This dissertation investigates the dynamics of cohesion within the US–South Korea alliance, along-lasting but asymmetric defense partnership, and aims to answer the question: What factors drive changes in the alliance’s cohesion? While enduring alliances are often viewed as cohesive, this study demonstrates that changes in cohesion can occur even within long-lasting alliances, necessitating an examination of cohesion dynamics separate from alliance durability. Two main arguments are presented. First, changes in the US–South Korea alliance’s cohesion were influenced by the interplay of international and domestic political-economic factors. The study argues that, in some cases, domestic constraints may outweigh international factors like shared threats and strategic interests in shaping the alliance cohesion. Second, in asymmetric alliances, the strategic interests and political-economic situation of the major power (the US) had a more significant impact on the alliance cohesion than the minor power (South Korea). Furthermore, the strategic interests and political-economic situation of the US, a major power, influenced not only its alliance policy but also the alliance policy of South Korea, a minor power closely monitoring the US situation. An in-depth historical case study of the US–South Korea alliance during the 1960s and 1970s is conducted, a salient period characterized by significant changes in the alliance’s cohesion. I select this period because it effectively demonstrates the inadequacy of existing literature focusing on threats and interests and validates the main arguments of this study. The study analyzes how evolving threat perceptions, strategic interests, and domestic political-economic developments contributed to changes in the alliance cohesion. It offers valuable insights into the driving forces behind these changes and the complex interplay among them, ultimately enriching our understanding of alliance dynamics. This study contributes to the existing research on the US–South Korea alliance and holds potential for further expansion into a general theory of alliance cohesion in asymmetric contexts.