I would much prefer to consider a counter-factual history where Patton continued on to destroy Russia, claimed the Middle East and instituted Pro-American capitalist regimes throughout the world. Buchanan is an embarassment to conservatism.
Agreed, but the question here is whether Britain's loss of empire was, more-or-less, inevitable.
The answer is: yes, given history as it actually happened.
But can we imagine a history where that did not happen?
The answer is: yes, according to Buchanan, if Britain had made peace with Germany, then it could keep it's empire forever.
Germany was, after all, all about building empire, and with Britain as their partner, unlike Americans, would never make empires, as such, more difficult to maintain.
Pure hypothetical speculation, but the larger subject is "loss of western civilization" and I'm merely making the point that if by "western civilization" you mean "the British Empire", well then, the alternative choices are not very attractive, regardless of Patrick Buchanan.