In 1984 it was discovered that the Soviet Union was spying on communications from US embassies. It was previously believed they only had audio bugs which could be swept for. However, the GUNMAN project revealed a remarkable new form of digital surveillance that used bugged typewriters to intercept plain text communications (typed on physical paper). They later found out this was in practice for the last 7 years.
The impact of the discovery was far reaching. The NSA became an important agency, developing anti-tamper devices. New groups formed to create offensive capabilities. Some would alter argue that this was the catalyst cyber-warfare and a digital arms race.
Read Learning from the Enemy by the NSA.