Created by Congress in 1947, the Central Intelligence Agency (CIA) is the primary intelligence agency of the United States government. Its activities range from political and economic analysis to spying and covert operations. By law the CIA is limited to foreign operations; it is not supposed to undertake activities within the US—an operation left to the Federal Bureau of Investigation (FBI). In the mid-1990s, “the Agency” as the CIA is sometimes called, was estimated to have about 17,000 employees—the number is classified.

Public and media interest in the CIA has varied over time, but from its inception the CIA’s very nature has raised perplexing questions about how a clandestine agency and a democracy can coexist. The secrecy that surrounds many CIA operations has also made it a favorite target of conspiracy theorists, a frequent inspiration for suspense novelists and a phobia for the pathologically paranoid.

The CIA came into existence at President Harry Truman’s urging with relatively little notice in the early days of the Cold War, its establishment being just one item in a massive law reorganizing the nation’s defense operations. The CIA’s forerunner was the Office of Strategic Services, which functioned during the Second World War, but the US had never had a permanent agency devoted exclusively to intelligence before the CIA was created.

Throughout the 1950s, the CIA engaged in numerous covert activities overseas that received little public notice and even less criticism. It helped overthrow the leftist prime minister of Iran in 1953, installing the Shah in his place. It also participated in a coup in Guatemala in 1954 to eliminate a reformist government there. Both actions were seen as part of an effort to stymie the Soviet Union.

One of the most successful CIA-sponsored efforts to spy on the Soviet Union itself— fly-overs by U-2 spy planes, which began in 1956—ended up shattering the agency’s sheltered existence when one of the planes was shot down and the pilot captured in 1960.

To make matters worse, Soviet leaders were able to prove that President Dwight Eisenhower had initially lied to the American public about the operation—an early inkling of the credibility crises that were to plague Eisenhower’s successors.

The U-2 affair was followed in 1961 by the far more disastrous Bay of Pigs invasion, an ill-fated CIA-sponsored attack on Cuba designed to overthrow Fidel Castro. The invasion was just the most public debacle in a series of CIA plans to get rid of Castro, which involved everything from fomenting unrest in Cuba to assassinating Castro.

Throughout the 1960s, the CIA was heavily involved in US operations in Vietnam, helping, for example, to overthrow the South Vietnamese government in 1963. But the CIA was also one of the most cautious federal agencies concerning Vietnam, its analyses frequently raising questions about the underlying assumptions of American policy.

In the end, the Vietnam War and the Watergate scandal (in which the CIA had abetted the cover-up) provoked the first detailed public and congressional scrutiny of the CIA. In 1975 special congressional investigations probed and publicized the darker side of CIA activities, including covert operations, assassination attempts and illegal spying on domestic dissidents. As a result, Congress set up permanent committees to oversee the CIA for the first time and put in place clear procedures to ensure that covert operations had presidential approval. In addition, President Gerald Ford issued an executive order prohibiting any federal employee from plotting or carrying out an assassination.

With its morale and prestige in tatters, the CIA limped through the remainder of the 1970s until President Reagan began strengthening the agency. But the CIA’s involvement in the Iran-Contra affair led to another series of charges and investigations as the 1980s drew to a close.

The CIA began to remake itself yet again in the 1990s as the Cold War came to an end—an end that the CIA should have better foreseen, according to the agency’s critics.

But, while putting its house in order, the CIA discovered that it had been the home of several double agents, most notoriously Aldrich Ames, who had revealed to the Soviets the details of fifty-five clandestine operations and the names of thirty-four secret agents.

Ames received a life sentence in 1994.