In the early 20th century, the US military faced a new challenge with the increased use of radio technology for military operations. Cryptology, the study of codes and how to crack them, emerged as an important tool for military intelligence. It was for this reason that during World War I, the US Army created [PDF] the Cipher Bureau within the Military Intelligence Division to assist with radio intelligence. The bureau was disbanded in 1929, and remained dormant until World War II. It was re-established as the core of the Signal Security Agency, and played a key role in intelligence gathering during the war.

The US military and intelligence agencies transformed in the aftermath of World War II. In 1947, the National Security Act passed [PDF] and created the Central Intelligence Agency and the National Security Council to combat new threats to American security. In 1949, the Armed Forces Security Agency was created within the Department of Defense for the purposes of organizing electronic communication throughout civilian agencies. After a few short years of operation, however, the effectiveness of AFSA came under scrutiny in the Brownell Report. In order to combat the ineffectiveness of the AFSA, President Harry Truman issued [PDF] a memo in 1952 that transformed the AFSA into the National Security Agency. According to Truman’s memo, the NSA was “to provide an effective, unified organization and control of the communications intelligence activities of the United States conducted against foreign governments.”

The Cold War era presented new threats and new opportunities for the NSA to increase intelligence gathering operations. The NSA and its predecessor managed to decipher Soviet codes for a brief period in the late 1940’s, until a mole notified the Soviets about the NSA activity. Soviet codes would not be successfully deciphered again until the 1979 Soviet invasion of Afghanistan. China became another target of NSA activity in 1964, when the Agency deciphered communications regarding the impending detonation of China’s first nuclear weapon. The NSA was also active in the Middle East. The USS Liberty was accidentally attacked [PDF] by Israelis in a joint air and sea assault on June 8, 1967 while on a mission to gather intelligence. A similar incident occurred in January 1968, when North Korean forces captured [PDF] the USS Pueblo during an intel gathering mission off the North Korean coast. Despite drawbacks, by 1970 the NSA had achieved success with deciphering foreign communications in numerous theatres.

The Agency remained relatively unknown to the American public. But during the course of a 1975 US Senate investigation, many Americans learned that not only did the NSA exist, but that it monitored Americans. The Director of the NSA testified that the Agency had monitored the phone calls of anti-war Americans to discover connections to suspected foreign criminals. Because the NSA had some success, the Foreign Intelligence Surveillance Act was passed in 1978. The Act authorized the creation of secret FISA courts to issue warrants for wiretaps when requested.

The mission of the NSA changed dramatically in the aftermath of the September 11, 2001 terrorist attacks. President George W. Bush ordered the use of warrantless wiretaps on American citizens in contact with foreign persons for information about potential terrorist attacks. President Barack Obama has allowed the NSA to continue gathering information about Americans under the Bush-era policies.