The History of Spies: From Ancient Rome to the CIA, How Intelligence Gathering Shaped the Modern World

Spying is one of the oldest professions in recorded human history. Not a metaphor. Egyptian hieroglyphs from roughly 3,500 years ago document the use of intelligence agents. The Hebrew Bible contains what scholars describe as one of the earliest detailed reports of a sophisticated intelligence operation, Joshua sending spies into Jericho before attacking. The Art of War, written by the Chinese general Sun Tzu around the 5th century BCE, devotes an entire chapter to the classification and management of spies. He identified five categories: local spies, inward spies, converted spies, doomed spies, and surviving spies. The categories map cleanly onto what modern intelligence agencies call, respectively, agents in place, double agents, turned agents, disinformation carriers, and case officers.

Sun Tzu was not describing something novel. He was systematizing something that had been happening for centuries. The need to know what your enemy is doing before they do it is not a modern insight. It is a feature of every organized human conflict on record.

Rome: Where It Got Sophisticated

No ancient civilization invested more heavily in espionage than Rome. The creation of the largest empire of the ancient world required governing an enormous infrastructure, and governing it required knowing what was happening at its edges before the edges collapsed.

Roman emperors such as Augustus maintained extensive intelligence networks to prevent rebellion, protect trade routes, and safeguard imperial authority. These early systems combined human observation, code words, and clandestine correspondence. The frumentarii, originally military grain collectors, evolved under later emperors into a secret police force that reported on governors, generals, and senators. The agentes in rebus, created in the 4th century, served as imperial couriers who also gathered intelligence and monitored the loyalty of provincial officials.

Rome’s most famous intelligence failure is also its most famous assassination. Julius Caesar died on March 15, 44 BCE, despite the fact that the Roman intelligence community had apparently received warnings about the conspiracy. The exact failure remains historically contested. What is documented is that the warning was there, and Caesar proceeded anyway. Some historians think he did not believe it. Others suggest the intelligence was not actionable enough to act on.

Elizabethan England and the First Modern Intelligence Service

The Renaissance saw European spycraft evolve into something recognizably institutional. The Venetian Republic maintained one of the most sophisticated intelligence networks of the medieval period, using its merchant travelers as a global information network. The Vatican used its clerical agents across pre-Reformation Europe similarly.

The operation that historians most often point to as the founding of modern intelligence services in England, however, belongs to Sir Francis Walsingham, Queen Elizabeth I’s spymaster from 1573 to 1590. Walsingham built a network of agents across Europe that intercepted correspondence, ran informants inside foreign courts, and broke ciphers at a level of sophistication that had not existed in English government before.

His most significant success was uncovering the Babington Plot of 1586, a Catholic conspiracy to assassinate Elizabeth and place Mary Queen of Scots on the throne. Walsingham did not simply detect the plot. He ran it. His agents had penetrated the conspiracy so thoroughly that he was able to allow it to develop until he had enough evidence to arrest and execute all the conspirators, including, crucially, proof sufficient to execute Mary herself.

Notably, his methods included forgery. He added a postscript to one of Mary’s letters asking for the names of the conspirators, ensuring he had evidence linking her directly to the assassination plan. The line between intelligence and manipulation, between detection and entrapment, runs through Elizabethan spycraft as clearly as it runs through modern operations.

World War II and the Intelligence Revolution

Both world wars transformed intelligence from an ancillary activity into a central feature of military strategy. The most significant development was cryptography.

The breaking of the German Enigma cipher machine by British codebreakers at Bletchley Park, led in significant part by the mathematician Alan Turing, gave the Allies access to German military communications throughout much of the war. Historians estimate that the intelligence advantage shortened the war by two to four years. The full extent of Bletchley’s achievement remained classified until 1974, meaning that for nearly 30 years, the people who won the war through cryptography received no credit for it.

On the human intelligence side, the Second World War produced Operations Mincemeat and Double Cross. Operation Mincemeat placed forged documents on a corpse dressed as a British officer, feeding the Germans false information about the planned invasion of Sicily. Operation Double Cross turned virtually every German spy in Britain into a double agent, feeding Germany a sustained stream of disinformation throughout the war.

The Americans, meanwhile, were starting from essentially nothing. Franklin Roosevelt turned to William Donovan to run a new agency, the Office of Strategic Services, which eventually became the CIA. The OSS reached 5,000 personnel by September 1943. In all, 35,000 men and women served in it by the time it closed in 1947.

The Cold War: Espionage as Central Policy

The Cold War institutionalized espionage as a feature of global politics rather than a supplement to military strategy. The most famous and active organizations were the American CIA, the Soviet KGB, and the British MI6. The East German Stasi was primarily concerned with internal security but its Main Directorate for Reconnaissance operated espionage activities around the world.

The Cold War produced the spy satellite, the U-2 reconnaissance aircraft, and systematic signals interception. It also produced some of the most consequential individual intelligence operations in history.

Oleg Gordievsky was a senior KGB officer who operated as a double agent for Britain’s MI6, providing a stream of high-grade intelligence that had an important influence on the thinking of Margaret Thatcher and Ronald Reagan in the 1980s. He convinced Washington and London that the fierceness and bellicosity of the Kremlin was a product of fear and military weakness rather than an urge for world conquest. This assessment directly influenced how Western leaders approached arms control negotiations in the final decade of the Cold War.

Aldrich Ames, a CIA officer who sold intelligence to the Soviet Union for money, identified Gordievsky to the KGB. Gordievsky was exfiltrated from Moscow in 1985 before the Soviets could arrest him. The human cost of Ames’s betrayal included the execution of numerous CIA assets in the Soviet Union.