The United States has been South Koreas (the Republic of Korea, or ROK) most influential ally since World War II. It helped defend the South from the Norths invasion during the Korean War, and South Korea helped the United States during the Vietnam War. Moreover, the United States and South Korea have come to mutually benefit from extensive economic ties. Nevertheless, the security relationship has shifted over time. In the early 2000s, public protests against U.S. military bases in South Korea soared. This thesis asks: Why has anti-U.S. base sentiment emerged and fluctuated in South Korea? It is argued that, since 1987, the democratization of South Korea affected the countrys politics, economics, military, and society. During this transition, a number of governments allowed anti-U.S. base sentiment to take root and grow, especially under the administrations of progressive party leaders. In addition, resurgent Korean nationalism during the early 2000s strained U.S.ROK relations, which also elevated anti-U.S. base sentiment in South Korea. Nonetheless, anti-U.S. sentiment has decreased since conservative party leaders won elections in 2008.