German submarine warfare against merchant shipping according to a report of the American Naval Attaché at Berlin, Germany Nr. R562 dated 15 September 1939

Diary note dated September 1939. An item reported that until now just a few reports of German submarine warfare against merchant shipping were (indirectly) received. In contarty was the belief that a considerable number of merchant ships already were sunk before the British became worried and planning a convoy system.

Source

National Archives. FDR Presidential Library&Museum Roosevelt PSFC000175. War diary Naval Attaché Berlin, volume 1 (1 September 1939-26 May 1940)