In 1973, a former CIA operations officer in Latin America walked into the KGB rezidentura in Mexico City with what he claimed was a tranche of invaluable secrets about the United States’ covert operations in the hemisphere. The “resident,” or Soviet station chief, was wary of too-good-to-be-true offers coming from seeming Western volunteers. So, believing he’d been sent an obvious double agent bearing conspiratorial gifts from Langley, the spymaster showed Philip Agee the door.

Oleg Kalugin, at that time the head of counterintelligence for the KGB’s First Chief Directorate, which handled the Soviet Union’s foreign intelligence, would later tell that tale with chagrin: “Agee then went to the Cubans who welcomed him with open arms. The Cubans shared Agee’s information with us. But as I sat in my office in Moscow reading reports about the growing list of revelations coming from Agee, I cursed our officers for turning away such a prize.”

That “prize,” as Kalugin further noted, had “reams” of actionable intelligence about ongoing CIA operations, including the names of 250 covert operatives in the Americas, and was a propaganda windfall. Agee published a book, Inside the Company: A CIA Diary, that became a best seller in 1975, and in the years to come Agee, by then a Soviet agent codenamed “PONT,” would be used to help compromise some 2,000 CIA officers whose identities were quietly provided by the KGB for publication in the Covert Action Information Bulletin (which was actually founded “on the initiative of the KGB,” as the agency’s former archivist Vasili Mitrokhin noted), and in a book he co-edited called Dirty Work : The CIA in Western Europe.

Still, it’s hard to blame the Mexico City resident retrospectively for suspecting what in spook parlance is known as a “dangle.” It’s exceedingly rare that a trove of genuine intelligence will ever cross the transom of a foreign embassy. More likely, if you think you’re being handed the keys to an enemy kingdom, you’re actually being invited to don its shackles. If you’ve read John le Carré’s Tinker, Tailor, Soldier, Spy, then you’ll recall that “Project Witchcraft,” the infamous operation that guides the plot of the novel, is in fact a beautifully orchestrated KGB dangle, using a Soviet cultural attaché who connives with a British double agent to defraud “the Circus,” or MI6, which thinks Alexei Polyakov is actually its man.

Both sides in the Cold War played this game, even before there officially was a Cold War, and now we know that the Soviets had ample case histories showing the lengths to which their democratic rivals would go to lure KGB officers into elaborate traps.

In 1943, “Guber,” a lieutenant in the Germany army, turned himself over to the Soviets at the Soviet-German front. Although they threw him into a POW camp, the Reds clearly saw potential in Guber. They educated him in “anti-fascist school” and he was recruited by what was then known as the NKVD, one of the many alphabet-soup precursor agencies of the KGB (and now the Russian FSB and SVR) to help educate his fellow imprisoned Germans. According to one retelling of this lost tale of international intrigue, which began at the height of World War II, “Guber exposed two underground fascist groups [in the Soviet Union] who had tried to contact Germany. He portrayed himself as a progressive opponent of the Hitler regime.”

So far, so good. Guber was released from the POW camp in 1950, whereupon he was trained “extensively for placement in West Germany as an illegal,” or undercover spy with a false identity and backstory accounting for why he’d spent so much time in the East Germany, or, as it was known, the German Democratic Republic. He would claim he was a refugee originally from West Germany who wanted to return home. Real facts of his biography were sprinkled into Guber’s “legend” or Soviet-concocted backstory: a correct address in Munich, his deceased father’s name (Wilhelm) and profession (dentist). He also would say that he was drafted into Hitler’s army in 1943 during his third year of medical school at Jena University.

It was just enough verisimilitude, or so his handlers thought, to pass inspection by any snooping West German intelligence officers who might become curious about this newly returned exile. Except that Guber’s handlers also checked up on what they’d been led to believe was his provenance. And while they found that a man called Wilhelm had indeed lived at the stated address and that his home had been bombed out during the war, killing him, they also found a neighbor who told them that Wilhelm had had no children.

Soviet spies dug further and learned that in 1945 the British removed the entire archive from Jena University, making a trawl through its past classes impossible. But the Soviet agents were able to locate a former university accountant who used to issue wartime ration cards to the students. The accountant kept his own list of ration card recipients. Not only was the name Guber not among them, but there was no evidence that any third-year students had been drafted into the Wehrmacht. Furthermore, students from the school who did get drafted were given “bonus bread cards,” records of which the accountant had also kept. No one matching Guber’s description could be cross-referenced with any draftees from this private archive either.

Guber was a German soldier—that much was true. But before he surrendered to the Red Army in 1943, he had been captured by the British army, turned into one of their own spies and instructed to head toward the front to offer himself up as a captive of Moscow. Given the number of fleeing Fritzes the Soviets were scooping up at the time, Guber would simply blend in as a “natural.” That he did, costing Stalin’s service several years of training and squandering untold resources for what was, in the end, a cleverly concealed agent of London.

Two years before Agee got turned away in Mexico City, the KGB had published a top-secret document for internal circulation about the looming danger of dangles. As this was a Soviet text, it was given a characteristically turgid title: “Exposure of the Enemy’s Set-ups in the Process of Development of Persons Who Are of Interest to Intelligence.” A copy of this document was passed to me recently by an officer from a Western security service. As with prior examples of leaked KGB training manuals I’ve published at The Daily Beast, this one is still classified in modern Russia owing to its tutorial value to the SVR, Putin’s foreign intelligence arm and the post-Soviet successor to the First Chief Directorate. Guber’s story, never before told, is contained in a cautionary brief for Soviet spies, as are other anecdotes about Western trojan horses.

Or, as was often the case, honey traps.

In 1969, János, a Hungarian spy who worked at the Hungarian trade agency in the U.K. under cover as an engineer, met “Gretta” at a reception hosted by Britain’s Foreign Office. As she presented herself, Gretta was a 26-year-old technical secretary at the West German embassy in London with “progressive views… unmarried, the daughter of rich parents” with whom she’d fallen out. She struck up a romantic interest in János, who was married and seemingly uninterested in a little side action—or at least not this particular side action. Which did not deter Gretta from finding ways to meet him in London and by offering more than just her good self.

First she flirted casually by telling János that her job at the embassy involved typing up all manner of strange documents, with which she wasn’t really familiar. János was unmoved. Then Gretta came on strong by saying that she could easily return to Germany and get a job at the foreign ministry. The Magyar was unmoved. Finally, she let slip that she “had access to important materials on Sundays,” which might have seemed to János the espionage equivalent of being handed a pair of unlaundered panties with a hotel room number written on them. Nevertheless, he wisely remained an oak of self-control.

Because the Warsaw Pact services were always subordinate to, and typically penetrated by, the KGB, Gretta’s tenacity did not go unnoticed by Moscow Center, which placed her under surveillance and found that she’d made contact with an official from the BND, West Germany’s foreign intelligence service.

Gretta was an easy-to-spot amateur who tried way too hard, but many other dangles are adroit at playing upon a spy’s psychology or personal vices. Always be on the look-out, the manual states, for those who appear cautious at first, knowing they’re being vetted. Anyone who promises the Pentagon Papers or the back-end code for Stuxnet at his first encounter is almost certainly a provocateur, Agee notwithstanding. Be mindful that prospective assets claiming to have access to foreign intelligence may provide useful and credible information initially to get you on the hook, but then their stuff quickly exhibits diminishing returns. Avoid charity cases or anyone creating conditions to become dependent on you or proffering quid pro quo.

But don’t be so cautious or paranoid that you miss out on the real deal when it presents itself.

Gretta’s bizarro-world double was a typist in a “West European” foreign embassy called Liviya, 25, single and the school friend of the wife of a “tested and loyal” agent of Bulgarian intelligence called Georgy, a doctor in private practice with no discernible ties to the unnamed West European government. The Bulgarian rezidentura had been duped before by trying to cultivate another foreign ministry typist who turned out to be a dangle and so, rather than work on Liviya directly, which would surely arouse the suspicions of the host country’s counterintelligence, the Bulgarians used Georgy to vet her.

The good doctor found out that her left-wing beliefs were sincere, as was her generally positive view of socialist countries. Like Gretta, Liviya had been indiscreet with a married man, her coworker, but not because she was ordered to by a handler. And unlike Gretta, her affections had been requited. A child was born of their affair and the coworker dropped Liviya. She was now low on funds. Georgy tried to appeal to her “progressive” leanings, and Liviya started to tell him about the documents she typed up at the ministry. Liviya was successfully recruited as an agent and she soon began passing those sensitive documents onto the Bulgarians.

The bold and the brave can make excellent spies, but not when they’re also careless about operational security. That suggests they have good reason not to fear a host country’s counterintelligence service: they’re working for it. Finally, the would-be recruit who keeps asking for new assignments without ever completely fulfilling old ones is a likely plant, as is the one who volunteers for “immoral actions that might compromise him.” No honest target seeks out life-threatening danger unless he or she is an adrenaline junkie or just plain psychotic and therefore of no use anyway. An asset must be convinced to sacrifice.

Countries where hospitality is part of the cultural DNA—the manual specifies those in Africa and the Middle East—may seem attractive for running agents, being invited to a target’s home is almost always bound to be a preliminary of recruitment. But such countries are also fraught with the hazards of getting to know too many of a target’s friends, colleagues and family members, who might give the game away even before it’s afoot.

Stasi agent Klaus worked in Rome under cover as the first secretary at the embassy. He “successfully developed friendly relations” with “Irwin,” the first secretary of the American embassy who, the Center’s background check had confirmed, had no connections to the CIA. But Irwin didn’t hide his acquaintanceship with Klaus, and the target of an East German recruitment operation was quietly turned into tool of an American one.

One night, “Stanley,” the second secretary of at the U.S. embassy in Rome, came to dinner with Irwin and Klaus. After that, Irwin shunned his East German counterpart, but Stanley kept up with Klaus and pumped him for information. Klaus’ cover had been blown.

One way to vet a target, according to the manual, is to task him or her with collecting “information on neutral persons” and then waiting to see if an enemy intelligence agency takes an interest in them or tries to develop or recruit them itself.

Take Staub, a Soviet agent in Vienna, who met Jack, a West German journalist who lived in Paris. Jack said all the right things—he disapproved of America’s “militarist policy”—and he was married, but in a state of financial difficulty. He wasn’t pushy or nosy and went back to France as he was scheduled to, having moved there a while ago with his wife.

Belev, a Stasi agent in Paris, was then assigned to meet Jack and vet him. Jack impressed Belev, who then asked him to “characterize” four West German trade officials in France. One of these was called “Chameleon,” whom the Soviets and Stasi believed was actually the BND station chief in Paris. Jack correctly identified Chameleon as a spy, thus earning Belev’s trust. But Moscow Center still wasn’t convinced and put him through a more elaborate trial. Jack would often travel to “a Scandinavian country” on business, and Staub was instructed to ask him to locate three West Germans there. The presumption was that if Jack were in fact a dangle, his interest in them would become known to the BND; and if the BND didn’t act to stop or check up on his probing, he was clearly acting as one their assets.

When Jack returned to Paris, he brought back “detailed information” on the three Germans, which confirmed what the Stasi had already known about them. However, the Center was right to put the journalist through this second exercise on third-party soil. The Stasi rezidentura in Bonn found out that the BND was aware of Jack’s inquiries about the Germans but did nothing to interdict them, or him.

Thus the KGB was able to determine that Jack was a plant, albeit a well shrouded one. After all, he’d been ordered to expose the West German station chief in Paris to the enemy just to prove his bona fides, a testament to just how high-value his mission of infiltrating the agent network of the Stasi was deemed, or simply a reflection of the fact that the BND knew Chameleon was compromised and therefore saw no harm in confirming his true identity through a dangle.

Using one’s own state secrets to try and snag an Eastern Bloc or Soviet one may sometimes be considered a necessary investment, but Western nations were not above using the the secrets of their allies to do so, as well.

Mavr was a code clerk at the Spanish foreign ministry. Mavr seemed appropriately “progressive;” he spoke with “admiration for the achievements of socialist science, industry and culture, and agreed with the Cuban line, criticizing American and British reactionary policy,” even though his views were unknown to others around him. He seemed to be valuable, passing along names of foreign ministry personnel who might be recruited, and selling the ministry’s code to the Cubans and, by extension, the Soviets. Moscow Center decided to test Mavr.

He was given a small suitcase to hand off to a certain someone who was passing through Spain, at a location two hours from Madrid. If the person didn’t show up, he’d simply return the suitcase to Gensolen. The person didn’t show up. But the suitcase he brought back had shown signs of being tampered with: specifically, someone had tried to open it but then stopped “so as not to leave a trace.” Or so they thought. Had it been opened, the tamperer would have discovered that the KGB had placed a bug inside. Undetected, the bug confirmed that Mavr was working for the CIA, which meant that he was also working undercover at the Spanish foreign ministry. He gave up Spanish intelligence to reel in the Cuban and Soviets on behalf of the Americans.

Falling for a dangle is a bit like falling for a scheming lothario more interested in worming his way into your well-heeled family than he is in you. Entire rezidenturas have been destroyed by letting in such a bad apple. Which is why letting him down must also be handled with subtlety and care. No ghosting. “If a target is discovered to have ties to [counterintelligence],” the manual instructs, “don’t break off relations with him as that will tip him off. Just meet less and less often and then end them.” In spy craft as in love, if you suspect you’ve been had by a player, the best revenge is turn him into the played.

Translation by Catherine A. Fitzpatrick.