The proprietor of the Daily Mail sent a series of supportive and congratulatory telegrams to National Socialist leaders, including Adolf Hitler, just months before the second world war.
Intercepted messages from Lord Rothermere to Berlin are among the first papers to be released from Foreign Office intelligence files.
The files also show how, as early as 1936, MI6 drew up detailed plans to plant agents in Europe "in the event of war with Germany". At the end of 1938 they were telling London that Hitler believed Britain was "enemy No 1".
Yet in the summer of 1939, Rothermere was still appealing to Hitler, saying that Britain and Nazi Germany must remain at peace. "Our two great Nordic countries should pursue resolutely a policy of appeasement for, whatever anyone may say, our two great countries should be the leaders of the world," he told Joachim von Ribbentrop, Hitler's foreign minister, on July 7 1939.
Ten days earlier, Rothermere had written to Hitler: "My Dear Führer, I have watched with understanding and interest the progress of your great and superhuman work in regenerating your country."
He assured Hitler that the British government had "no policy which involves the encirclement of Germany, and that no British government could exist which embraced such a policy".
He added: "The British people, now like Germany strongly rearmed, regard the German people with admiration as valorous adversaries in the past, but I am sure that there is no problem between our two countries which cannot be settled by consultation and negotiation."
If Hitler worked to restore the old friendship, he would be regarded by the British as a popular hero, in the same way they regarded Frederick the Great of Prussia, said Rothermere. "I have always felt that you are essentially one who hates war and desires peace."
Rothermere appealed to the NS leadership to convene a conference to sort out what he called the "misunderstanding" - concerns about Germany's intentions, particularly with regard to Poland and, as he called it, "the Danzig problem".
On July 6 1939, he appealed to Rudolf Hess, Hitler's deputy, to help settle "all outstanding problems" by organising an international conference. "Could I ask you to use your influence in this direction. There is really no cleavage between the interests of Germany and Britain. This great world of ours is big enough for both countries."
Rothermere made clear he sympathised with Germany's grievances over the peace settlement after the first world war.
He referred Ribbentrop to the grave iniquities of the Versailles treaty.
"I am optimistic enough", he wrote, "to believe that even before the end of this year, glaring grievances can be redressed."