This paper examines the divergent attitudes of Taiwan and South Korea towards Japanese colonialism and their impact on regional security and cooperation in East Asia. While both countries share a history of Japanese colonial rule, their collective memories and national narratives have evolved differently, shaping distinct political, social, and diplomatic approaches to Japan. Taiwan's generally pragmatic and conciliatory stance contrasts with South Korea's more confrontational and memory-sensitive approach, reflecting differences in domestic politics, cultural identity, and the role of historical grievances in national policy. By analyzing key factors such as historical memory, public opinion, and government rhetoric, we explore how these attitudes influence bilateral relations with Japan and broader regional dynamics, including trilateral security cooperation with the United States. The findings highlight the complexities of reconciling historical grievances with strategic imperatives in East Asia, underscoring the importance of nuanced historical dialogue and trust-building for fostering stability and collaboration.