Starting in 1968 Hata headed a team of scholars with a task from the Ministry of Education to analyze all available sources and documents on the workings of the wartime and prewar armed forces of Japan. The fruit of their research was "Nihon Rikukaigun no Seido, Soshiki, Jinji" ("Institutions, Organization, and Personnel of the Japanese Army and Navy"), released in 1971, which Mark Peattie called "the authoritative reference work in the field". Soon after Hata was tasked with coordinating another collaborative research project, this one for the Finance Ministry, on the subject of the occupation of Japan by the United States after World War II. John W. Dower, Sadao Asada, and Roger Dingman credited Hata for the key role he played in producing the multivolume project, which began to be published in 1975, and deemed it the best work of scholarship on the occupation produced until that point.