Richard Nixon created a paranoid image of America as a "pitiful, helpless giant" if we did not prevail in South Vietnam. That idea was almost entirely military. That is, small countries around the world would pick on our navy, our merchant ships, possibly even our few colonies. They could do so because if a little country like Vietnam could outlast us, then any other country could also.
This paranoia was well accepted by an audience who grew up on military images of Czechoslovakia dismembered, on the dominos falling in Europe before Nazi aggression, and a possible Pearl Harbor style attack with nuclear weapons. But there was a counter-narrative. Even serious conservatives knew that the great Empires of the past had fallen from within.