The Memorial Hall used to be the main building at Camp Ichigaya and has a storied history. It has been housed the Imperial Army Academy, Eastern Army Headquarters, and the International Military Tribunal of the Far East (otherwise known as the Tokyo War Trials), and it was the place where Award winning novelist Yukio Mishima committed suicide. All elements of this history are fascinating (particularly Mishima's failed attempt to stir a coup d'etat), but since we're following MacArthur's footsteps, we will focus on the use of the hall for the Tokyo War Tribunals.
MacArthur once remarked that “There is a popular misconception that the achievement of victory in modern war is solely dependent upon victory in the field…There must be a complete spiritual reformation such as will not only control the defeated generation, but will exert a dominant influence upon the generation to follow as well. Unless this is done, victory is but partially complete.” Among the important initiatives to achieve MacArthur's vision of a truly transformed Japan was the prosecution of Japan's war criminals, both as a means to provide justice for those who lost so much during the war, but to provide a formal and legal end to the bloody chapter of Japan's history. The tribunal lasted two and a half years, trying 28 defendants from senior levels of the Japanese government and Imperial military.
The main hall at Ichigaya was selected for the War Tribunals, in part because it could house a facility similar to that which existed at Nuremberg. Seeking to follow the precedents set at Nuremberg, the facility was modified to accommodate the proceedings.
Thanks to the Japanese government, everyone now has a chance to visit this important place. The Ministry of Defense (Camp Ichigaya) is located in between Yotsuya and Ichigaya station (about a 10 minute walk from each). If you want to secure a tour of Ichigaya and the storied Memorial Hall, visit the Ministry of Defense website here: http://www.mod.go.jp/e/p_affair/misc/I_tour/