This article covers an important historical precursor to contemporary NSA surveillance: Project SHAMROCK, the name for the NSA’s interception of telegrams passing over US soil between 1945 and 1975.
Three telegraph companies handed over tapes of their telegram data to the Fort Meade spy agency. This wholesale “upstream surveillance” was then processed and passed on to other law enforcement agencies, such as the FBI, under Project MINERVA. Civil rights activists in particular were targeted and placed on an extensive “watch list.” This was the time of J. Edgar Hoover’s FBI after all, during which time “national threats” could be little more than civil disturbances.
The pivotal Senate Church Committee report can be found here, and it provides an impressive amount of detail and context on the first instance of mass (electronic) surveillance in U.S. history.