Question

What does the author’s first use of the key term illusion suggest about American foreign policy?

Answer 1

I believe that the author used the term 'illusion' in the beginning to suggest that The United States could not remain separate. 
The 'illusion of isolation' means that America tried to pretend to be an isolationist continent, meaning that it would have nothing to do with the politics of other countries, however, that turned out to be a lie, especially when it openly entered WWII. 

Answer 2

Answer:

The US could not remain separate.

Explanation:

The passage is saying that the US was kind of fooling itself to believe that it could be detached from world problems.