"South Korea [...] is one of the United States' most important strategic and economic partners in Asia, and since 2009 relations between the two countries arguably have been at their most robust state in decades. Several factors drive congressional interest in South Korea-related issues. First, the United States and South Korea have been military allies since the early 1950s. [...] Approximately 28,500 U.S. troops are based in the ROK [Republic of Korea] and South Korea is included under the U.S. 'nuclear umbrella.' Second, Washington and Seoul cooperate in addressing the challenges posed by North Korea. Third, the two countries' economies are closely entwined and 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 Korea has repeatedly expressed interest in and consulted with the United States on possibly joining the U.S.-led Trans-Pacific Partnership (TPP) free trade agreement, which has been signed, but not yet ratified by the current 12 participants. [...] Dealing with North Korea is the dominant strategic concern of the U.S.-South Korean relationship. [...] On broad strategic matters in East Asia, while South Korean and U.S. perspectives overlap, there are areas of significant differences. For instance, South Korea often hesitates to take steps that antagonize China and has shown mistrust of Japan's efforts to expand its military capabilities. North Korea's 2016 nuclear weapons tests and missile launches, however, potentially have shifted the geopolitical dynamics in Northeast Asia in ways that could bring the United States and South Korea closer together on the best approaches to China and Japan."
CRS Report for Congress, R41481
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html