The study begins with questions about Japan s colonization of the Korean Peninsula under international law. Currently, the international community is actively debating the establishment of comfort facilities for Asian women, including the peoples of the Korean Peninsula, who were colonized by Japan during the Sino-Japanese War, and were put into forced labor following the national recruitment order. In South Korea, the research focuses on the treaties signed between Japan and South Korea from the beginning of the Russo-Japanese War in 1914 through 1910, as well as comfort women, and forced labor. However, Japan continues to argue that while its annexation of Korea was unjustified, it was not a violation of international law due to the absence of multilateral treaties on colonial expansion and few judicial bodies or international conferences. In order to elucidate the Japanese annexation of the Korean Peninsula, which is closely linked to Korea s identity, international law research on colonial expansion in the 19th and early 20 centuries is needed. Furthermore, given the fact that international law evolved in the process of the expansion of European colonies, such research is crucial for the correct understanding of contemporary international law. This study analyzes the “European International Law” of colonial expansion by examining 19th and early 20th-century bilateral treaties, state practice, and the views of international jurists. The definition of colonies as defined in international law at the time is explored, and the concept of territorial acquisition and trust to colonial peoples of European countries are also discussed. Additionally, it reviews a non-annexation declaration agreed upon at the Paris Peace Conference. In addition, it examines the human rights norms applied to the mandate system, a territorial management system operated by the League of Nations. Furthermore, this study applies the international law on colonial expansion discussed in Chapter 2 to cases involving Japanese colonial expansion. Japan began colonial expansion at the end of the 19th century when it joined the “Family of Nations”, comprising “European Christian countries.” The international community criticized Japan s ambition to expand its colonies against European international law for its colonization of the Korean peninsula, the Manchurian Incident, the Sino-Japanese War, and the mandate of the South Sea Islands.