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 U.S.-ROK Mutual Defense Treaty, signed in 1953 at the end of the Korean War, commits the United States to help South Korea defend itself, particularly from North Korea (officially the Democratic People's Republic of Korea, or DPRK). The alliance also helps the United States promote its interests in East Asia and around the globe, including by deploying ROK troops to U.S.-led military conflicts in the Middle East. Approximately 28,500 U.S. troops are based in the ROK. [...] Policy cooperation between the United States and South Korea was inconsistent under the administrations of Donald Trump and Moon Jae-in. President Moon, a progressive, was elected in May 2017 after a decade of conservative rule. President Biden has called for reinvigorating the alliance, suggesting he would abandon President Trump's repeated use of punitive tariffs against allies like Seoul and his demand for a five-fold increase in South Korea's payments for the costs of hosting U.S. troops. Unlike Trump, Biden has not signaled an interest in demanding new concessions in the KORUS FTA [U.S.-South Korea Free Trade Agreement] or withdrawing U.S. troops from South Korea. Bilateral difficulties could surface, however, over North Korea policy. Moon's government favors easing sanctions against Pyongyang, a stance that could create tensions with Washington."
CRS In Focus, IF10165
Congressional Research Service: https://crsreports.congress.gov/