Comment by compiler-guy

Although the Rand corporation did contribute some ideas theoretically connected to nuclear survivability (packet switching in particular). All that work was pre-ARPAnet and don’t really motivate the design in that way.

It was designed to handle partial breaks and disconnections though. Wikipedia quotes Charles Herzfeld, ARPA Director at the time as below. And has much ore discussion as to why this belief is false. https://en.wikipedia.org/wiki/ARPANET

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The ARPANET was not started to create a Command and Control System that would survive a nuclear attack, as many now claim. To build such a system was, clearly, a major military need, but it was not ARPA's mission to do this; in fact, we would have been severely criticized had we tried. Rather, the ARPANET came out of our frustration that there were only a limited number of large, powerful research computers in the country, and that many research investigators, who should have access to them, were geographically separated from them.[113]