Robert Kagan is not wrong about many points, but it is worth noting that he is one of the leading neocons, husband of Nuland, and architect of PNAC:
https://en.wikipedia.org/wiki/Project_for_the_New_American_C...
The think tank (Brookings Institute) which he is part of laid out the Iran strategy "Path to Persia" something like 15 years ago.
I think he does recognize that the US needs Europe for projecting power, so that part is genuine. The US could not prosper against a hypothetical Eurasian union.
Not sure what to make of this. Maybe it is supposed to appease the EU (you still have allies in Washington), maybe it is a wakeup call for the deep state to pull the brakes.