The origins of the interstate highway system in defense is well-documented in the historical record, and the title of the bill authorizing it was the "National Interstate and Defense Highways Act":

https://en.wikipedia.org/wiki/Federal_Aid_Highway_Act_of_195...

There were of course other civilian concerns as well, but Eisenhower's arguments at the time of the plan primarily cited the importance of Germany's autobahn system during WW2 as a justification.