“Secretary Morrice did this day in the House, when they talked of intelligence, say that he was allowed but £700 a year,” wrote Samuel Pepys in his diary in 1668, “whereas, in Cromwell’s time, he did allow £70,000 a year for it; and was confirmed therein by Colonel Birch, who said that thereby Cromwell carried the secrets of all the princes of Europe at his girdle.”

Charles d’Éon de Beaumont went to Russia in 1755 as a secret correspondent of French king Louis XV. Disguised as a woman, he obtained an appointment as reader to a Romanov empress; he returned for a mission the next year dressed as a man. By the 1770s speculation about his gender reached such fervor that odds were quoted by London brokers. An 1810 postmortem finally confirmed that he was anatomically male; Marie Cole, his companion of fourteen years, reportedly “did not recover from the shock for many hours.”

Union general William T. Sherman believed newspaper correspondents to be liabilities. “A spy is one who furnishes an enemy with knowledge useful to him and dangerous to us,” Sherman wrote in an 1863 letter. “I say—in giving intelligence to the enemy, in sowing discord and discontent in an army—these men fulfill all the conditions of spies.”

In ancient Indian espionage networks, a sattri was an orphan-spy trained from youth in palmistry, magic, omens, and augury. 

To label a program conceived in 2007 that declared as its purpose the tracking of “every user visible” on the Internet, collected data from over a trillion Web events in a repository called the Black Hole, and targeted sites broadcasting recitations of the Qur’an, the UK Government Communications Headquarters chose the name Karma Police, after a song by the band Radiohead whose chorus is: “This is what you’ll get / when you mess with us.” 

About 40 percent of employers peer at the social-media profiles of prospective candidates, and 40 percent of party guests admit to snooping through a host’s medicine cabinet or drawers. The percent of women who admit to having looked through a partner’s phone without permission is 34 percent; of men, 62.

New York Giants pitcher Christy Mathewson wrote in 1912 that he had heard the Philadelphia Athletics “had a spy” who stole signs and “tipped the batters by raising and lowering an awning a trifle.” In Philadelphia for the World Series the year before, Mathewson had looked for the culprit. “In the enemy’s camp, I kept watching the windows of the houses just outside the park for suspicious movements,” he wrote. “But I never discovered anything wrong.”

Walter Kirke, British deputy head of military intelligence in France, noted in his diary in October 1915 that the chief (“C”) of the Secret Intelligence Service had come upon a solution for how to send secret messages: “Heard from C that the best invisible ink is semen,” Kirke wrote. The substance, it turned out, was hard to detect by the common revealing method of iodine vapor. The chief’s name: Mansfield Cumming.

In July 1990, one year before the collapse of the USSR, scholar Nicholas Eberstadt testified before a Senate committee about a CIA study of the Soviet economy, which showed high Soviet meat production and per-capita milk output—exceeding U.S. levels—though shortages were widely reported by tourists and Soviet citizens. “The Soviet government routinely hides many of its efforts from outside views,” Eberstadt granted. “But where, one wonders, are the hidden stockpiles and reserves of Soviet meat?”

While stationed as a secret agent in Bern in 1917, future CIA director Allen W. Dulles received a phone call from a Russian exile with an urgent message to deliver to the United States. Having already arranged a rendezvous with Swiss twin sisters at a country inn, Dulles demurred, finding out only later that the caller was Vladimir Lenin, who returned to Russia in a sealed train the next day. 

Michael Hayden, former director of the CIA and NSA, claimed while discussing the NSA’s collection of telephone-call metadata, “We kill people based on metadata,” quickly qualifying, “But that’s not what we do with this metadata.” When declining an interview about alleged U.S. cyberattacks on Iran, he sent a one-line email that read, “Don’t know what I would have to say beyond what I read in the papers.”

In 480 bc, with the Persian army on the cusp of defeating Greece, Athenian general Themistocles sent a trusted slave to convey a message to Persian king Xerxes; the note professed allegiance to Persia and reported many Greek ships prepared to defect. The Persians, acting hastily on this false intelligence, sailed into the Strait of Salamis, where the Greek fleet was waiting and gained a decisive victory. 

Seventh-century Persian king Khosrow II is said to have tested the loyalty of courtiers whom he believed were becoming too close. Telling one of his decision to execute the other, he would swear the man to secrecy and then watch the friend’s behavior. If it went unchanged, he knew the first man was loyal and had kept silent; if different, he was a traitor and dealt with accordingly. 

When asked why he didn’t use intelligence agents, Alp Arslan, sultan of the Seljuq Empire in the 1060s, replied that his favored subjects would trust the spies, while his opponents would curry favor and bribe them; he’d end up hearing damaging reports about his friends and positive ones about his enemies. “Reports good and bad are like arrows,” Arslan said. “If you shoot enough of them, at least one will hit the target.”

At Super Bowl XXXV in Tampa police used cameras and biometric face-recognition software to survey fans. The system identified nineteen subjects of outstanding warrants, far more than anticipated. None were arrested. “We thought we were ready to use it,” said a detective.