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But he would also be heard by tens of ­millions of people listening at home on the radio, as well as Nazi officials in Berlin who would soon be picking over his speech in microscopic detail. None of the guests in the Mayflower Hotel that night imagined they were to witness an elaborate act of mass deception. None of them, that is, apart from the man due to introduce the president. This was Colonel William “Wild Bill” Donovan, newly installed as chief of America’s leading intelligence agency.

Another who was in on the secret – because he had come up with the idea in the first place – was the maverick Canadian inventor and businessman Bill Stephenson, the most senior MI6 officer in the United States. Once Donovan finished his introduction, Roosevelt began to address the nation. After a steady start, the president changed tack. “I have in my possession a secret map made in Germany by Hitler’s government,” Roosevelt announced. “It is a map of South America and a part of Central America, as Hitler proposes to reorganise it.” He went on to describe this extraordinary Nazi map. It showed South America as it would look after a successful German ­takeover, with the continent divided into five colonies ruled from Berlin with the Panama Canal under Nazi control. “This map makes clear the Nazi design not only against South America,” Roosevelt growled, “but against the United States itself.” That was not all.

Franklin D. Roosevelt and Winston Churchill

“Your government has in its possession another document made in Germany by Hitler’s government. It is a plan to abolish all existing religions – Protestant, Catholic, Mohammedan, Hindu, Buddhist and Jewish alike.” As Roosevelt explained, part of this plan involved replacing the words of the Bible with those of Mein Kampf, and for the Christian cross to be superseded by the swastika. It was an explosive speech. Most Americans cared deeply about Christianity. But they were, if anything, even more shocked by the idea of a Nazi plot to ­colonise and conquer South America. Opinion polls at the time showed the majority of Americans would call for war against Germany if there was evidence of a Nazi incursion within South America – a finding that had not been lost on British intelligence. These two Nazi documents were sinister, shocking, disturbing – and completely fake. Both had been cooked up several weeks ­earlier at the behest of Bill Stephenson, one of the most unusual wartime appointments to Britain’s foreign intelligence service.

Most of his new colleagues were privately educated with family ties to the old firm. Stephenson, for his part, was of working-class Icelandic stock, and had grown up in extreme poverty in the red-light district of a remote Canadian town. By the age of five, his father had died, and he had been given up for adoption by his mother. But Stephenson proved to be a ­master of reinvention. During the First World War, just out of his teens, he became a ­decorated flying ace. By the start of the Thirties, he had moved to London, glossed over his past and emerged as a maverick “tech” millionaire working in radio.

Now in his early forties, Stephenson had been sent to New York by MI6. Since the summer of 1940, he had run a vast British influence campaign out of a sprawling office deep inside Manhattan’s Rockefeller Center. At its peak, his operation employed just under 1,000 staff, making it considerably larger than the alleged Russian influence campaign launched in America some 75 years later in the run-up to the 2016 US ­presidential election. Among Stephenson’s colleagues was a British intelligence officer named Ian Fleming, who a decade later used the Rocke­feller Center as a location in his first James Bond novel, Casino Royale. Fleming later paid tribute to Stephenson as “one of the great secret agents of the last war”, and described his martinis as “the most powerful” in Amer­ica. They were so good, in fact, that Flem­ing was inspired by Stephenson’s recipe: “Booth’s gin, high and dry, easy on the vermouth, shaken not stirred”.

Bill Stephenson proved to be a ­master of reinvention. Bill Stephenson

Stephenson’s wartime British operation infiltrated American pressure groups, hacked into the private communications of US congressmen and senators, subsidised and directed political protest groups, manipulated opinion polls, operated wiretaps, organised protests, and harassed political opponents. They also produced what was called at the time, as it is today, “fake news”, which they fed into the American news cycle. By late 1941 this undercover British campaign had two distinct objectives: to change American public opinion about going to war; and to provoke Hitler into a declaration of war on the United States. The fake Nazi ­documents served both ends. They could ­further shift American public opinion towards war, and at the same time infuriate the Nazi hierarchy in Berlin.

Once Stephenson had approved the plan, the job of producing these fakes was passed on to his specialist forgery unit in Toronto, called Station M after the unlikely Briton he had put in charge. This was Eric Maschwitz, the lyricist behind the hit song A Nightingale Sang In Berkeley Square, who was also a first-class forger. With Maschwitz’s help and guidance, a team of forgers in Station M had produced the two fake Nazi documents. From Toronto they were couriered to the MI6 station in New York.

American military commander Colonel William Donovan

From here it was up to Stephenson to get them on to Roosevelt’s desk in the White House. But how? This was where “Wild Bill” Donovan came in. By then Stephenson and Donovan had an exceptionally close working relationship. Unknown to most Washington insiders, Stephenson was feeding a stream of British intelligence to Donovan, who would pass it on as US work. Usually, the first person to see Donovan’s latest batch of intelligence was the president himself.

Hitler imagined Roosevelt to be incapable of persuading Congress to go to war

So the two fake Nazi documents were passed to “Wild Bill”, who gave them to Roosevelt. But was the president in on the ruse? Almost certainly. Based on everything we know today, including memoirs, diaries and a number of recently declassified documents, there is ­little doubt that the president knew these were British fakes. But he went ahead with the speech anyway. What was the effect? Not only did it help push American public opinion towards going to war, it also caused a storm in Berlin.

In the febrile days that followed, the state-controlled Nazi media launched a volley of denials and accusations at the White House. There was even a formal complaint. This speech riled many Nazi officials, especially Hitler himself. Less than a fortnight later, when the German leader next spoke in public he could talk of little else. Indeed this made such a deep impression on the Nazi leader that it appears to have influenced what has since been described as his costliest mistake – his decision one month later to declare war on the United States. After the Japanese attack on Pearl Harbor on December 7, 1941, Germany did not immediately declare war on the United States. Hitler had no obligation to do so, and almost none of his senior military officers were calling for it. Germany remained at peace with the US until four days later, December 11, 1941, when the German leader surprised the world by declaring war.

Winston Churchill wanted to get the US into the war

In a speech given on the same day, Hitler tried to justify his decision. He accused Roosevelt of “intolerable provocations”, “baseless allegations” and “shameless misrepresentations of truth”. We will never know exactly what he was ­referring to here. But it is highly possible the two British fakes played a part. Hitler had always wanted to declare war on the United States first, rather than have Roosevelt beat him to it. Until reading about the president’s speech, Hitler imagined Roosevelt to be incapable of persuading Congress to go to war against Germany.

But what happened in the Mayflower Hotel changed all that. Suddenly, it was as if he was up against a different type of opponent, one capable of lying to his people in order to take his ­country to war. From that moment, it was no longer a question of if Hitler would declare war on the US, but simply when. Put another way, an MI6-led operation appears to have played a part in provoking Hitler into declaring war on America. Churchill may have revealed too much, several months later, when he said America’s entry into the war was, “dreamed of, aimed at, and worked for, and now it has come to pass”. Soon after Hitler’s declaration of war, Stephenson flew back to London.