Byzantines Reunite the Roman Empire - Then What?

There was a thread a few weeks that asked what the effects would have been had Belisarius not been recalled from Italy and had the plague of Justinian been butterflies and/or delayed. The consensus seemed to be that this would set the Byzantine Empire up for further expansion/consolidation in Italy and Iberia, but I want to take this a bit further. What if those two PoDs, coupled with a lot of luck for the Byzantines in the West, had allowed the empire to fully reach old imperial borders in the West, with the probable exceptions of anything too far to the north like Anglo-Saxon England or Frankish territory like Austrasia, Neustria and Swabia.

So, for at least a brief period of time in the late 500s/early 600s, the Roman Empire has been fully restored. What would happen next? How long could this Empire have lasted before suffering major territorial losses in the West or East? Would the Roman Empire be once more split into a formal Eastern and Western Empire, as was suggested in the thread I linked to? How would the presence of this empire affect the development of states in central and Eastern Europe? Any other effects?
 
I don't see it holding for long. There's simply too many issues coming down the pipeline against the empire that it's going to have to weather against.

Even if you avoid a Gothic war scenario in Italy, it only takes one major fuckup to get a repeat of that against the Visigoths or, god forbid the Franks, except that this time you're much further from the imperial center of power and stretching out your military even more. This will be exacerbated in the early seventh century when the next round of migrating tribes from the north begin to really push against the empire, and there's still the matter of the Sassanids to the east who the empire is still paying a hefty sum to keep quiet. And while Phocas and everything after is likely butterflied, I would still argue that the Byzantines and Sassanids are due for a major knock-down-drag-out brawl eventually considering that hostilities had been increasing for some time. And that's not even getting into if some alt-timeline counterpart to Islam happens to be birthed to the south or not within the next century.

Without a ruinous Gothic war, I would say Constantinople would do well to consolidate its holdings in Italy and Africa and then bunker down for the coming storm. To do otherwise would be to invite a similar catastrophe to what happened anyway.
 
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