As the War drew to a close in Europe, arrangements were being made for the new United Nations Organisation and its commissions to be moved from San Francisco to London, where they were lodged at the Church House. This move caused annoyance to some members of the Church Assembly who had anticipated an early return to their building.
The War Crimes Commission was allocated half of the third floor, and the British War Crimes Executive was given offices on the floor below. They were later joined by American and French teams of lawyers, and at the last moment by the Soviet delegation. The London Conference of the Commission opened twenty-four hours later on 26 June 1945. During its fourteen lengthy sessions it examined the possible cases against the Nazi leaders and discussed the 'organisation and methods of procedure for the trial of major war criminals'. Discussions were often very difficult and the situation was not helped by the attitude of the Soviet delegation, who at times seemed to feel that there was no need to establish guilt, only to determine punishment.
Agreement was finally reached on 2 August, and the document, signed six days later, states that an International Military Tribunal would be established 'for the trial of war criminals whose offences have no particular geographical location, whether they be accused individually or in their capacity as members of organisations or groups or in both capacities'. The British War Crimes Executive had particular responsibility for collecting evidence and preparing the cases against the defendants. So Church House became a collecting point for documents, testimonies and even gramophone records, which were later to be submitted at the Nuremberg trials and at other war crimes tribunals.
The Executive Committee of the Commission that was to prepare for the first meeting of the General Assembly of the United Nations was also housed in the building, and their deliberations were held in what was later to become the Bishop Partridge Hall. The Preparatory Commission itself, a much larger body, commenced its second session on 25 November 1945, attended by officials and journalists, in a tightly packed Hoare Memorial Hall. It was in this Hall also that the Security Council sat while the full General Assembly was meeting in January 1946 at the Methodist Central Hall.