Answer:
The whole of the American continent
Step-by-step explanation:
In the same way (European) kings and clergyman used to reason that their absolute power was a divine right, O´Sullivan makes a similar statement that the (divine) destiny of Americans - meaning those who happen to live in the U.S., therefore, ignoring large parts of the American population - is to own the entire continent.
I remember a little guy in Germany with a moustache who expressed the same thoughts some 80 years ago...