From its inception, the United States has engaged in espionage and intelligence-gathering activities in order to defend against its enemies. Intelligence gathering was initially a function of the Department of Defense, which has several agencies under its umbrella: the Defense Intelligence Agency, the intelligence agencies of each arm of the military, the highly secret National Security Agency, the National Reconnaissance Office, and the National Imagery and Mapping Agency. Over the years other executive departments have also created intelligence divisions. The Department of Energy, for example, has an intelligence arm to protect nuclear secrets. The Department of State and the Department of the Treasury also conduct intelligence to safeguard national interests under their protection.

Probably the most well known of the departmental intelligence agencies is the FBI, which is part of the Department of Justice. The FBI was initially a law enforcement agency. Prior to World War II, the...