Interesting read.
Here's an excerpt of one of his theories.
when a society becomes a great civilization (and, presumably, the dominant culture in its region), its high point is followed by a period of decay. This means that the next cohesive group that conquers the diminished civilization is, by comparison, a group of barbarians. Once the barbarians solidify their control over the conquered society, however, they become attracted to its more refined aspects, such as literacy and arts, and either assimilate into or appropriate such cultural practices. Then, eventually, the former barbarians will be conquered by a new set of barbarians, who will repeat the process.
He was then, of course, referring to infidels as the barbarians.
I wonder if his theory allowed for role-reversals.