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Easy - Preserve the Roman Empire.
Or at the very least, Roman control of Egypt. There was a compelling argument I've read on the forum somewhere that insisted that there was a Roman identity throughout Europe, at least in the ERE and former WRE territories, that was only destroyed by the invasion of the Caliphate, and the rise of Charlemagne
There are alternatives suhc as the Islamic Caliphate conquering Europe, or a Mega-HRE. (One my favorites is a long-lived Macedonian Empire that conquers Europe).
But the most likely IMO, is in some way, shape, or form, preserve, or maintain a pan-European Roman Empire. It doesn't matter how you do it - it could be the perfect combination of using the Moors and the Berbers to be foederati against the Germanic ones, the heavy plow coming into northern Europe VASTLY earlier to increase the population and development of Germania to make it more worthwhile to conquer, wipe the Huns out in a plague, have Theodosius live longer, have constitutional reform/reform of the succession to help stabilise the Empire, have Islam be a Christian sect and find war against the Christian Romans aberrant and only invade Persia, have Justinian and Belisarius have more trust and some good luck, prevent the Plague of Justinian by having highly improved sanitation throughout the Empire, have a plausible invention of the stirrup, sort out the tax and enlistment issues in the West, have Majorian successfully invade North Africa.
Whatever it is, have it reach the point that Europe sees itself as Roman - which will inevitably require a Roman Germania, Marcomannia, Pannonia, and Hibernia to complete the conquests. Long term stability and significant prosperity, and you have Roman Germans, and Roman Italians, Roman Greeks, and Roman Moors(? I'm not sure about this one), etc, etc.
Have a link to the argument.
Even with a surviving Rome I am not sure that this is possible. Rome would need to adopt a more inclusive mindset in regards to integrating large swaths of the empire to Roman culture. The reason China has what it does is because it is freaking old and has shared cultural values and traditions even across it's many ethnic groups. Roman government, religion and culture doesn't lend itself to centralization and cultural establishment the same way Confucianism does.
You mean the Roman Empire that specialised in dispersing other cultural groups and settling them down to individual families across the Empire to overwhelm them in Roman and Greco-Roman culture? That Rome?
That Rome that prior to Christianity just shrugged and may have well have hung a sign saying that "Oh, that God is one of ours too, another aspect of one of our Gods". Even afterwards, the Church and Emperor did try and create cultural orthodoxy, as it was convenient.
Considering that many of the Empires rulers weren't 'Roman' as per the city, but from all over the Empire, that their armies were designed to integrate non-citizens into the citizenry, I'm not sure WHICH Roman Empire you're talking about - because IOTL the Romans worked very hard to Romanize their Empire. It is why we have "Romance" languages!
A European style mandate of Heave would be nice, could it be possible with a Christian Europe?
Literally Christianity was what kept Europe together against all "foreign incursions" - until the bad bad Protestants came along, of course. There's a reason why so many cities wished to be called "a nth Rome".
The empire that was so bad at centralized government it repeatedly split itself into pieces? The empire that constantly had to fight off rebellions due to a lack of Roman identity? The empire that had such a chaotic government that it could never found a true dynasties like China? The empire that changed religions on a dime because it had so little established culture of it's own?
The empire that was so bad at centralized government it repeatedly split itself into pieces? The empire that constantly had to fight off rebellions due to a lack of Roman identity? The empire that had such a chaotic government that it could never found a true dynasties like China? The empire that changed religions on a dime because it had so little established culture of it's own?
This was a problem in China too, right up to modern day if you look at the mainland/Taiwan split.