Plausibility check: Pan-Orthodox Empire after merging the ERE and Russia

If the ERE could have managed to survive a bit longer or Russia would have unified earlier, is it possible that both could have merged in a sort of pan-Orthodox Empire (with capital in Constantinople)?
 
I think it depends on when the union is occuring. If it is happening around 1700 I don't see Russia joining with byzantium unless there is something to gain from the union.
 
If it ever where to happen, it would probably be completely unwieldy, it also depends on the time. If it's Pre-Mongol Invasion Russia you would have a bureaucratic state with a northern area with feudal dynastic states. I still think it would never happen.
 
If Russia is strong and the ERE is weak, the latter is definitely getting subsumed within Russia, especially if the latter owns Constantinople/the Dardanelles.

If Russia is strong and the ERE is strong, they are not going to unite and will likely go to war with each other. It's not like the Rus' have never attacked Byzantium before, and Russia's quest for warm water ports will almost-inevitably bring it into conflict with nations to its south.

If the ERE is strong and Russia is weak, nothing much happens. I can't see Byzantium wanting a vast, unruly 'wasteland' full of barbarians in their Empire, especially as it means bringing more Russian noble families (and thus contenders for the throne) into Constantinople.
 
If Russia is strong and the ERE is weak, the latter is definitely getting subsumed within Russia, especially if the latter owns Constantinople/the Dardanelles.

If Russia is strong and the ERE is strong, they are not going to unite and will likely go to war with each other. It's not like the Rus' have never attacked Byzantium before, and Russia's quest for warm water ports will almost-inevitably bring it into conflict with nations to its south.

If the ERE is strong and Russia is weak, nothing much happens. I can't see Byzantium wanting a vast, unruly 'wasteland' full of barbarians in their Empire, especially as it means bringing more Russian noble families (and thus contenders for the throne) into Constantinople.

Well, my idea was basically this: Russia is a rising power while Byzantium is declining, so the union would be based mostly on the need of Byzantines of Russian protection against Turks and other powers, not by pure will.
As you said, other scenarios of union are unlikely.

A declining Byzantium that would have realized that its own existence was in real danger because of i.e. the Turk expansion could have requested Russian protection in order to save themselves, even if finally puppetized or absorbed by Russia. Obviously Russians would have accepted as it would grant access to the Aegean and control over Constantinople.
 
Well, my idea was basically this: Russia is a rising power while Byzantium is declining, so the union would be based mostly on the need of Byzantines of Russian protection against Turks and other powers, not by pure will.
As you said, other scenarios of union are unlikely.

A declining Byzantium that would have realized that its own existence was in real danger because of i.e. the Turk expansion could have requested Russian protection in order to save themselves, even if finally puppetized or absorbed by Russia. Obviously Russians would have accepted as it would grant access to the Aegean and control over Constantinople.

You can imagine something without any Mongol invasion when a strong Kievan state with a heavy influence on the Black Sea and strong ties to the ERE can help the ERE in the 1200's...

You need a Kievan state which expand down the Dniepr river and settle the Black Sea coast, and east to the Dniepr to border the Georgian and Armenian states in the Caucasus...
 
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