Files found by Dr Thomas Weber, of the University of Aberdeen, show how Fritz Wiedemann unsuccessfully tried to persuade the British and then the Americans to facilitate Hitler's removal from power in 1940.
Wiedemann was a near-paternal figure for Hitler in World War One and backed his nomination for the Iron Cross. He went on to become the Fűhrer's personal adjutant in the peacetime years of the Third Reich but after a fall out was sent as German Consul General to San Francisco in early 1939.
Wiedemann was a controversial figure when he arrived in America and was feared by many to be a spy for the Nazi regime. But new evidence which has come to light following the publication of Dr Weber's Book, Hitler's First War, paints a very different picture of Wiedemann. It is included in an updated paperback version of the book published in Germany this month.
Dr Weber said: "I showed in Hitler's First War how Wiedemann had secretly met up with a senior British intelligence representative in a San Francisco Hotel in 1940.
"Following the publication of the book, I stumbled, amongst the collections of Yale University, across the private papers of Sir William Wiseman, who met with Wiedemann, which really give us a true flavour of the lengths he was prepared to go to in acting against Hitler.
"The notes in the files say Wiedemann was convinced that Hitler "has a split personality. He is always kind to children and dogs, but can be the most cruel man in the world." According to Wiedemann Hitler "believes himself to be a superior Napoleon and is convinced that he can conquer the world."
Yet, as Wiedemann informed British intelligence, following the Battle of Britain, Hitler was "'off balance' meaning that for the first time his plans have gone wrong." According to Wiedemann, Hitler "was completely convinced that the British could not stand mass bombing" and "that the British Empire [was] in the process of breaking up." Furthermore, Hitler was most frustrated that from his inner circle only Goering supported his invasion plans, while the commander of the German navy, Erich Raeder, had told him to his face that invading Britain "was mad and would end in a massacre."
After the fiasco over the skies of Britain, Wiedemann stated, Hitler was "like a sleepwalker who bumped into a wall and woke up" and did not know what to do next." Wiedemann therefore advised the intelligence officer that "now was the time to strike back at Germany hard".
Hitler, according to Wiedemann, was even more ignorant about the United States, believing that "the United States will come into the war but does not believe it will make any difference," as Hitler viewed the United States "as a country of gangsters, without any military importance whatever" and as a country on the "verge of social revolution."
Wiedemann also informed Wiseman that the true level of popular support for Hitler and his war aims was much lower than generally believed because the authors of internal reports for Hitler would regularly and deliberately inflate the level of support amongst Germans for Hitler in an attempt to write only what Hitler wanted to hear.
Wiedemann's assessment supports Dr Weber's argument that the scholarly consensus about Hitler's popularity during the early phases of the war is 'based on faulty pieces of evidence and needs to be reconsidered'.
He added: "Fascinatingly, the intelligence files also mention a number of Wiedemann's associates in Germany who also were prepared to act against Hitler in 1940 including Hjalmar Schacht, the president of the German Central Bank and Hitler's economics minister.
"Schacht and many others did claim after 1945 that they had been prepared to act early on in the war, and not only once Hitler's war effort turned sour, but their post-war claims have been dismissed as apologetic attempts to save their skins.
"This new evidence shows that there may have been truth in these claims. It also reveals the lengths Wiedemann was prepared to go to – stating that he would be willing to go public and speak to the press – in order to stop Hitler. However, senior figures in the Roosevelt administration stopped Wiedemann from speaking out against Hitler."
In Hitler's First War, Dr Weber also revealed how Wiedemann, who was posted to China 'as far out of Hitler's way as possible' following the closure of German diplomatic missions in America in July 1941, helped a former Jewish comrade to escape the Third Reich. Further information about others he helped has also come to light and is included in the new postscript of the German paperback edition.
Dr Weber added: "I have been approached by three Jewish families over the last year who all told me how Wiedemann helped their relatives to escape National Socialist persecution. Wiedemann intervened on behalf of Alex Wormser, a Jewish company commander from Hitler's World War One unit.
"He also helped to protect another Jewish company commander from Hitler's regiment, Ernst Hess, from persecution during the Holocaust.
"Rather implausibly, some reports suggested earlier this summer that Hitler personally intervened on behalf of Hess, while all evidence strongly suggests that, in fact, Hess survival was solely a result of Wiedemann's intervention.
"Finally, Wiedemann also helped a young Jewish woman, Kate Hoerlin, to escape from Germany and settle in Binghampton, New York. Rather movingly, Hoerlin intervened, in turn, with U.S. authorities in 1945 to make sure that they were aware of Wiedemann's brave actions. This demonstrates Hoerlin, unlike historical scholarship to date, knew what to make of the behaviour of one of Hitler's closest associates.
"Wiedemann's case invites us to reconsider the actions and political belief systems of German diplomats and conservatives during Hitler's reign and the levels of popular support for him at a time when Hitler was at the height of his power."