crs_reports: R48877

Congressional Research Service reports with summaries, authors, and topic classifications.

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R48877 U.S.-South Korea Alliance: Background and Issues for Congress 2026-03-05T05:00:00Z 2026-03-06T21:23:15Z Active Reports Daniel J. Longo Chemical, Biological, Radiological & Nuclear (CBRN) Weapons, East Asia & Pacific, Security Assistance, Security Cooperation & Arms Exports, South Korea, Strategic Forces, CBRN, Arms Control & Nonproliferation In the wake of the Korean War (1950-53), the United States and South Korea (officially the Republic of Korea, or ROK) forged an alliance that remains one of the United States’ most significant military arrangements in Asia. Under the Mutual Defense Treaty that took effect in 1954, the United States and South Korea committed to defend each other against armed attack. Today, about 28,500 U.S. troops are stationed in South Korea. The alliance has traditionally focused on perceived threats from North Korea (officially the Democratic People’s Republic of Korea, or DPRK), which have changed as North Korea has developed weapons of mass destruction (WMD) and missile capabilities. South Korea is included under the U.S. “nuclear umbrella,” also known as extended deterrence. Over the last few decades, the United States and South Korea have taken steps to reform the alliance. These steps unfolded as South Korea emerged as a wealthier country due to its rapid economic growth. For example, in 1991, bilateral negotiations led South Korea to begin defraying the cost of hosting U.S. troops. Both sides also are preparing a change in operational control in the event of war, moving from the existing structure of U.S. and South Korean soldiers operating under a binational command led by a U.S. general, to a binational command led by an ROK general with a U.S. deputy. Since 2022, the allies have upgraded their consultations over the U.S. extended deterrence commitment, partly to respond to growing calls within South Korea for the acquisition of an independent nuclear weapons capability. South Korea also has helped bolster trilateral security cooperation with the United States and Japan, particularly since a trilateral summit at Camp David in 2023. The U.S.-ROK alliance may have reached an inflection point, some observers say, as the administrations of Donald Trump and Lee Jae Myung seek to further “modernize” the alliance. Amid rapidly changing threats by the People’s Republic of China, Russia, and North Korea, Presidents Trump and Lee appear ready for South Korea to take a more active role in the alliance and to broaden the alliance’s mission. President Lee has stated an intention for South Korea to acquire nuclear submarines, which President Trump says he supports. South Korea potentially could employ such submarines to counter North Korea and possibly China, though the wisdom of deploying ROK military assets against China is debated among ROK policymakers and experts. In addition, in November 2025, U.S. Secretary of Defense Pete Hegseth reporetedly stated that U.S. forces in South Korea could be used for a “regional contingency,” which may include a China-Taiwan conflict. Historically, Congress has supported the U.S.-ROK alliance on a broad, bipartisan basis. Through legislation, oversight, and other tools, Congress may direct and influence the executive branch’s handling of the alliance. For instance, Members insert provisions related to South Korea and North Korea into the annual National Defense Authorization Act (NDAA). In addition, the House and Senate Armed Services Committees generally hold hearings the first quarter each year on U.S. military activities in the Indo-Pacific that include the commander of U.S. Forces Korea. During these hearings, Members may raise questions about U.S.-ROK joint war planning, the evolution of perceived regional threats, and other alliance issues. As the U.S.-ROK alliance potentially enters a new period, Congress may face several key issues related to its legislative, oversight, and appropriations roles that could affect how the United States and South Korea navigate decisions, including: Should the United States alter its force presence in South Korea, and if so, how? What impact would increasing, reducing, or maintaining the U.S. force presence in South Korea have on U.S. national security interests, South Korea’s security, and regional and global security? What might an alliance response look like in potential contingencies involving China, including around Taiwan or the South China Sea? What role, if any, would U.S. forces in South Korea potentially play in these scenarios? Should the United States promote U.S.-ROK-Japan cooperation, and if so, to what extent? In what ways do U.S.-ROK alliance planners incorporate the North Korean nuclear threat into policymaking, relative to other concerns such as China? Should the United States and South Korea adjust combined military exercises and other aspects of the alliance’s posture to complement outreach to North Korea? If so, to what extent? Should Congress approve the Trump and Lee governments’ plans for South Korea to acquire U.S. nuclear-powered submarine technology and U.S. approval to enrich and reprocess nuclear fuel? https://www.congress.gov/crs_external_products/R/PDF/R48877/R48877.2.pdf https://www.congress.gov/crs_external_products/R/HTML/R48877.html

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