Plausibility and consequences of a (re)unification between the HRE and the ERE

Hello guys, today I want you to think about the likelihood and the consequences of some kind of union between the eastern Roman empire and the holy Roman empire, precisely I'm thinking about a unification between the 9th and 11th century. How would this new empire look like?
It would be extremely unlikely.
For what matters the IXth to Xth century, Carolingia is far too busy collapsing and dividing itself in kingdoms itself divided in large principalties to really be able to unify with anything, especially ERE.
Note that Carolingian Empire shouldn't be confused with HRE, in spite of the claims of continuity : institutionally and socially, we have two quite different (if not alien to each other, of course) entities.

Ottonian HRE doesn't seems a good option either : Ottonians really had other, more urgent, matters do deal with (namely Hungarians, Slavs and dealing with Late Carolingians in France). When they treated with Byzantines, they generally were at odds for what mattered south Italy. The matrimonial agreement between Otto I and John Tzimiskes was already a diplomatical victory for Ottonians, and it didn't put them anywhere close to uniting with Byzzies.

For what matter Constantinople, unifying with a bunch of semi-barbarous, emperors-wannabee polities far from their actual focuses...

Alcsentre Calanice

Gone Fishin'
Well, if Otto III had lived longer and married a Byzantine princess like his father did, there would have been a slim possibility that he inherited the Byzantine Empire, or at least that he claimed the Byzantine throne. I could even imagine him enforcing this claim, given that he was a German emperor focussing on Italy and knowing the east Roman culture due to his mother's origin.

However, I doubt the result would be a unification of the HRE and Byzantium. To conquer Byzantium, Otto III would have had to stay in the Mediterranean for a long time, maybe for his whole life. Meanwhile the German heartland of the HRE would have been without any ruler, and the local dukes would have done anything to expand their autonomy and undermine the emperor's authority. Sooner or later, Germany would have broken away from the Empire under some usurper, and Otto, concentrating on Italy and Greece, would have been unable to do anything about it.

The result of a successful imperial scheme of the Ottonians would have been a Saxon ruler in Constantinople and a Byzantine Italy, but no reunification of the HRE and the ERE. An empire ranging from the North Sea to Cyprus would just have been indefensible.
Well, if Otto III had lived longer and married a Byzantine princess like his father did, there would have been a slim possibility that he inherited the Byzantine Empire
Theophanu, while definitely a prestigious noblewoman, was in no position to give claims or pretention to the imperial title. Byzantine emperors at this point were particularily cautious to not bring even remotely this kind of situation.
There were talks of sending Theophanu back to her uncle for this precise reason (among others, see below) but Otto knew that was as much he could get.

Note that John I was widely seen both in the empire and outside as an usurper, (due to a mix of military takeover and bedroom promotion, without even a matrimonial union to tie himself with the ruling dynasty) in need of diplomatical and political legitimation.
That was so definitely not the case with Basil II, who had little issue with meddling with Ottonian's dominance in Italy, for instance supporting overthrow of Gregory V as pope in favour of an Italo-Byzantine candidate. Simply said, there was no ground of an union between Otto III and a Byzantine princess even less someone that could give him the remotest claim on Byzantium.

EDIT : at best, you'd need neither Basil or Constantine inheriting the title, and a period of relative dynastic unstability to both prevent Byzantine meddling in Italy and allowing such an union. But, at this point, it wouldn't nearly as relevant and promising politically.
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What @LSCatilina writes is correct. The only viable option I have ever seen is, in fact, Charlemagne marrying Irene and producing an heir (and agreeing to drop Frankish inheritance law and proclaiming that heir the undisputed inheritor of the re-united Empire). Even that is very unlikely, but it could happen and would produce a re-united Empire. Whether that Empire would last very long is, of course, quite another matter!
The only viable option I have ever seen is, in fact, Charlemagne marrying Irene and producing an heir
The whole proposal doesn't looks entierely credible : it's mentioned by Theophanes the Confessors IIRC, in an hostile outlook against Irene. It simply doesn't appears in Carolingian sources.
Assuming this was genuine (and I don't think it was), there's no way Byzantine court would let her do this : if we consider Theophanes' account, she was chased of power as soon as it was out of the bag.

There's a possibility that it was mixed (willingly or not? It might be a case of rumor getting written down too) with the proposed union of Rotrude and Constantine VI which seemed to be workable enough that Rotrude was sent to Constantinople she was taught greek by a Byzantine monk, until the whole thing broke off in 787.
Peventing this matrimonial project's abandon is doable, not especially obvious, but doable : it wouldn't give much in the way of an imperial unification, this being said.

(and agreeing to drop Frankish inheritance law and proclaiming that heir the undisputed inheritor of the re-united Empire)
Until the death of Pepin of Italy in 810, the imperial title of Carolingians wasn't really supposed to outlive Charlemagne. Louis being sole survivor and steeped in a clerical culture putting the stress on a Christian imperium did changed things on this regard.
Note that "Frankish inheritence laws" (which we probably agree shouldn't be confused with actual inheritence customs) were an evolution from late Roman practices of shared/split rulership (as co-imperialship in Constantinople evolved from, even if without the territorial part) that was expected by Latin societies (not just Frankish, but Italian or Aquitain as well, for instance).
It would be hard to get rid of something ingrained in institutional conceptions since centuries in most of Latin Europe : historically it asked for the collapse of Carolingia, and it did survived partly into the custom of giving the leading title to the heir before the death of the king (as in France before Philipp II) or duke/count/etc. (such as in Toulouse until the end).

That Carolingia was, eventually, a relatively weak state overall (even compared to Merovingian Francia in some regards) that was built right from the beggining trough aristocratic takeover and support (Carolingian having put great effort building up first forms of feudalities) doesn't make me things that "dropping" strong institutional customs and pulling a centralized empire would be doable.
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No, I was speaking of a situation in which Otto III marries a Byzantine princess, giving him and his children a claim.
I'm using the exemple of Theophanu to explain to you why a byzantine marriage was certainly not going to give Ottonians a claim, as Otto III didnt have a claim to Constantinople IOTL.
Using close historical exemples to clarify a specific situation, basically.

Byzantines emperors didn't gave away their daughters and their nieces, and especially claims, away easily : Otto I did obtain Theophanu's hand for his son because she wasn't considered able to give or support a claim to the purple. At the very best, any successor to John wouldn't do differently.

Then, John I was in a delicate situation and in need of legitimization, something that Basil wasn't : the odds of Basil giving a random "Byzantine princess" to Ottonians are particularily low.

Even if Basil II is butterflied away by a succession of military/political coups in Constantinople, you'd be in a similar situation than with Otto and John, meaning a marriage with a Byzantine noblewoman removed from claims to the purple, even if part of the imperial family.
And frankly, such an union with a Byzantine Empire ruled trough dynastic hotseat wouldn't have much interest.
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