Western and Eastern Roman Empire AH Challenge

Create a scenario where the Eastern Roman Empire collapses in the late 5th century and the Western Roman Empire survives until round about the 15-16th century.
 
Fairly easy. Just convince Attila to concentrate the blunt of his forces on Constantinople and after a few sacks by various Germanic tribes, the Eastern Empire has fallen, while the Western Empire stays around in Iberia, Italy, the Western Med, and possibly southern Gaul (I wouldn't expect them to keep all of it).

I can only wonder what the migration patterns would be like in TTL. Franks settling in Greece? Anglo-Saxon Croatia? Vandal Anatolia?
 
I'm not sure it's as easy as all that. It would be fairly trivial to get the eastern empire to collapse in the fifth century or thereabouts, but considerably trickier to get the western empire to survive another thousand years. Economically, the empire relied on its eastern provinces both for its food and its eastern trade routes. Without those regions, the western empire is going to be hard pressed. There's also the cultural aspect: the eastern half of the empire was the cultural and intellectual heartland of the empire. Without it serving as a unifying force, the western empire is going to be considerably different, if it can even survive at all.

On the other hand, if the goal is simply to have a kind of "mirror, mirror"/inverted take on OTL, then we need not be so demanding. Byzantium survived until the fifteenth century, but Byzantium in 1400 was by no means the same entity as it was in 600. I can envision a gradual wasting away of the outlying provinces of the western empire, until it's basically just a medieval Italian state (maybe not even all of Italy), maybe with a handful of outposts in North Africa and on the coast of Iberia. This probably also serves to make it easier for something at least recognizably descended from classical Roman culture to survive, in the same way that Constantinople served as something of an oasis of classical culture (though I'm perhaps being too generous here...) throughout the middle ages.

The problem, though, is that Rome wasn't Constantinople, and barring a very early POD, it's going to lack both the strategic significance and population that was so important to Constantinople's survival in OTL. By the fifth century, the capital of the western empire had already been relocated to a succession of other cities, ultimately settling on Ravenna. But Ravenna was no Constantinople, either, and though it was preferable to Rome, there are probably cities that would have fit the bill even better. Venice is a good possibility.

The butterflies from a fallen eastern empire are interesting, though. Assuming it's defeated and partitioned a la the western half of the empire in OTL, what about the other major or minor powers of the region? Without Rome, western Europe was a pretty sparse place, politically speaking. But the eastern Mediterranean had its share of established powers, most notably Persia. Do the Persians swoop in on the remains of the eastern provinces? Can the barbarian invaders hold them off? And does anything even remotely resembling Islam emerge in the absence of the Byzantine/Persian wars? What, in the end, are the demographics of the east? In the issue of religion alone, it's difficult to be certain: Christian? Pseudo-Muslim? Zoroastrian? Resurgent paganism? Or something else entirely?
 
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