See, that gets a lot of play but it isn't the driving reason. Here is his letter to Congress, the one military mention is the need to evacuate cities in the event of atomic bomb attack. That would seem indicative of wanting roads into cities.Ike focused on military logistics. One of the purposes of Ike’s road system was internal movement of military units. The PR signs posted throughout the construction areas included references to national defense highways. The Act authorizing construction also noted defense purposes. In one of Ike’s books, he wrote about his 1919 experience and the difficulties in the coast to coast convoy he commanded. Thus, Ike intended an inter-urban system.
When President Dwight D. Eisenhower took office in January 1953, however, the states had only completed 6,500 miles of the system improvements. Eisenhower had first realized the value of good highways in 1919, when he participated in the U.S. Army's first transcontinental motor convoy from Washington, DC, to San Francisco. Again, during World War II, Eisenhower saw the German advantage that resulted from their autobahn highway network, and he also noted the enhanced mobility of the Allies, on those same highways, when they fought their way into Germany. These experiences significantly shaped Eisenhower's views on highways and their role in national defense. During his State of the Union Address on January 7, 1954, Eisenhower made it clear that he was ready to turn his attention to the nation's highway problems. He considered it important to "protect the vital interest of every citizen in a safe and adequate highway system."
If the old saw that the military was why, I'm not sure why it isn't mentioned at all. It certainly would have sold the concept as congress wasn't going to turn down a "we must have this to defeat communism" request from Ike.