On Oct. 27, 1941, President Franklin Delano Roosevelt took the stage at The Mayflower Hotel in Washington, DC, to speak in honor of Navy Day. With Britain under Nazi siege, Roosevelt wanted the United States to join the fight. The American public was not convinced.

“I have in my possession a secret map made by Hitler’s government. It is a map of South America and part of Central America, as Hitler proposes to reorganize it,” Roosevelt told the shocked assemblage.

The president then revealed another German document that pledged to eliminate the world’s religions.

The reaction was explosive, but the facts were not. Neither the map nor the religious proclamation was real.

As author Henry Hemming outlines in his new book, “Agents of Influence: A British Campaign, A Canadian Spy, and the Secret Plot to Bring America into World War II,” (PublicAffairs), the documents were commissioned by an unlikely Canadian spymaster named William Stephenson, who ran a British propaganda operation out of Rockefeller Center with the intention of getting the United States into the war by any means necessary.

Stephenson, born in 1897 in Winnipeg, Manitoba, was a World War I flying ace who later became a millionaire by developing and selling radios in Britain.

He started an investment fund called British Pacific Trust and assembled his own intelligence service to burnish it, a team of advisers and paid informants he called The British Industrial Secret Service.

In 1939, Member of Parliament Sir Ralph Glyn learned of Stephenson’s organization, and introduced him to Military Intelligence, Section 6 or MI6, Britain’s government Secret Intelligence Service.

“Stephenson came across as discreet, motivated and shrewd, and MI6 offered to provide him with assistance and some funding,” Hemming writes. “In return, he would name his sources and share the intelligence they provided.”

After Stephenson met with FBI head J. Edgar Hoover at MI6’s request and they hit it off, Stephenson was named head of MI6 operations in the United States.

With Britain enduring intense German bombing, its only hope for survival was getting the United States to enter the war — something only 7% of Americans favored in 1940.

British officials gave the job of changing those minds and counteracting Nazi propaganda to Stephenson. His first victory was small, but informative. He learned from a source that a German diplomat, Dr. Gerhard Westrick, had met with American businessmen, including the CEO of Texaco.

“Stephenson decided to weaponize this intelligence,” Hemming writes. “He instructed a member of his staff to turn it into ‘a first-class news story.’”

The story ran on Aug. 1, 1940, in newspapers across the country under headlines such as “Hitler’s Secret Agent.” Texaco’s CEO resigned, and Westrick was forced to leave the country.

Plucked from what was essentially a nonstory, it was a significant victory for the newly christened spy.

“Bill Stephenson had learned the first and most basic lesson of covert propaganda,” Hemming writes. “How to encourage millions of people to listen to your story sympathetically? Disguise where it has come from.”

Later that month, Roosevelt asked a lawyer and former military commander, William Donovan, to travel to London to assess whether Britain could successfully resist a Nazi invasion.

Recognizing a potential asset, Stephenson advised MI6 to pamper Donovan, who met King George IV, the queen and Prime Minister Winston Churchill on his trip. They and others fed Donovan information what they wanted him to believe about British military strength.

Upon his return, at Stephenson’s urging, Donovan gave radio interviews and wrote articles talking about Britain’s mighty military, and also mentioned a sizable Nazi “fifth column,” or a spy operation, in America.

“In reality, the German ‘Fifth Column’ in the United States amounted to no more than a handful of agents, many of them incompetent,” Hemming writes.

“Donovan’s articles were littered with exaggerations and half-baked rumors. Almost all of these had been fed to him by Bill Stephenson.”

In September 1940, Stephenson realized that the United States needed an MI6 of its own, and hopefully one friendly to Britain.

After discussing the idea with Donovan, the two wrote a proposal for just such an agency, with Donovan in charge.

But after Roosevelt approved it, Donovan had a change of heart, deciding he’d rather join the fighting on the ground.

To help persuade him otherwise, MI6 sent two intelligence officers to the United States: Britain’s director of naval intelligence, Adm. John Godfrey, and his assistant, Lt. Cmdr. Ian Fleming.

Through Stephenson, Fleming, who would later invent the fictional spy, James Bond, would play an instrumental role in the creation of the organization that would eventually become the Central Intelligence Agency, while also taking inspiration from Stephenson and his team for his future novels. (Godfrey was one of Fleming’s models for the character M.)

After convincing Donovan to run the new agency — first called the Office of the Coordinator of Information, or COI, then it would later morph int the Office of Strategic Services (OSS) before becoming the CIA after the war — Godfrey and Fleming toured Stephenson’s Rockefeller Center operation and were impressed.

“One [detail] which stuck in [Fleming’s] mind was the location of Stephenson’s clandestine operation just three floors below the Japanese consulate,” Hemming writes. “In Fleming’s first novel, ‘Casino Royale,’ James Bond is dispatched to Rockefeller Center with orders to kill a Japanese cipher clerk.”

But Stephenson’s greater influence on the James Bond character was his preference for drink. Fleming was enamored of Stephenson’s martini recipe, which made what Fleming considered “the most powerful martinis on Earth.” He loved them so much that he wrote down the recipe: “Booth’s gin, high and dry, easy on the vermouth, shaken not stirred.”

While Donovan struggled to build the agency, Stephenson provided him with “a stream of material from MI6 sources.”

“By writing this material up as if it had come from [the COI] his own agency,”,” Hemming writes, “Donovan rapidly burnished his agency’s reputation.”

But MI6 didn’t just provide the intelligence; it taught Donovan how to build the COI. British scientists were assigned to work with him, and Fleming “wrote detailed reports on how to run an intelligence agency, for which Donovan later rewarded him with a Colt .38 revolver engraved with the words, ‘For Special Services.’ ”

With contacts throughout the American media and his puppet running US intelligence, Stephenson’s message was everywhere, in whatever form he sought.

In June 1941, American newspapers reported on a successful and daring British parachute raid within Germany that captured 40 Nazi pilots. The story helped Americans see that the British had the resolve to defeat the Nazis.

But the raid was fiction. Conceived by MI6 in London, the article was written by one of Stephenson’s agents and spread via the Overseas News Agency, a well-known and “reputable” agency that was secretly on Stephenson’s payroll.

Knowing that Adolf Hitler was obsessed with astrology, Stephenson even had his propaganda hit the horoscopes. MI6 had “pet astrologers” around the world, and Stephenson had them “start predicting the Nazi leader’s sudden demise, which they duly did.”

Just before the Dec. 7, 1941, bombing of Pearl Harbor, the number of Americans in favor of intervention had risen to 72% — a tenfold increase from 1940 — due at least in part to Stephenson’s manipulations of the public mind.

The fictional map of Hitler’s South America, for example, came from Stephenson’s team. The idea, Hemming writes, originated with a British stockbroker friend of Fleming’s, Ivar Bryce. When rumors spread that Hitler had his sights on South America, Bryce let his imagination wander.

“Crawling into Hitler’s mind, Bryce enlarged or shrank each country, and in the case of Uruguay abolished it altogether, until he was left with one German colony and four vassal states covering the whole continent,” Hemming writes.

Stephenson approved the idea. To create the map itself, he relied on his crack forgery squad, which was headed by Eric Maschwitz, a songwriter and screenwriter who earned an Oscar nomination the year before for co-writing 1939’s “Goodbye, Mr. Chips.” In just two days, Maschwitz and his team produced a picture-perfect map, down to using including inks consistent with those used by the Nazis. in just two days, Stephenson got the map to Roosevelt through Donovan. It’s unknown if Roosevelt was genuinely fooled or knew it was a useful forgery.

Once the US entered the war, Stephenson’s mission was accomplished. He did more traditional intelligence gathering during the conflict, and was honored with a knighthood.

Stephenson spent the rest of his life working on various business interests from his homes in Jamaica and Bermuda. He died in 1989, at age 92, having had a greater effect on history than his island neighbors could have imagined.

“In June 1940, this quiet, sturdy-looking 43-year-old arrived in Manhattan as a first-time spymaster with a threadbare staff and little sense of the job in hand,” Hemming writes.

“Nine months later, he had launched an assault on American public opinion unlike anything which had gone before. [His] operation . . . played a critical part in leading the US away from isolationism.”