This study focuses on Japan’s aggression and war crimes in Southeast Asia, with a particular emphasis on the International Military Tribunal for the Far East (IMTFE) held in Tokyo following World War II. The Pacific War, initiated by the Japanese Empire, was waged with the intention of incorporating Southeast Asian colonies into Japan’s sphere of influence under the pretext of liberation. Despite Southeast Asia’s central role in the Pacific War, the IMTFE placed limited emphasis on Japan’s aggression in the region. This study examines the legal and political discourse surrounding the indictments, proceedings, and judgments of the IMTFE to analyze how Japan’s aggression and war crimes in Southeast Asia were interpreted within the framework of Western colonialism and Japanese imperialism. By examining the cases of the Philippines, French Indochina, and the Dutch East Indies, this paper highlights the processes through which the occupation and control of Southeast Asia were addressed and reveals how the trials exposed the dual imperialist dynamics in the postwar international order. Slogans such as the Greater East Asia Co-Prosperity Sphere and the New Order, which symbolized Japan’s wartime ideology, lost their legitimacy as evidence of Japan’s brutality surfaced during the trials. However, while the IMTFE aimed to prosecute Japan’s war crimes, it did not serve as a justification for the continued legitimacy of Western colonial rule. Ultimately, the IMTFE not only condemned Japan’s imperialist aggression but also functioned as a mechanism that reinforced Western imperial dominance, illustrating the multilayered complexities of colonial rule in postwar Southeast Asia