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). Approximately 28,500 U.S. troops are based in the ROK, which is included under the U.S. 'nuclear umbrella.' The U.S.-ROK economic relationship is bolstered by the U.S.- South Korea 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, behind China."
CRS In Focus, IF10165
Congressional Research Service: https://crsreports.congress.gov/