Cool title, huh? It always feels good to type this kind of stuff. "Empire". Pronounced with a 1900s British accent. Feels good man. Insert happy frog pic.
Anyway. The most interesting, shall I say "official" theory of historical geopolitics of the reaction must surely be Peter Turchin's theory of meta-ethnic frontier armies pumping up their asabiya and conquering the civilizational center.
The theory basically says that to run a civilization you need a strong army. To run a strong army you need cohesion, discipline, i.e. asabiya. To produce this cohesion and discipline you need your soldiers to feel its need. Discipline isn't nice. You'd rather slack off and drink beer and be merry. The kind of discipline an army runs off is produced by massive amounts of violence and unreasonable demands. You can only get people to do so if they feel is absolutely necessary. And they will only feel it's necessary if they get to the realization that either they behave like good soldiers, or they're dead, and they will lose everything they hold dear.
The way you get your soldiers to feel that is to stack them against a different civilization. People so alien to you that they you have nothing in common. If they win, everything you are accustomed to, all your life, all those little habits of behavior that form your identity: all that will be destroyed. And you don't like that. It's taken a while for you to adapt to that culture. Starting at birth. Your brain unconsciously...