South Korea is one of the United States' most important strategic and economic partners in Asia, and for the past five years relations between the two countries (known officially as the Republic of Korea, or ROK) have been arguably at their best state in decades. Members of Congress tend to be interested in South Korea-related issues for a number of reasons. First, the United States and South Korea have been allies since the early 1950s. The United States is committed to helping South Korea defend itself, particularly against any aggression from North Korea. 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 over how to deal with 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' sixth-largest trading partner. The United States is South Korea's second-largest trading partner. In late 2013 and early 2014, South Korea took the first steps toward possible entry into the U.S.-led Trans-Pacific Partnership (TPP) free trade agreement negotiations.