From the Summary: "South Korea (known officially as the Republic of Korea, or ROK) 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. The United States is committed to helping South Korea defend itself, particularly against any aggression from North Korea. Approximately 28,500 U.S. troops are based in the ROK 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, though not yet ratified by the current 12 participants."
CRS Report for Congress, R41481
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html