Also your got the additional problem that the empire and neighbouring lands are overwhelmingly Christian by now. That means that the lands of the former empire are riven by deep and probably unbridgeable divisions. This makes it far easier for groups who want to stay outside the emperor's control to do so. During the period of Roman expansion that was not the case and religion was far less of a barrier to such expansion.
Steve
I am not sure I would agree that Religion is a factor preventing the reconquest of the Western Empire.
That being said, I think we need to keep in mind that the most remarkable fact about the Roman Empire is that it retained so much of its territory for as long as it did. It simply was too large to remain stable. A reconquest of the Western Empire would not have changed that basic fact.
As far as I can tell, the Empire could never be really stable. It either had to be growing or it was in a state of decay. When it was growing, strong ambitious generals had an outlet for their ambitions. When it was in a state of decay the generals turned their ambition to the throne. In a world where it would take months to get from one end of the empire to the other, it was simply impossible to react to all the threats that were occurring.
When the Byzantine Empire had to face the challenge of Islam it contracted fairly quickly to a state that was a fraction of its size under Justinian and then managed to stay fairly stable for several centuries (and it may have persisted if not for the 4th Crusade).
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Bill