The Advanced Research Projects Agency network, abbreviated to ARPAnet, was the first system able to connect computers in geographically diverse locations.
Originally intended as a network to connect research universities (as seen on the map above), ARPAnet soon became a centerpiece of military intelligence. Coast to coast transmission of classified information quickly moved onto the ARPAnet, with the program becoming more readily accessible through the 1970s. As the military became more invested in the technology, improvements began to focus on the ruggedness and reliability of the network. By 1975, the military saw the ARPAnet as the main tool of communication in the event of a nuclear attack.
As the 1980s came to a close, ARPAnet fell away to more advanced technologies. However, the IP (Internet Protocol) technology that was originally developed for ARPAnet remains in use today.
SAGE
SAGE, or Semi-Automatic Ground Environment, was a system of computers that coordinated data from hundreds of radar sites to produce a single unified image of the airspace over a wide area. The system, consisting of just over twenty separate cubic-acre housing structures, was able to track and calculate flight patterns of enemy aircraft. Adjusted for inflation, these computers cost $67 billion to construct, three times the cost of the Manhattan Project.
Although mainly used for intelligence purposes, the system was able to guide defensive missiles to intercept enemy aircraft. In the event of a nuclear war, SAGE also had several autonomous nuclear warheads at its disposal.