged to purchase $25 billion worth of U.S. military equipment by 2030.

South Korea's Potential Acquisition of Nuclear-Powered Submarines

Since the mid-1990s, South Korean officials have expressed interest in acquiring nuclear-powered attack submarines in response to North Korean technological advancements. At an October 2025 summit with Lee, President Trump announced U.S. support for South Korea to acquire nuclear-powered submarines. A White House joint fact sheet states, "The United States will work closely with the ROK to advance requirements for this shipbuilding project, including avenues to source fuel." One top U.S. Navy admiral reportedly said it is a "natural expectation" that South Korea would utilize the submarines to counter China. ROK and U.S. officials initiated consultations in June 2026 in Seoul and have not yet finalized details of the acquisition. South Korea's cooperation with the United States on nuclear-powered submarines may require a new agreement under the Atomic Energy Act, which would be subject to congressional review.

Congress's Role in the Alliance

Congress has largely supported the alliance and has acted to constrain the executive branch's ability to make major changes to U.S. force structure and operational leadership on the Korean Peninsula. Section 1268 of the FY2026 NDAA (P.L. 119-60) prohibits the use of funds to reduce U.S. forces deployed to South Korea below 28,500 until 60 days after the Secretary of Defense, who is using "Secretary of War" as a "secondary title" under Executive Order 14347 dated September 5, 2025: (1) certifies to Congress that such a reduction serves U.S. national security interests and that South Korea and Japan were consulted, and (2) submits a report to appropriate congressional committees analyzing the impact that such a reduction may have on U.S., ROK, and Japanese national security, the interoperability of U.S. and ROK armed forces, and other considerations. Section 1268 of the FY2026 NDAA further prohibits the use of funds to complete the transition of wartime OPCON "in a manner which deviates from a bilaterally agreed plan to effectuate such a transition" until 60 days after the Secretary of Defense, among other things, certifies to Congress that the transition is in the national security interest of the United States. A marked-up House committee version of an FY2027 NDAA (H.R. 8800) would extend the Korean Peninsula-related provisions set forth in Section 1268 of the FY2026 NDAA.