This study examines the evolution of alliance dynamics under the Trump administration, with particular attention to the South Korean case and its implications for U.S. alliances. Drawing on developments in U.S. foreign policy and allied responses, it argues that recent shifts have significantly altered perceptions of alliance reliability. The administration’s approach—marked by unpredictability and unconventional communication—has intensified allied uncertainty, reshaping strategic expectations and behavior. In the case of South Korea, these dynamics have contributed to a reassessment of strategic dependence and a growing emphasis on self-reliant defense capabilities. The study conceptualizes alliance dilemmas through the twin risks of abandonment and entrapment, where allies must balance the danger of being left unprotected against the risk of being drawn into unwanted conflicts. Applying this framework to the South Korean case, the analysis shows how heightened uncertainty under the Trump administration reinforced both concerns: fears of abandonment encouraged greater consideration of autonomous defense strategies, while concerns over entrapment highlighted the potential costs of alliance commitments, particularly in crisis scenarios involving regional escalation. Building on these findings, the study argues that alliance dynamics under conditions of uncertainty have broader implications for U.S. hegemony and the stability of the alliance system. The erosion of trust in U.S. commitments has contributed to a shift toward strategic autonomy among allies, challenging the cohesion of the hub-and-spokes structure. At the same time, the emergence of coordinated allied responses suggests that alliance politics are increasingly shaped by perceptions of leadership credibility rather than material asymmetries alone. These trends underscore the centrality of predictability and trust in sustaining alliance stability and, by extension, the durability of U.S. hegemonic leadership.