Have you heard the rumor that Donald Trump is mentally ill? Did you hear that President Obama wiretapped Trump Tower? With the help of British intelligence?  Or that a child-sex ring connected to Democrats was being run out of a Washington, D.C. pizza restaurant?

American society is being bombarded by rumors. Fake news websites push stories like the aforementioned “Pizzagate.” Russian has an army of Twitter trolls who blast out all sorts of wild rumors. Even Donald Trump’s own tweets deluge us with confusing and contradictory information.

It seems awful hard to know what’s true and what’s not these days. Where is the antidote for the epidemic of fake news? Many of us may feel like we can’t even trust our own judgment. And maybe, that’s the point.

The post-truth era, as it’s been called, might feel very familiar to American spies operating behind enemy lines in World War II. Back then, U.S. operatives were coming up with creative ways to damage morale and divide the leadership of Nazi Germany. One of their best weapons was the use of carefully crafted, well-timed rumors.

Rumors were a specialty of the Morale Operations Branch of the Office of Strategic Services (OSS), the predecessor of today’s CIA. One of the most famous of the OSS’ rumor campaigns was “Where Is Hitler?” The OSS would broadcast a fake report that Hitler was supposed to appear at an upcoming rally.  When Hitler inevitably failed to show, the OSS would float rumors that Hitler was ill or suffering from a mental breakdown. These rumors spread so widely that they became the subject of articles in American newspapers, including The New York Times.

Creating a loss of confidence in leaders was just one was just one the tricks dreamed up by the OSS Morale Operations branch. Others are spelled out in a now declassified field manual, which is a guide on how to use rumors, forgeries, blackmail and bribery to destabilize a country. What the OSS called “subversive rumors” could be used to cause enemy populations to distrust their own news sources, create division among racial, political and religious lines, to create confusion and dismay with a welter of contradictory reports, and to tip the balance when public opinion was in a precarious state, among other things.

Viewed in this light, fake news seems less a nuisance and more like something that would trouble our intelligence community. And indeed, they do appear concerned. The U.S. intelligence community recently concluded that Russia mounted an “influence campaign” during the 2016 presidential election that blended covert intelligence operations with overt efforts by Russian Government agencies, state-funded media, third-party intermediaries, and paid social media users or “trolls.” Russia influence campaign sought to undermine faith in U.S. democracy and denigrate Hillary Clinton’s campaign.

In essence, Russia has created a modern version of the OSS Morale Operations branch. Social media gives the modern operative powers the likes of which his or her OSS forerunner could only have dreamed. Whereas the OSS had to send operatives into enemy territory to plant rumors, the modern influence campaign can without leaving home harness the power of social media sites. Facebook, Twitter and YouTube are machines for the rapid transmission of rumors.

While the technology behind rumor campaigns has evolved, the nature of rumor itself hasn’t changed much in the 74 years since the OSS wrote its field manual. The OSS defined a rumor as “an unauthenticated, unofficial story or report, represented and transmitted as fact.” This distinguishes it from propaganda, which stamps its authorship on its message. Anybody can start a rumor. Crafting a good one is an art form.

The old OSS characteristics of what makes a good rumor still hold true: A good rumor still must be simple, consisting of a single idea. It must be plausible. It is tied to some known facts, yet is impossible to completely verify. It frequently appears as an “inside” story.  The best rumors to spread are existing ones. “In many cases, the most effective rumor policy will be to spread further rumors that have arisen spontaneously in enemy territory,” the field manual advises.

A good rumor must also be vivid. Rumors with “strong emotional content” are extremely effective. (Case in point: the unforgettable, unverifiable story of Trump cavorting in a Moscow hotel room with prostitutes.) A suggestive rumor was well adapted to spreading fear and doubt, by doling out limited but tantalizing bits of information that allow the audience to formulate conclusions (“FBI Director James Comey made an unexpected trip to the White House.”)

Robert Knapp, who developed the section of the OSS’ Field Manual on rumors and wrote academic papers on the subject, likened a rumor to a torpedo. “Once launched, it travels of its own power,” he wrote. Knapp had an insight into what gave rumors their power: They expressed and gratified the emotional needs of the community, just as daydreams and fantasies expressed the needs of the individual. Rumors gave sense and direction to fears, resentments or hopes. ”No rumor will travel far unless there is already a disposition among those who hear it to lend it credence,” he wrote in a 1944 paper.

Among the many coincidences involving Russia and Donald Trump, one that goes unnoticed is their mutual grasp of the power of rumor. Trump used rumors to stunning effect in his campaign, beginning with the suggestion that President Obama was born in Kenya. This rumor tapped into deeply-held beliefs about President Obama that many people were not comfortable expressing publicly. Outright racism is unacceptable to most Americans. However, many found the disguised racism of a rumor about the African-American president’s birthplace more palatable. There is frequently a racist undertone to many of Trump’s rumors: Muslims celebrating Sept. 11 in New Jersey, illegal immigrants voting, terrorist incidents that didn’t happen, and so on.

Rumors may also help explain Trump’s appeal. In a recent interview, Time magazine’s Michael Scherer pressed Trump on his use of rumors. “What am I going to tell you? I tend to be right,” the president told him. “I’m an instinctual person, I happen to be a person that knows how life works.” In other words, Trump’s rumors feel true to him, even if they can’t be verified. Trump’s words also feel true to his supporters, almost like an article of faith. He is making a connection on a deep emotional level that, once established, is difficult to break.

However, Trump’s predilection for rumors over facts is dangerous, for it leaves him wide open to manipulation. Unwittingly or not, Trump has spread rumors that originated in Russia. The story spread by the White House that President Obama used British intelligence to spy on Trump and his associates started as a story on RT, the Kremlin-backed propaganda outlet.  On the campaign trail, Trump quoted a report that appeared to originate on Sputnik, another Kremlin-backed media outlet.  At a March 30 Senate intelligence committee hearing, Clint Watts, a former FBI agent and an expert on Russian disinformation, explained  in striking terms the problem with having a rumor-monger for a president:

Rumors do work on the campaign trail, but they are toxic to the presidency. Credibility is one of the president’s strongest assets, never more so than in moments of crisis. Trump seems not to understand that, as president, he is the authority, and the White House is the place where rumors end, not where they begin. If President Trump truly wants to make America great again, he must stop spreading rumors.

If Trump won’t quash rumors, others must do it for him. Many news organizations are now regularly refuting the president’s rumors. This effort harkens back to World War II, when rumors were an even bigger problem then they are now. Robert Knapp, the OSS’ rumor expert, founded a “rumor clinic” in Boston that collected rumors and sought to put and end to them. A column first published in the Boston Herald in 1943 quoted the rumor in italics followed by the word FACT. Rumor clinics opened in many cities, but quickly faded following a clash with the Roosevelt administration’s Office of War Information. Government bureaucrats wanted to smother rumors with facts, rather than call attention to them by singling them out for disproof. (For more on this click here.)

Knapp proposed that rumors could serve as an “index of morale.” They may be a better gauge of the true state of public opinion than any poll or survey. Rumors allow expression of the deeply held beliefs and fears that won’t be repeated to a stranger. A look at the rumors prevalent in American society show we are a deeply divided along racial, political, and religious lines. Many Americans have little or no confidence in our elected leaders. We distrust our own news sources.

In sum, American morale has been deeply wounded. We are much weaker than we think we are.