In 1945, the Hoover Institution began collecting Japanese materials with the specific purpose of documenting the social, political, and economic changes in modern Japan. The Kōshikan records relate to the Japanese occupation of Korea and include basic source material for the study of the history of modern Korea, international relations in the Far East, and Japanese policy and actions in Korea during the critical fifteen years preceding the annexation. They also portray, in some detail, the political and economic policies and activities of Russia, the United States, Great Britain, France, and other countries in Korea, as well as Korean domestic politics. The Japanese modern history manuscript collection contains rich holdings of archival materials relating to various aspects of Japanese history, ranging from the state building period known as the Meiji period (1868–1911) to the post–World War II reconstruction period. Notable items include the records of the Japanese Communist Party and the Japanese cabinet, the proceedings of the Imperial Diet from 1890 until 1946, and materials on the Sino-Japanese War of 1894–95, pre–World War II domestic affairs, the post–World War II American occupation, the International Military Tribunal for the Far East, and the U.S.-Japan Security Treaty. The Rikugun records contain a rich body of Japanese military surveys of lands, buildings, and other assets seized by the Japanese from China during World War II.
Other featured collections include