Trilateral security cooperation between South Korea, the United States, and Japan will significantly impact the security order of the Korean Peninsula and Northeast Asia. The final obstacle to the realization of trilateral security cooperation between the three countries has been the historically strained relations between South Korea and Japan. The Yoon Suk-Yeol government strove to improve relations with Japan despite potential domestic backlash. This paper applies Hermann’s theory of foreign policy change and the prioritization of national interests to explain the substantial shift in South Korean foreign policy since President Yoon took power in May 2022. The paper argues that among external factors, North Korean military provocations against South Korea, which pose both immediate and existential threats, are the primary drivers behind the foreign policy shift. This contrasts with the belief that strategic competition between the United States and China or the rapprochement with Japan may influence policy decisions. Rather, the analysis finds that the interpretation of national priorities by the leadership plays a more decisive and direct role in foreign policy changes. Moreover, this paper finds that South Korean public opinion on trilateral cooperation with the United States and Japan remained constant between the Moon and Yoon governments, suggesting that it is the interpretation of public opinion by the chief policymaker that matters more than public opinion. The translation of opinion into policy is what shifted dramatically between the Moon and Yoon administrations. Furthermore, President Yoon’s reinterpretation of South Korea’s top national interest – ensuring South Korea’s safety from North Korean nuclear threats – also demonstrates how leadership, rather than structural factors, has a greater impact on foreign policy orientation. Based on these findings, South Korea will maintain trilateral security cooperation with the United States and Japan as a national security measure as long as North Korean provocations continue. Consequently, there are substantial implications for the United States, Japan, and China, compelling nuanced responses to the evolving security landscape in Northeast Asia.