Piffle. This is precisely the argument used by leftists to denegrate the Founders of the Republic (they stole the land from the natives and some of them owned slaves; therefore, they were no-good Dead White Males).
Buchanan's point is a legitimate one to be considered. If American culture seems decadent to the extreme to some of us that live here, how must it seem to the bass-ackwards folks we're trying to sell it to? Insinuating that Buchanan is now a pro-terrorist anti-American indicates more of a personal resentment of Buchanan for ditching the republicans more than it is a reasoned response to the article.
Buchanan is not saying that America should not be defended. He is saying that we should not go to war to impose such defective "values" as abortion on demand on the rest of the world.