위안부
Hata would expand his research into the 1999 book "Ianfu to senjō no sei" ("Comfort women and sex on the battlefield"), described by Sarah Soh as "a 444-page treatise on the comfort women issue". "Ianfu to senjō no sei" was noted for its extensive compilation of information, being praised by historian Haruo Tohmatsu as "probably the most well documented study on the question" and by Mainichi Shimbun reporter Takao Yamada as "an encyclopedia-like collection of facts on comfort women". In "The International History Review", A. Hamish Ion stated that with this work Hata has succeeded in creating "a measured evaluation in the face of sensational and supposedly ill-researched studies by George Hicks and others". The book was also favorably reviewed by political scientist Itaru Shimazu and the journalist Takaaki Ishii. By contrast, historian Hirofumi Hayashi criticized the work for faulty use of documents, such as where Hata cites a document listing 650 comfort women allocated in five prefectures, when in fact the document said 400 comfort women.