In June of 1941, Americans read about an extraordinary British mission into Nazi-occupied France. Newspapers, including the Baltimore Sun and New York Post, detailed how the British parachuted into an airfield with tommy guns and hand grenades, overpowered the guards and destroyed about 30 planes. All of the team members made it back to Britain alive via torpedo boats, along with 40 German prisoners in tow. It was an incredible story.

It was also completely made-up.

Unbeknownst to the United States, the British foreign intelligence service known as MI6 had planted the story in the press as part of a covert influence campaign to convince the country to enter World War II. With Hitler aggressively gaining ground across the continent and dropping bombs over London, British Prime Minister Winston Churchill had been anxiously lobbying Franklin D. Roosevelt for reinforcements against the Germans, but America firmly resisted being drawn into another bloody war on the European continent. In May 1940, after the Nazis invaded the Low Countries and France, a Gallup poll reported that only 7% of Americans thought the U.S. should declare war on Germany. In April 1941, the aviation hero Charles Lindbergh and the America First Committee led a massively popular campaign against U.S. entry into WWII, a conflict many Americans didn’t see as winnable.

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“Americans generally did not see Britain as some close, beloved ally at the start of the Second World War,” says Henry Hemming, author of Agents of Influence: A British Campaign, a Canadian Spy, and the Secret Plot to Bring America into World War II. “Britain was instead one of the major economic rivals to the United States.” In addition, the colonialist British Empire, which America had proudly detached itself from, “was hugely unpopular, and understandably so.”

By November 1941, though, polls suggested that a majority of Americans now favored entering the war to help defeat Germany. Why the shift? Earlier that year, according to Hemming’s book, William Stephenson, a decorated WWI fighter pilot and source of inspiration for James Bond (Ian Fleming noted Stephenson’s martinis were “shaken, not stirred”), was installed as the head of MI6’s U.S. office. A personal friend of Churchill, Stephenson (code name: “Intrepid”) began to employ new tactics to sway public opinion about the war—and convince the U.S. to come off the sidelines.

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“There are three major strands to what he does,” Hemming says. One was to persuade the U.S. to establish its first intelligence office and convince William “Wild Bill” Donovan to run it. Both of these events unfolded in July 1941, when President Roosevelt created a new intelligence organization called the Office of the Coordinator of Information, or COI (a predecessor to the CIA), and appointed Donovan—whom Stephenson had been courting as a sympathetic ally—to lead it.

“Most of the material that [the COI is] passing on to the White House…originates with MI6 and British sources, and that givens Stephenson enormous power in terms of what American government officials are reading about the stakes of the war,” he says. “That plays a significant and sometimes overlooked part in helping to precipitate this shift towards the idea that the British are doing alright, that the war in winnable, that Nazi Germany should be taken on.”

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Another part of the covert campaign involved infiltrating U.S. pressure groups that were already trying to get the U.S. to enter the war. MI6 operatives influenced these organizations’ campaign strategies and made sure they had adequate funding.

In April 1941, MI6 operatives helped organize a protest of an America First rally in New York City. When a female protester approached the mostly male ralliers that day, one of the men charged at her and punched her in the face, sparking violent clashes between the groups, Hemming writes. MI6 operatives used the media attention to promote their messages about the war.

“Reports in the next day’s papers focused on the violence, with most articles also listing the different interventionist groups involved in the march and what their spokespeople had to say about Lindbergh and America First,” Hemming writes in his book. “Anyone reading these with a keen eye might have noticed that some of the activists used very similar language. It was almost as if they were reading from the same script: which, as it happened, some of them were.”

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The third part of the campaign involved setting up an office for MI6 operatives to distribute fake news. These were stories like the one about the bogus British raid meant to convince the public that the war against Germany was winnable and the U.S. should join Britain in the fight.

At its peak, the office planted more than 20 stories a week. For one, Stephenson’s office drew a fake map purporting to show Adolf Hitler’s plans to invade South America, and made sure this map ended up on FDR’s desk at the White House.

It did. In October 1941, FDR gave a speech declaring that the map “makes clear the Nazi design not only against South America but against the United States as well.”

“When Hitler hears about this, he’s furious, he’s outraged, because he knows that this map is a fake,” Hemming says. “And when Hitler gives his next public speech, he can talk of almost nothing but this particular map.”

READ MORE: How South America Became a Nazi Haven

The map, Hemming argues, not only influenced America’s decision to go to war against Germany. It also influenced Hitler’s decision to declare war on the United States on December 11, four days after the Japanese attack on Pearl Harbor. This was something Germany had no obligation to do after the U.S. declared war on Japan.

A few hours after declaring war on the United States, Hitler explained his reasons for doing so in the Reichstag, Nazi Germany’s pseudo-parliament. “A lot of the reasons are about Roosevelt,” Hemming says.

“First he incites war, then falsifies the causes,” Hitler declared on December 11, 1941. “Then odiously wraps himself in a cloak of Christian hypocrisy and slowly but surely leads mankind to war.”

Both the United States and Hitler’s Germany were now primed for the fight.