This paper argues external threat perception and internal friction dominate a government's cost-benefit analysis for hosting foreign troops. As a result, security cooperation between the Republic of Korea and United States varies with the degree of threat Seoul perceives from North Korea and the intensity of incidents related to the Status of Forces Agreement (SOFA). The ROK government's domestic political costs and perceived security benefits of the alliance produce a mix of nationalist and cooperative behavior toward the United States. The level of cooperation Seoul offers Washington shapes how effective and efficient the United States considers the alliance, which determines the ratio of American unilateral and cooperative behavior. How coordinated U.S. actions are with the ROK in turn affect Seoul's calculations of the costs and benefits of the alliance. This basic model of forward-deployed and host nation interaction is presented to account for post-Cold War patterns of cooperation between South Korea and the United States. It is argued that this process explains variation in U.S.-ROK cooperation better than popular arguments focused on the level of amity between leaders or the strength of national capabilities. The paper suggests how recent U.S. unilateralism and South Korean nationalism are related, producing negative feedback that can inhibit future cooperation. To successfully update and transform the alliance, Washington and Seoul need to be more attentive to how national policies affect the costs and benefits the other side associates with the alliance. Given the urgency of foregoing American unilateralism and Korean nationalism for alliance cooperation in the face of a nuclear North Korea, this analysis demonstrates the importance of Washington regaining the trust of the South Korean people and Seoul making accurate assessments of the North Korean threat.