From 1934-1939, as the Imperial Japanese military attaché in Berlin, Colonel Hiroshi Ōshima, who spoke excellent German, became well acquainted with Adolf Hitler and his foreign policy advisor Joachim von Ribbentrop.

Returning to Berlin in February 1941 as Ambassador, Ōshima reportedly discussed with von Ribbentrop the possibility of a joint German-Japanese initiative for war against the British Empire and the United States.