The American troops in South Korea are a contested issue in inter-Korean relations. While the opinion of South Korea and the United States is that they are essential for the South’s defense, North Korea’s view is that they hinder reunification. The South Korea-United States alliance, which was formed during the Korean War (1950-1953), is analyzed here on the basis of alliance theory. The alliance was strengthened by the signing of a Mutual Defense Treaty in 1953 which is the legal basis for the American troops’ presence. The United States prevented South Korea from retaliating against North Korea following assassination attempts against South Korean presidents in 1968 and 1983. Troop reductions in 1970-1971, 1990-1992 and 2004-2008 caused U.S.-ROK tensions. These tensions peaked due to President Jimmy Carter’s (1977-1981) troop withdrawal policy, until the policy was reversed due to strong opposition and an underestimation of North Korea’s armed forces. American troops have contributed to maintaining peace by building a joint South Korean-American fighting force, providing quality intelligence, and serving as a force that both countries regard to be of the utmost importance for the South’s defense.