"South Korea (officially the Republic of Korea, or ROK) is one of the United States' most important strategic and economic partners in Asia. Congressional interest in South Korea is driven by both security and trade interests. Since the early 1950s, the 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.' Washington and Seoul cooperate in addressing the challenges posed by North Korea. 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. Between 2009 and the end of 2016, relations between the two countries arguably reached their most robust state in decades. Political changes in both countries in 2017, however, have generated uncertainty about the state of the relationship."
CRS Report for Congress, R41481
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html