Hemming calls the plot the largest state-sponsored influence campaign ever run on American soil. "Covert, sophisticated eye-wateringly expensive," he writes, "this undercover British operation has been described as 'one of the most diverse, extensive, and yet subtle propaganda drives ever directed by one sovereign state at another'."

The Washington Post called the campaign "arguably the most effective in history", "a virtual textbook in the art of manipulation" and one that "changed America forever". Interestingly, the man running it, Bill Stephenson – in a way similar to the Knight/"M" connection – was hailed by Bond author Ian Fleming as one of the inspirations for licensed-to-kill, secret agent 007.

Cloak-and-dagger stuff, then. And this is how it all came about: Stephenson, a Canadian of Icelandic stock, grew up in poverty in the red-light district of Winnipeg on the prairies. Aged 20 he got away in 1917 by joining the Canadian Expeditionary Force. He was sent to France, was gassed and wounded, convalesced in Britain and recovered to become a Great War air ace with the Royal Flying Corps, notching up 14 "kills" in his Sopwith Camel.

Stephenson returned to Winnipeg as a war hero and became an entrepreneur who rode high before going bankrupt in 1922. He moved to London, married an American heiress and became a millionaire with his own investment fund by the mid-1930s.

Much of his wealth was tied up in northern European companies, so he had a loose private network of contacts to tell him about the economic situation in countries such as Sweden and Germany. With World War Two under way, this network piqued the interest of a British government official and led to Stephenson starting a new career as a professional intelligence officer.

Stephenson was only too willing to gravitate towards the world of spies, and despite having no previous experience he joined MI6, Britain's foreign intelligence agency, and was appointed Head of Station for the United States. Aged 43, he docked in New York on June 21, 1940 with the task of opening up a channel of communication between the Federal Bureau of Investigation, headed by J. Edgar Hoover, and MI6.

This was about the same time that Adolf Hitler was laying down the terms of the armistice with France. Italy had also just joined the war. Britain now stood alone and unable to defeat Nazism wihout assistance. Doing so would be partly reliant on Stephenson organising a secret influence campaign to win hearts and minds, and persuade the reluctant Americans to enter the war on Britain's side.

Stephenson had been given the job without either asking for the position or having the chance to turn it down. This first-time spymaster with no experience of running an MI6 station, let alone manipulating public opinion on a nationwide scale, had a task of burgeoning responsibility.

It was part of a desperate last-ditch effort to save Britain from defeat. With the Dunkirk evacuation over, Britain was on the verge, in dire need of weapons and other munitions, ships, food, industrial raw materials and aid of every kind. British Prime Minister Winston Churchill warned: "We shall lose unless you come in – and with all you have."

But the ability to wage war rested on the thoughts and feelings of ordinary Americans, who were obdurate and anti-war. Only a scant 8 per cent of the population was keen to help. After Dunkirk, many Americans didn't think the British worth saving. In 1937, in a statistic unearthed by Hemming, a Gallup poll showed that 70 per cent of the American people believed with hindsight that their nation's involvement in the First World War had been a big mistake.

Joseph Kennedy, the ambassador to Britain from 1938 to 1940 and father of the future president JFK, told US President Franklin D. Roosevelt that "the RAF would be obliterated by the Luftwaffe" and British surrender was "inevitable".

Roosevelt, in fairness, was doing his best, despite opposition, to assist Churchill. He sent warships and planes to Britain in return for 99-year leases on naval and air bases in the Caribbean, Bermuda and Newfoundland, plus he moved US troops to Greenland and Iceland, pushed through the Lend-Lease aid package, instructed the US Navy to escort Atlantic convoys and engage German U-boats, and ordered the closure of German consulates and the freezing of German assets.

He argued that by giving more aid it would improve America's chances of staying out of the war, while Churchill hoped that a US destroyer might be accidentally torpedoed by a German submarine, and thus provoke America into joining the war. Hitler was not fooled, though.

Stephenson's headquarters in the Rockefeller Center, New York, housed up to 1000 "agents, intermediaries, analysts, clerical staff, pressure-group leaders, journalists, pollsters". Their job was to place articles in the press, make radio appeals, manipulate opinion polls, carry out wiretaps and infiltrate protest groups, to convince Americans it was going to be in their interest to give Britain a helping hand.

Stephenson's agents flooded the American media with fake news, infiltrated pressure groups, conducted offensive operations against isolationists and spread false anti-Germany rumours. He used forged maps and documents to "prove" that the Nazis were going to invade Latin America, having first occupied Bolivia.

Ranged against Stephenson was the America First Committee, a grass-roots organisation that believed in protectionist tariffs and an isolationist foreign policy, and at its peak had close to a million members. One of their spokesmen was the American hero Charles Lindbergh, the first aviator to fly solo non-stop across the Atlantic, who demanded rhetorically in many a radio broadcast: "Shall we submerge our future in the endless wars of the Old World? Or shall we build our defences and leave European war to European countries?" Lindbergh supported Nazi aggression and he was an anti-Semite.

Nazi propaganda was also directly at work. Members of Congress, military officers and businessmen were bombarded with pamphlets extolling the virtues of Germany. Hitler made it clear that "his country posed no threat to America", and many Republicans, who "had no desire to fight Germany", quite accepted this avowal.

Henry Hemming has accessed hitherto private and classified documents, including the diaries of his own grandparents, who were briefly part of Stephenson's extraordinary campaign. The Canadian is described as a flawed maverick, full of contradictions, but one whose work changed the course of the war, and whose gripping story is now told in full.

And on December 7, 1941, came the Japanese attack on Pearl Harbour, and the subsequent United States declaration of war against the Japanese Empire. Four days after Pearl Harbour, Nazi Germany declared war against the US in response to what was claimed to be a series of provocations by the US government when the US was still officially neutral. Roosevelt opted to concentrate on defeating Germany first, then turn to Japan.

Shortly before the end of the war, Stephenson was knighted at Churchill's request. Post-war, Stephenson moved to a large house in Jamaica and renewed his business interests. He died in 1989 aged 92.

Ian Fleming, who was a British intelligence officer during the war, had been taking notes. Sir Bill had liked his martinis shaken not stirred.

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