Yoshimi has done major research on war crimes perpetrated by the Imperial Japanese Army and the Imperial Japanese Navy during the first part of the Shōwa period. He has published his studies on the use of chemical weapons by Japan, ordered by Emperor Shōwa himself. Yoshimi is mostly noted for his work on comfort women. He found the first documented evidence at the Defense Agency Library of Tokyo that the Imperial Japanese Army established and ran "comfort stations." One piece was a notice written on 4 March 1938 by the adjutants to the Chiefs of Staff of the North China Army and Central China Expeditionary Army titled "Concerning the Recruitment of Women for Military Comfort Stations." The gist of the document is as below: "Many agents should have required special attention. Some of them accentuated the name of the armies as much as they might hurt the credibility of the armies and cause misunderstanding among the public, others recruited women without control through war correspondents or entertainers, and others selected the wrong agents who took a kidnapping approach to recruit women so that the police arrested them. In the future, the armies in the field should control recruiting and select the agencies circumspectly and properly, and should build up a closer connection with the local police and the local military police in the implementation of recruiting. Take special care not to have problems which have the potential to damage the armies' credibility or are not acceptable to social standards." The publication of these documents led to admission statements by the Chiefs of the Cabinet: Secretary Koichi Kato on 12 January 1993 and Yōhei Kōno on 4 August 1993. The Kono statement has been especially important in the comfort women issue. In July 2004, Yoshimi and historian Yuki Tanaka announced the discovery of documents at National Archives of Australia that demonstrated that cyanide gas was tested on Australian and Dutch prisoners in November 1944 on the Kai Islands. On 17 April 2007, Yoshimi and fellow historian Hirofumi Hayashi told a news conference that they had discovered documents at the archives of the Tokyo Tribunal that demonstrate that Tokkeitai members coerced or otherwise forced women from Indonesia, Indochina, and China into sexual slavery. Although Yoshimi has previously made statements in which he conceded "there is no physical evidence to support the forced draft of comfort women by the military in the colonies," he has unequivocally stated that what documentation he has unearthed makes it clear that "there was coercion of those women into sexual slavery and it is the Japanese authorities who decided to set up wartime brothels".
