Hiroshi Ōshima Baron was a general in the Imperial Japanese Army , Japanese ambassador to Nazi Germany before and during World War II — and unknowingly a major source of communications intelligence for the Allies . His role was perhaps best summed up by General George C . Marshall , who identified Ōshima as " our main basis of information regarding Hitler ' s intentions in Europe " . Biography . Early life . Ōshima was the son of a prominent Japanese family from Gifu Prefecture , his father having served as Minister of War from 1916 to 1918 . Ōshima graduated from the 18th class of the Imperial Japanese Army Academy in June 1905 and was promoted to second lieutenant in June 1906 and to lieutenant in June 1908 . He graduated from the 27th class of the Army War College in May 1915 , and was promoted to captain the following year . From 1918 - 1919 , he served in Siberia with the expeditionary forces , and was appointed assistant military attaché in the Japanese embassy to the Weimar Republic . Promoted to major in January 1922 , he served as a military attaché to Budapest and Vienna from 1923 - 1924 . After his return to Japan , he was promoted to lieutenant colonel in August 1926 ; following a promotion to colonel in August 1930 , he served as commander of the 10th Field Artillery Regiment from 1930 - 1931 . Military career . In 1934 , Colonel Ōshima became Japanese military attaché in Berlin . He spoke almost perfect German , and was soon befriended by Joachim von Ribbentrop , who was Adolf Hitler ' s favorite foreign policy advisor at that time . Although Hitler ostensibly used the Foreign Ministry ( " Auswärtiges Amt " ) for his foreign relations , he was in fact more dependent on the " Dienststelle Ribbentrop " , a competing foreign office operated by the ex - champagne salesman . Öshima ' s importance for Hitler during that period can be seen in the fact that following the conclusion of the Anti Comintern Pact , US Ambassador in Japan , Joseph Grew estimated that the agreement was more the result of Ōshima ' s work done even without the participation of the Japanese Ministry of Foreign Affairs . Promoted to major general in March 1935 , under Ribbentrop ' s guidance , Ōshima met privately with Hitler that fall . With the support of the Nazi leadership and the Imperial Japanese Army General Staff , Ōshima progressed rapidly while in Berlin , attaining the rank of lieutenant general and being appointed ambassador to Berlin in October 1938 , upon which he entered the reserves . During his early months as an ambassador , according to evidence presented later at the Nürnberg Trial of Major War Criminals , he was plotting the assassination of Joseph Stalin through Russian agents that were sympathetic to his cause . In a conversation Ōshima had with Heinrich Himmler on 31 January 1939 , he expressed the hope that German - Japanese cooperation in the field of intelligence would lead eventually to the disintegration of the Soviet Union . In September 1939 , Ōshima was recalled to Japan ( with Saburō Kurusu succeeding him ) - as relations between the German and Japanese governments were strained following the conclusion of the German - Soviet non - aggression pact - returning via the United States . Upon the insistence of the Nazi government , he returned to Berlin as Japanese Ambassador in February 1941 , and remained in that position until the German surrender in May 1945 . He dedicated his efforts until the end of the war in Europe to build closer relations between the two countries , including military cooperation in the Indian Ocean area ( in the form of anti - merchant submarine warfare ) ; thus he was instrumental in the forging and signing of the Anti - Comintern Pact on 25 November 1936 and the Tripartite Pact on 27 September 1940 . Such was his fanaticism to the Nazi ideology that he created an impression in American journalist William L . Shirer , writing in his book " The Rise and Fall of the Third Reich " that Ōshima " is more Nazi than the Nazis " . Ōshima ' s close relationship with Hitler and Ribbentrop gave him unparalleled access for a foreigner to German war plans and national policy , comparable to that of Winston Churchill with the American war leadership . In turn , Hitler admired the militaristic Japanese and made Ōshima a personal confidante . Ōshima made visits to the Eastern Front and the Atlantic Wall , and he met periodically with Hitler and other Nazi leaders . Being a meticulous military officer in training , he wrote detailed reports of the information provided to him by the Nazis — and promptly reported by radio to Tokyo in the Purple diplomatic cipher . Unknown to the Japanese , the PURPLE code was broken by American codebreakers in 1940 ; thus Oshima ' s reports were being read almost simultaneously by those who had access to Magic intelligence . Often , they were able to read them before the Japanese did , as transmission problems between Germany and Japan often held up the cables for hours . Pacific War . Already on 13 February 1941 , Ōshima discussed with Ribbentrop the possibility of a joint German - Japanese initiative for war against the British Empire and the United States , and agreeing with him the time was ripe to strike at the British Empire in Asia . On 23 February 1941 , Ribbentrop urged him to press the Japanese government to attack British possessions in East Asia . On 28 November 1941 , in a conversation with Ribbentrop , the German Foreign Minister , he was given an assurance that the Third Reich would join the Japanese government in case of war against the United States . Such was Hitler ' s high esteem that Ōshima was one of only eight recipients of the Grand Cross of the Order of the German Eagle in Gold . Hitler awarded the medal following the attack on Pearl Harbor in 1941 . The award ceremony was attended by Reich Foreign Minister Ribbentrop and the secret notes of the conference were revealed at the Nuremberg trials in 1945 . In addressing Ōshima , Hitler reportedly said : Germany presses for Japanese attack on Soviets . Despite Ōshima ' s anti - Soviet positions , the Japanese government in April 1941 concluded a non - aggression pact with Moscow . The German armed forces invaded the Soviet Union in June and the German government was interested in a simultaneous Japanese attack on the USSR . However , prior to the invasion itself , the German government did not update Ōshima about plans of attack . In a conversation held on 17 May 1941 , Ernst von Weizsäcker , State Secretary in the German Foreign Office , denied there was any tension with the Soviet government . Following the invasion of the USSR on 22 June 1941 , pressure was placed on the Japanese government to join the invasion . On 9 July 1942 , Ribbentrop tried to convince Ōshima to urge his government to join the attack on the Soviet Union . Ribbentrop ' s main argument being that " never again would Japan have such an opportunity as existed at present to eliminate once and for all the Russian colossus in eastern Asia " . On 6 March 1943 , Ōshima delivered Ribbentrop the following official statement from the Japanese government : Supporting war crimes . Ōshima ' s high esteem with Hitler made him privy to some of the planning relating to actions later to be defined war crimes and atrocities . One example was a meeting held between him and Hitler on 3 January 1942 , where they both agreed on the action of sinking life - boats working for the rescue of Allied naval personnel fleeing ships sunk in military action . The official text reads : At a meeting Ōshima had with Hitler and Ribbentrop on 27 May 1944 , Hitler advised that the Japanese government should publicly hang every captured US pilot who was involved in air raids , with the hope this would deter further such attacks . Intercepted dispatches . Virtually all of Ōshima ' s dispatches were intercepted : approximately 75 during the 11 months of 1941 , some 100 in 1942 , 400 in 1943 , 600 in 1944 , and about 300 during the just over four months of 1945 when Germany was at war . For example , in a dispatch decoded on 19 January 1942 , Ribbentrop agreed to supply daily intelligence reports to Ōshima , which he could pass on to Tokyo . He warned that " any leakage of these reports due to our fault would be of grave consequence , so all the handling of these reports should be strictly secret " . This despite the fact that the Germans often reproached him of the unreliability of the Japanese codes , although Ōshima assured them of its security . This laxity proved to be fatal to Japanese espionage efforts , as even much of the intelligence gathered by the Japanese spy network codenamed " TO " in Spain ( with implicit support given by the Spanish authorities ) was channelled through him . This evidence halted the loading of petroleum by the United States onto Spanish tankers in 1944 . While some of his predictions were wrong — Ōshima predicted that Britain would surrender to Germany before the end of 1941 — his reporting of the Nazi leadership ' s plans and policies and his factual data were invaluable to the Allies . For example , on 6 June 1941 , he advised Tokyo that Germany would invade the Soviet Union on 22 June ( see Operation Barbarossa ) . Another example was in November 1943 , when Ōshima was taken on a four - day tour of the Atlantic Wall fortifications on the coast of France . Upon his return to Berlin , he wrote a detailed 20 - page report of his visit , giving an account of the location of every German division , as well as its manpower and weaponry . He described tank ditches in detail , armament of turrets located close to the shore , and available mobile forces . This provided valuable intelligence to the planners of the D - Day assault . Connected to this was that the Allies knew that Operation Fortitude was working because just one week before D - Day , Hitler confided to Ōshima that while the Allies might make diversionary feints in Norway , Brittany and Normandy , they will actually open up " an all - out second front in the area of the Straits of Dover " . Thus Ōshima dutifully reported that the bulk of German forces would not be waiting in Normandy , but mistakenly , at the Pas - de - Calais area . His dispatches also proved to be valuable to those who were involved in the bombing campaign in Europe , as Ōshima provided details on the effect of Allied bombing raids on specific German targets , giving valuable and relatively unbiased bomb damage assessments to the Allies . During and after the war . As the war progressed and Germany began to retreat , Ōshima never wavered in his confidence that Germany would emerge victorious . However , in March 1945 he reported to Tokyo on the " danger of Berlin becoming a battlefield " and revealing a fear " that the abandonment of Berlin may take place another month " . On 13 April 1945 , he met with Ribbentrop — for the last time , it turned out — and vowed to stand with the leaders of the Third Reich in their hour of crisis . " I do not wish to be treated in the same manner as other diplomats merely by reason of great danger from the ravages of war " , he announced . But he was informed that evening by the Foreign Ministry ' s chief of protocol : all diplomats were to leave Berlin at once by Hitler ' s direct order . Ōshima had sent his wife to Bad Gastein , a mountain resort in Austria , and the next day left to join her , together with most of the Japanese diplomatic staff . Less than a month later Germany surrendered and Ōshima and his staff were taken into custody . They were brought to the United States by ship , arriving on 11 July 1945 . After interrogation and internment in Bedford Springs Hotel , a resort hotel in the heart of the Allegheny Mountains , Pennsylvania , Ōshima was returned to Japan in November 1945 . Although he enjoyed freedom briefly in his devastated country , he was arrested on 16 December 1945 and charged with war crimes . When brought before the International Military Tribunal for the Far East , he was found guilty of conspiring to wage aggressive war on 12 November 1948 and sentenced to life imprisonment . Ōshima was paroled in late 1955 and granted clemency three years later . Ōshima died in 1975 , not knowing that he provided the Allies with invaluable intelligence during the war .