This study examines the dynamics of trilateral cooperation among South Korea, the United States, and Japan in addressing the North Korean nuclear issue, focusing on the centrality of U.S.-North Korea relations. The two cases of the Korean Peninsula Energy Development Organization (KEDO) and the Trilateral Coordination and Oversight Group (TCOG) shows this power dynamics in the negotiation of the North Korea nuclear problem. The analysis demonstrates that the core of the trilateral cooperation lies not only in institutional or policy alignment among the three countries but in the management of the concrete conflicts that arise between each member state of the trilateral cooperation and DPRK. Both KEDO and TCOG cases reveal that the continuity and effectiveness of trilateral cooperation hinges fundamentally on the political will of the United States. North Korea’s behavior or requests that the United States deems unacceptable serve as dealbreakers that terminate negotiations and dismantle the cooperative mechanism. Once the U.S.’s commitment becomes wavers, cooperation among the three countries becomes unstable, and the institutional foundation that has been built overtime starts to collapse.