The Central Intelligence Agency was created on September 18th, 1947, following the passage of the National Security Act of 1947. It succeeded the wartime Office of Strategic Services, which had been disbanded at the end of the war, only to be replaced by an intelligence force divided between the War Department and State Department. This divided system was quickly abandoned and replaced with the Central Intelligence Group, the CIA's immediate predecessor. In 1947 the current incarnation of the CIA was formed.
In 1948 this new intelligence agency was given authority to carry out covert intelligence operations in friendly and hostile countries. In 1949, the CIA was granted freedom from having to report on its activities and from the usual budgetary restrictions. The new agency gained stability in 1952, when the Directorate of Plans was created within the CIA to coordinate human intelligence (HUMINT). Also during this time the US Army Special Forces was created; the missions of the two new groups occasionally overlapped, and they sometimes shared resources.
During the Cold War, the CIA was mainly involved in covert operations against communist groups both within America and without. In 1953, Operation Ajax marked the first time they became involved in the overthrow of a democratic government that was in some way an enemy of the United States. They were also involved in the overthrow of Cuba's Fulgencio Batista and, later, the repeated assassination attempts on Batista's replacement, Fidel Castro. In 1954 the CIA became involved in Vietnamese affairs as part of the US government's policy of communist containment. Mainly they acted in an analytic and advisory capacity, attempting to project the future balance of power. They remained involved in Vietnam until the American withdrawal in the mid-1970s.
The mid-1970s also marked a trying time for the CIA, as public revelations about the less savoury aspects of their involvement in world affairs began to surface, including their involvement in assassinations or attempts at such, illegal and uninformed experimentation, and domestic spying operations conducted against American citizens. This public distrust was only heightened by the Watergate scandal, in which ex-CIA agents were involved in an attempted burglary at the Watergate Hotel. 1975 saw the passage of an executive order that forbid CIA personnel from assassinating any foreign leaders, and the government at the time, lead by President Gerald Ford, distanced itself from the previous actions of the CIA. In 1991 a further act, the Intelligence Authorization Act, was passed, defining covert operations and bringing the previously secretive and unaccountable CIA into account before the president.
The CIA received increasing criticism after the 9/11 attacks. In 2004 the commission on the attacks lay some of the blame at the feet of the CIA, along with the FBI, who were determined to have been remiss in their duties. This led to the creation of the position of Director of National Intelligence, which oversees the actions of all of the members of the US intelligence community, including the CIA. The CIA is currently involved in some capacity in the War on Terror and the War on Drugs, among other covert or intelligence-related operations.
Currently, the CIA is headquartered in Langley, Virginia. It contains a number of directorates and offices dedicated to specific elements of intelligence, such as the Directorate of Science and Technology, or the Office of Middle East and North Africa Analysis. The current number of CIA employees, like the CIA budget, is classified, but estimated at about 20,000 people.