The more powerful the U.S. became, the more fearful Americans became as a result.
I disagree that Americans have some innate fear of the rest of the world. Where is the polling that shows a majority of Americans actually fear an attack from North Korea, Iran, Vietnam, Iraq, etc.? Americans are told by our ruling elites that we are threatened and so we, who are lead by the nose by the MSM, willingly go along with the wars, the dronings, the coups, and the sanctions.
When it comes to foreign policy there is no democracy in America. That is why our elites never allow Congress to vote on a declaration of war before proceeding, and they proceed a lot.
The citizens have to be kept insecure, fearful and on edge, lest they ask why it is necessary to surrender what remains of their freedoms or start to clamor for reforms.
"We don't have time for that now! Don't you know we gotta fight Saddam/Bin Laden/The Taliban/Saddam again/Ghadaffi/Assad/Putin/Kim/Xi etc. ad nauseam?"
Fear is how all governments manipulate the populace, at least the weak-minded ones. And the people in the mushy middle are the ones who will cast the deciding votes.
The thought of evil recoiling upon oneself can make one very paranoid.