However, you might then wonder what evidence there is that the US is losing the war on terror, and that Islamist radicals will hold the initiative in what looks like a clash of civilisations.
In order for there to be a clash, there must be at least two things in collision.
There is only one civilization.
Maybe an analagy of the Roman Empire and the barbaric European tribes is more apt. By comparison, those tribes weren't too civilized, but Rome was eventually overrun nevertheless by various Goths and Vandals.