Over the past six decades, Washington has recognized that its own security is closely connected with that of its allies around the globe. As a part of this policy, U.S. leaders and policy makers have endeavored to protect South Korea by extending deterrence against North Korea. Despite its ongoing promises, U.S. security guarantees have occasionally shown signs of faltering credibility. Moreover, given North Koreas evolving nuclear capability, the credibility of U.S. security commitments, especially those tailored to the Korean Peninsula, still remain uncertain in the minds of some South Koreans. This thesis explores South Koreans recent perceptions of U.S. extended deterrence since 2010, and examines two key factorsU.S. nuclear strategy and North Koreas ICBM capabilitythat might affect South Koreans confidence in U.S. extended deterrence. The thesis then reveals the correlation between South Koreans perceptions and the two key factors. It argues that, contrary to the conventional wisdom, these two key factors do not significantly affect South Koreans confidence, and the impact of those two factors on South Koreans perceptions is not considerable. In fact, existing South Korean doubt about U.S. extended deterrence has been overestimated, and U.S. security guarantees have generally remained credible in South Korean eyes. Nonetheless, the U.S.-ROK alliance must continue to strive for the elimination of any lingering doubt about U.S. extended deterrence.