Espionage, or spying, refers to the covert surveillance of another government aimed at gathering information about the behavior and intentions of the opposing state. Espionage encompasses the following activities: undertaking covert operations in foreign countries, recruiting foreign nationals to work as double agents, monitoring the mail and telecommunications of foreign nationals, and using reconnaissance flights for aerial surveillance of enemy territory. Although espionage has existed for more than 2,000 years, it was not until the 20th century that most countries developed permanent espionage programs, creating government agencies and training professional agents. Espionage is common in international politics, but it remains a controversial practice. This entry provides an overview of espionage, detailing the legal and political debates over covert surveillance programs. It then considers the multitude of surveillance tools that countries have employed to gather intelligence about enemies and allies alike, in an effort to gain a strategic advantage in international affairs.

Espionage and National Security

Although there is no single definition of espionage, scholars agree that all espionage is covert; in this view, espionage is different from other categories of intelligence gathering, such as the analysis of open source materials. Commonly, espionage involves the recruitment of double agents. An effective double agent is a high-ranking official in another country. This person can be a source of valuable information on the internal politics of his or her country, by providing copies of classified documents. Double agents are also valuable for counterespionage; for example, if Country A recruits a double agent in Country B, the double agent can then inform Country A if any of its own top officials are secretly working on behalf of Country B.

Another type of espionage relies on the use of electronic surveillance. Here, a government can gather information on another regime using reconnaissance flights, satellite photographs, or the monitoring of electronic data, such as phone calls and Internet traffic. Advances in technology, such as the development of unmanned aerial vehicles, or drones, have made this form of intelligence gathering much more common.

Most countries see espionage as a policy tool, allowing them to gather information about the actions and intentions of enemies and allies alike. Specifically, governments can use the information gathered from spying to predict and prevent military attacks, discern the motives of a negotiating partner, or gain knowledge on new military technologies.

Nearly every country in the world has an agency dedicated to foreign intelligence gathering. Notable agencies include the Central Intelligence Agency (CIA) in the United States, MI6 in Britain, the Ministry of State Security in China, the Mossad in Israel, and the Foreign Intelligence Service (formerly the KGB) in Russia.

Traditionally, espionage has been defined as spying on a foreign government. For many countries, however, especially those in the developing world, the greatest security threats come not from foreign powers but, rather, from internal forces, such as separatist movements or terrorist groups. To combat these asymmetric threats, governments can also employ the tools of espionage, for example, monitoring the communications and financial transactions of a targeted group.

The Legality of Espionage

Espionage occupies a legal “gray area” as it is neither expressly forbidden nor permitted by international law. Several major international treaties and conventions do provide insight into the permissibility of the activity. For example, although the founding Charter of the United Nations does not speak directly of the issue of espionage, Article 51 of the Charter codifies the idea that states are sovereign entities with the right to self-defense. In this view, espionage is permitted as one tool states can use to protect their national security.

Two other legal documents contain clearer statements on the legality of espionage. The 1949 Geneva Convention, which outlines standards for humane conduct in wartime, defines espionage in Article 29; the treaty notes that a spy is an individual who enters a foreign nation under false pretenses for the purpose of gathering information. Although the convention holds that a captured spy may be punished, under the laws of war, this individual retains the right to a fair trial.

The 1961 Vienna Convention on Diplomatic Relations, which outlines modern standards of diplomatic practice, also addresses the issue of espionage committed by diplomats. More precisely, the convention specifies that diplomats, as credentialed personnel of foreign nations, are granted diplomatic immunity; as such, a diplomat who is caught spying may not be arrested or prosecuted by the host country. The host country, however, retains the right to declare the diplomat a persona non grata and expel him or her.

Finally, customary international law, namely the traditions and practices of states that are not codified into treaties, suggests that espionage is viewed as a legitimate though much disparaged tool of international politics. Specifically, while most regimes denounce espionage when it is committed against them, the same governments are willing to provide asylum to foreign nationals who commit espionage on their behalf. Overall, there is a moral relativism inherent in espionage: A citizen who spies for another country is reviled in his or her home country but is treated as a hero in the country to which he or she provides the intelligence.

In addition to debating the legality of espionage, scholars are also divided over the utility and wisdom of using espionage as a policy tool. Critics of espionage, such as Herbert Scoville, point to a litany of problems associated with the practice. Most basically, he questions whether or not espionage actually produces useful information. He notes that many double agents are motived by monetary gain, and they are unlikely to provide high-quality, actionable intelligence that will aid a country in making policy decisions.

Moreover, Scoville notes that espionage deepens distrust between countries and, when it is discovered, can precipitate an international crisis. For example, he points to the fallout that occurred in 1960 when the Soviets shot down a U2 spy plane piloted by the American Gary Baker. Following the incident, Soviet leader Nikita Khrushchev became less willing to work with the United States; he walked out of an international summit attended by representatives from the United States and refused to engage in arms control negotiations.

Supporters of espionage, however, argue that the practice actually decreases the probability of international conflict by providing countries with a more complete picture of the intentions of their enemies. For example, Glenn Sulmasy and John Yoo argue that the information gathered through espionage allows a country to adjust its strategy accordingly so as to prevent conflict. For example, if a country gains information about an imminent attack on its territory, it can take deterrent action to prevent military conflict. Alternately, espionage may reveal that a perceived enemy actually has benign intentions; in this case, a government may decide to open up peace talks. Overall, Sulmasy and Yoo argue that espionage provides intelligence that helps mitigate the uncertainty inherent in the international political arena.

Domestic Prohibitions Against Espionage

Although espionage is not clearly banned by international law, many countries have passed domestic laws criminalizing the behavior and outlining harsh punishments for captured spies. The United Kingdom was one of the first nations to formalize this prohibition into law, with the 1889 Official Secrets Act. The law has been amended multiple times, most recently in 1989. Specifically, the Official Secrets Act made it a crime for a U.K. citizen living anywhere in the world to obtain state secrets, transmit state secrets, or harbor an individual who is acting as an agent of a foreign power.

The United States modeled its own anti-espionage law, the 1917 Espionage Act, on the U.K. law. President Woodrow Wilson signed the Espionage Act into law during World War I; the act updated existing law, making it a crime to provide intelligence to the enemy. The act was later broadened, making it a crime to disrupt the war effort by urging draft resistance or disseminating false information.

Like the United States and the United Kingdom, Russia also maintains laws criminalizing espionage; during the Cold War, the Soviet Union meted out harsh punishments to officials who acted as double agents. Typically, captured double agents were executed and buried in unmarked graves.

Domestic espionage laws have been the subject of much criticism, as governments often use these laws to punish political dissidents or silence whistle-blowers. Famously, during World War I, the United States used the Espionage Act to jail Eugene Debs, an outspoken socialist who opposed U.S. involvement in the conflict. The law was also used to prosecute Daniel Ellsberg, a defense analyst for the RAND Corporation. In 1969, Ellsberg gave members of the press classified information about U.S. strategy during the Vietnam War; Ellsberg hoped that the release of the information would promote a robust public debate about continued U.S. involvement in the conflict. More recently, some members of the U.S. Congress have argued that the act should be used to prosecute Edward Snowden; Snowden, an intelligence analyst, leaked information to the press about the National Security Administration’s surveillance of domestic phone calls.

The Tools and Techniques of Espionage

Espionage, as commonly defined, is a form of intelligence gathering. There are four categories of intelligence that can be gathered: (1) human intelligence, (2) imagery intelligence, (3) communications intelligence, and (4) measurement and signals intelligence.

Human intelligence is defined as the collection of data by individuals; this is the form of intelligence gathering most commonly associated with espionage. To obtain this form of information, a country must first recruit a double agent who has access to valuable intelligence. Next, the double agent is assigned to a handler, namely an employee within an intelligence agency who manages the transfer of this information and, if applicable, provides the double agent with payment for his or her services.

Under the guidance of a handler, a double agent may employ a variety of techniques to gather and transfer information to a foreign government while remaining undetected. Since smuggling physical documents is risky, governments have often provided their double agents with document cameras. During the Cold War, both the United States and the Soviet Union provided double agents with a version of the Minox camera for photographing documents. Moreover, the Soviets developed document cameras that resembled books and notebooks, allowing agents to easily smuggle them into secure U.S. facilities.

In addition to document cameras, miniature hidden cameras were also common in 20th-century espionage operations. During the Cold War, the Soviets proved adept at engineering small cameras that could snap photos without detection. For example, by 1948, the KGB had developed a miniature robotic camera; this device, the F21, could be hidden on an agent’s clothing, disguised as a button or pin. The Soviets also developed the Tokya 58-Ml; this camera was the size of a cigarette case, and it was often hidden behind an agent’s necktie.

Once a double agent had gathered intelligence, he or she then faced the task of passing it to the handler. Typically, both sides would agree to exchange information at a “drop site”; this was a predetermined location, often in a public place. Most commonly, intelligence agencies used “dead drops.” To avoid direct contact between the double agent and the handler, the agent would leave the information in a public place. Later, the handler would retrieve the drop and often leave a payment. Famously, John Anthony Walker Jr., a U.S. Navy chief warrant officer who spied for the Soviets from 1968 to 1985, would hide classified documents inside a bag of ordinary household trash. Later, his Soviet handler would pick up the information and leave a payment for Walker in a trash bag at the same location.

In addition to human intelligence, a second type of intelligence, communications intelligence, can be obtained through espionage operations. Communications intelligence includes information gathered by monitoring foreign mail, phone conversations, and online activity. Although it may be relatively easy for a country to tap into the communication streams of an enemy government, most regimes guard against this type of spying by encrypting sensitive information. Prior to the Digital Age, governments used ciphers to encode communication concerning national security; many ciphers, such as those used in the 17th and 18th centuries, were based on simple substitution codes, where one letter or number in the encrypted message corresponded to another letter in the alphabet. Recruiting a double agent who possessed a cipher key was a boon for a country, allowing the government to decrypt virtually all of the enemy’s messages.

By the time of the Cold War, most countries relied on cipher machines to create complex codes that could not be cracked through trial and error. Today, ciphers have become obsolete, as most data are sent digitally and are heavily encrypted. To decode this information, intelligence agencies now rely on supercomputers.

Another type of intelligence is imagery intelligence, or photographic data. Here, states can use spy planes or spy satellites to gather data about enemy troop movements and military installations. Governments have been gathering imagery intelligence for more than a century; for example, in the early 1900s, countries used hot air balloons and rudimentary cameras to gain information about enemy activities. By the time of World War I, states had developed spy planes, such as the U.S. B-17 Flying Fortress, that could photograph enemy territory. Subsequently, during the Cold War, spy satellites became common. For instance, the United States used the CORONA satellite, originally designed for geological surveys, to photograph large surface areas of the Soviet Union.

Increasingly, surveillance by unmanned aerial vehicles has replaced piloted planes as a tool of espionage. Drones provide several advantages over traditional surveillance flights; most obviously, since drones are unmanned, they can be sent into areas of ongoing conflict where the airspace has not been secured. In addition, some strategic drones, such as the Global Hawk, can fly at 60,000 feet, evading detection while gaining high-resolution photographic information.

The History of Covert Surveillance

Since the beginning of recorded history, statesmen and military strategists alike have considered espionage to be a legitimate tool of statecraft. In 544 BCE, when Sun Tzu wrote his famous treatise The Art of War, he discussed the necessity of espionage-style activities. Specifically, he wrote that a good general is one who is able to gain intelligence on the other side, by intercepting communications, eavesdropping, or recruiting informants. In his view, espionage allowed a general to gain the upper hand when making both strategic and tactical decisions. Niccolo Machiavelli offered similar advice in The Prince, his guidebook for effective statesmanship. Writing in the 16th century, he counseled European leaders to embrace espionage as a means to detect and disrupt plots against their regimes.

Historians have recorded instances of espionage in many ancient regimes, including Mesopotamia, Greece, and Egypt. Of these ancient societies, Rome had the most sophisticated espionage program. In fact, many historians believe that Roman spies discovered the plot to assassinate Julius Caesar; the Roman leader, however, elected to ignore the warnings from his intelligence community.

By the 16th century, with the emergence of the modern state system, rulers used espionage to root out domestic treason plots and also monitor the behavior of neighboring powers. For example, in Britain in the mid-1500s, Queen Elizabeth I, a Protestant, developed a spy network to infiltrate Catholic groups who were plotting to overthrow her. Francis Walsingham, the head of her spy network, intercepted messages sent by the Spanish ambassador to England and discovered that the Catholic country planned to invade Britain and install Mary, Queen of Scots, as the new leader. Mary was eventually arrested for her role in the plot and beheaded for treason.

Espionage became increasingly common in Europe in the 18th and 19th centuries, in the era of wars between the continent’s great powers. For example, in France during the reign of King Louis XIII, the regime created the Cabinet Noir (“the black room”); this was a group of officials led by Cardinal Richelieu who monitored the mail of aristocrats suspected of plotting against the king. Subsequently, during the French Revolution, France used espionage to improve its fortunes on the battlefield. For instance, as an officer during the Italian campaign of 1796 to 1797, Napoleon relied on a network of French sympathizers to inform him about Italian troop movements. Notably, many of these informants were average Italian citizens, including doctors, lawyers, and merchants. Once in power, Napoleon tasked one of his trusted advisors, Joseph Fouché, with gathering information about plots to overthrow his government; some historians credit Fouché with the development of the first police state. Fouché, however, was eventually exiled after Napoleon discovered that he was acting as a double agent, conspiring to restore the monarchy to France.

On the other side of the Atlantic, espionage played an important role in several American conflicts. During the American Revolution, the Continental Army found itself at a significant military disadvantage. To gain the upper hand on the battlefield, General George Washington relied on information provided by civilian spies. One spy ring, the Culper Ring, was based in New York and headed by Benjamin Tallmadge, a soldier in the Continental Army. The members of the spy ring were trusted friends of Tallmadge; most were merchants who interacted with British officers on a daily basis. Washington eventually appointed Tallmadge to head the Continental Army’s intelligence-gathering unit. Another similar spying operation, the Clark Ring, was based in Philadelphia. The intelligence gained from these civilian spy rings had a significant impact on American military planning; for example, members of the Clark Ring informed Washington that the British were planning a major surprise attack at White Marsh in Pennsylvania. In response, Washington massed his troops there and was able to repel the British offensive.

By the time of World War I, most major powers, with the notable exception of the United States, had developed permanent intelligence agencies dedicated to the surveillance of foreign governments. This changed during World War II, when President Franklin D. Roosevelt appointed William J. Donovan to head the Office of Strategic Services (OSS); the OSS was tasked with providing intelligence for the Joint Chiefs of Staffs. In the fall of 1945, at the conclusion of the war, the then president Harry S. Truman dissolved the OSS, delegating its responsibilities to various executive branch agencies.

Subsequently, in 1947, Congress passed the National Security Act; this law created the CIA, a civilian agency dedicated to gathering foreign intelligence. The CIA quickly became the dominant organization in the U.S. intelligence community; the agency saw its size and budget grow steadily during the Cold War, when it became deeply involved in traditional intelligence gathering as well as engaging in covert actions against communist regimes in the Third World. Although Congress sought to curtail the powers of the CIA in the 1970s and 1980s, the agency gained renewed prominence during the War on Terror.

Espionage During the Cold War

In many ways, the Cold War can be considered the golden age of espionage. During this protracted conflict, the two superpowers went to great lengths to recruit double agents; the Soviets proved to be more adept than their American counterparts in this pursuit. One of the most notorious double agents, Aldrich Ames, spied for the Soviets while working for the CIA. Ames first became a double agent in 1985; he provided documents to a Soviet agent and was paid $50,000 for the information. During the next two decades, he continued to work with the Soviets, providing the KGB with information on more than 100 covert U.S. operations. Moreover, Ames also disclosed the names of 30 Russians working on behalf of U.S. intelligence agencies; the Soviets later arrested and executed many of these individuals. By 1993, the Federal Bureau of Investigation (FBI) was actively investigating Ames’s activities; in 1994, he was arrested and eventually sentenced to life without parole. His wife was also charged with abetting his activities.

At the same time when Ames was spying for the Russians, another American, Robert Hanssen of the FBI, was also providing intelligence to that country. Hanssen, who joined the FBI in 1976, worked for the Russians from 1979 to 2001; he appeared to be motivated solely by financial gain and received more than $2 million in total from the Russian government in exchange for classified information. In 2001, Hanssen was arrested, and he received a life sentence after being convicted of 13 counts of espionage. In 2002, a Justice Department report classified the failure to detect Hanssen’s spying as the “worst intelligence disaster in U.S. history.”

During the Cold War, the British were also victims of Soviet espionage. One of the most notorious cases involved the so-called Cambridge Five. More precisely, in the 1930s, the Soviet Union successfully groomed five Cambridge undergraduates to become double agents; the KGB anticipated that these men would eventually go on to hold high-level positions in the British government, allowing them to access top-secret information. The five men, Harold “Kim” Philby, Guy Burgess, Donald Maclean, Anthony Blunt, and John Carincross, all eventually secured government positions, including placements at MI5, MI6, and the Foreign Office.

Philby, the leader of the group, directed anti-Soviet counterintelligence at MI6; by 1949, he was stationed in Washington, D.C., where he had access to highly classified U.S. intelligence. During this time, Philby worked on the Venona Project, a U.S. counterintelligence operation that intercepted messages from Soviet intelligence agencies; he informed the Soviets about the existence of this monitoring, ultimately compromising the American project. Guy Burgess, another member of the ring, secured a position at MI5 and informed the Soviets about American plans to form the North Atlantic Treaty Organization to prevent Soviet expansion into Western Europe.

All of the Cambridge Five were eventually discovered by British intelligence. Maclean and Burgess escaped punishment by defecting to the Soviet Union in 1949. After a Soviet informant later exposed Philby, he too defected. Blunt and Carincross elected to cooperate with British intelligence and were not prosecuted.

The United States also successfully recruited well-placed Soviet officials into its service, although these Soviet double agents were fewer in number than the Americans who collaborated with the KGB. One, Oleg Penkovsky, a colonel with Soviet military intelligence, provided information to both the CIA and MI6. While working for Western intelligence agencies, he provided them with thousands of pages of classified documents, including information about the degree to which the Soviets were exaggerating the country’s missile stockpile. Moreover, he provided U.S. officials with information about the presence of Soviet missiles in Cuba, eventually triggering the Cuban Missile Crisis. Jack Dunlap, a National Security Administration agent who was working for the KGB, alerted the Soviets to Penkovsky’s activities, and the Soviet colonel was eventually arrested and executed in 1963.

Another double agent, Dmitri Polyakov, was considered by U.S. intelligence to be the “crown jewel” of Soviet informants. Polyakov began working for the United States in 1961, when he was assigned to the Soviet Mission at the United Nations. Polyakov, who was referred to by the code names “Top Hat,” “Bourbon,” “Donald,” and “Roam,” rose quickly through the Soviet ranks, eventually becoming a general. In this capacity, he was able to provide the United States with highly classified information on Soviet foreign relations; for example, he provided the United States with information about the growing conflict between China and the Soviet Union. This intelligence helped convince President Richard Nixon to actively exploit the growing Sino-Soviet split and open up diplomatic relations with China in 1972.

Polyakov claimed to be motivated by disillusionment with Soviet communism; as such, he refused any payment from the United States, accepting only small items such as power tools and fishing gear. Aldrich Ames eventually alerted the KGB to Polyakov’s espionage; subsequently, in 1986, Polyakov was arrested. He was convicted of treason and executed in 1988.

Spying for an Ally: The Case of Jonathan Pollard

Most espionage occurs when countries seek to gain information about their enemies. In some cases, however, countries will attempt to spy on their allies. One example of this is the espionage of Jonathan Pollard; Pollard, a U.S. citizen and an analyst at Navel Intelligence, received a life sentence after pleading guilty to spying on behalf of Israel. Notably, he became the only person to receive a life sentence for spying on behalf of an American ally.

Pollard was raised in a Jewish household and was a strong supporter of Israel; he believed that the United States was failing to adequately aid its ally in its struggle against Palestinian terrorism. As a result, in 1984, Pollard began providing classified documents to Colonel Aviem Sella, a veteran of the Israeli Defense Forces. Although Pollard initially refused any payment for his services, the Israeli government eventually provided him with a stipend of $2,500 a month. Some of the intelligence provided by Pollard had an impact on Israeli policy; for instance, information from Pollard led to Israel’s 1985 decision to bomb the Tunisian headquarters of the Palestinian Liberation Organization in the hope of killing Palestinian leader Yasser Arafat.

In 1985, fearing that he was about to be arrested by U.S. officials, Pollard and his wife traveled to the Israel Embassy, seeking asylum in Israel. The Israeli government, however, denied ever working with Pollard and refused to aid in his legal defense. In exchange for clemency for his wife, Pollard pleaded guilty to espionage and was sentenced to life in prison. In 1995, Israel did eventually grant him citizenship, and Israeli activists have continued to lobby for a presidential pardon for Pollard. Presidents Ronald Reagan, George H. W. Bush, Bill Clinton, and George W. Bush, however, have all refused to commute Pollard’s sentence.


Kelly McHugh and Kira Ramirez

See also Counterintelligence ; Diplomacy ; Intelligence Community ; Spies ; Surveillance During the Cold War

Further Readings

Baker, Christopher D. “Tolerance of International Espionage: A Functional Approach.” American University International Law Review, v.19/5 (2003).

Bennett, Richard. Espionage: Spies and Secrets. New York, NY: Random House, 2012.

Crowdy, Terry. The Enemy Within: A History of Spies, Spymasters and Espionage. New York, NY: Osprey, 2006.

Scoville, Herbert, Jr. “Is Espionage Necessary for Our Security?” Foreign Affairs, v.54/3 (1976).

Sulmasy, Glenn and John Yoo. “Counterintuitive: Intelligence Operations and International Law.” Michigan Journal of International Law, v.28 (2006–2007).