This is a good amount of info for people who are not familiar with the subject. To illustrate just one part of the pre-ARPANET hardened communications networks,

This is a good amount of info for people who are not familiar with the subject. To illustrate just one part of the pre-ARPANET hardened communications networks, I'll focus on a few typical usage scenarios. The DOD for the most part did not build their own domestic communications networks spanning multiple states and across the country. A lot of it was carried by the AT&T Long Lines network, which had a mix of barely-hardened, not hardened, and some extremely hardened sites (L4 coaxial, project offices, etc). If one finds, for instance, the location of all of the SAGE direction centers and all of the telecommunications links that fed the remote radar sites' data to the SAGE direction centers, you'll also find a ton of AT&T Long Lines mountain top, hilltop sites, mixed in with the ordinary central offices located in mid sized to large sized cities. In other places in the world the US DOD built its own bespoke communications networks for very specific command and control purposes. At one point in time there was a massive troposcatter network spanning nearly the entire Mediterranean sea, with important radar sites in Italy, Cyprus, linked to air bases and the DOD's backbone communications links from europe to the USA. Similar massive troposcatter data links were built through the Aleutian islands. Most of these were replaced by C-band geostationary satellite by the mid 1970s as that became a higher performance and more viable technology than the extremely massive, power hungry and expensive to maintain troposcatter stuff.