Submarines all but guaranteed a retaliatory strike capability, since they were always moving and invisible, and therefore impossible to target. Bombers flew rotating shifts, so that some were airborne all the time, and by the late 1970s the United States was developing "stealth" bombers that would be just as invisible as submarines. Railcars, hidden silos, submarines, and bombers all combined to form a swirling maze of missile platforms, making them very difficult to hit.

Emlen, Douglas J, Animal Weapons_ The Evoluti…, loc. 568