From the Overview: "South Korea (officially the Republic of Korea or ROK) is one of the United States' most important strategic and economic partners in Asia. The 1953 U.S.-ROK Mutual Defense Treaty commits the United States to help South Korea defend itself. Approximately 28,500 U.S. troops are based in the ROK, which is included under the U.S. 'nuclear umbrella.' The two countries' economies are joined by the Korea-U.S. Free Trade Agreement (KORUS FTA). South Korea is the United States' seventh-largest trading partner and the United States is South Korea's second-largest trading partner. South Korean President Moon Jae-in, a left-of-center politician elected in 2017, is due to leave office in May 2022. In recent years, Members of Congress have expressed interest in and conducted oversight over U.S.-ROK cooperation over North Korea policy, the U.S. military presence in South Korea, bilateral trade and investment flows, the state of political and civil rights in South Korea, and other issues. South Korea's rise into the ranks of the world's top industrialized economies and most powerful militaries has prompted many Members to explore whether and how the ROK government, companies, and organizations can serve as partners in many U.S. Indo-Pacific initiatives, particularly on those that involve the People's Republic of China (PRC, or China). ROK officials generally are reluctant to raise objections about PRC behavior that does not directly affect South Korea, though this may be changing due to rising negative attitudes towards China among South Koreans."
CRS Report for Congress, R41481
Congressional Research Service: https://crsreports.congress.gov/