This collection contains approximately 2,000 pages of typed notes and preparatory materials that Paul K. von Bergen compiled as chief prosecutor for several cases at the Yokohama War Crimes Trials (1946-1949). Concurrent to the larger International Military Tribunal for the Far East in Tokyo, the Yokohama Trials operated under U.S., rather than international, authority and focused on lower-level military personnel and civilians accused of Class "B" and "C" war crimes (conventional war crimes and crimes against humanity).

The documents in this collection represent von Bergen’s research as chief prosecutor for U.S. v. Kajuro Aihara et al., U.S. v. Kiyoharu Tomomori et al., and a few other cases related to the mistreatment of American Prisoners of War in the Japanese city of Fukuoka. The cases revolved around the extra-judicial execution of POWs at Japan's Western Army Headquarters and medical experiments conducted at the nearby Kyushu Imperial University. Owing to the size of these cases (Aihara et al. had 30 defendants, Tomomori et al. 25), and the nature of the crimes, they received considerable attention from the U.S. press at the time.

The papers were transferred from eight original binders (labelled A-G, I, J-Ko, Ku-Miz, Mo-Nu, O-Sa, Se-Tan, U-Z) to 22 folders preserving the original alphabetical organization. The documents consist of von Bergen's research and notes, including: full and excerpted segments of accused and witness statements, short biographies of the accused, petitions to the SCAP Legal Section for the apprehension of suspected war criminals, diagrams, sketches, and captioned photographs of the locations of the incidents, and other notes. Pages are typewritten with occasional handwritten marginalia and corrections.