AHC: Tripartate Roman Empire split - West, Central, East

I'm sure this has been asked before, but I was inspired by this thread (https://www.alternatehistory.com/forum/threads/wi-ahc-sites-for-a-western-roman-empire’s-version-of-constantinople.444795/) to ask if when/how the Roman Empire could have been split into three instead of two, under the following borders:

East, based in Constantinople - Same as OTL: Moesia, Greece, Macedonia, Thrace, Anatolia, Syria, Palestine, Egypt.
Central, based in Milan/Ravenna - The Praetorian Prefecture of Italia plus the Diocese of Pannonia
West, based in Augusta Treverorum - Same as OTL: Gaul, Britain, Spain and Mauretania.

Any thoughts? My first thought was one of Thedosius I's sons by his second wife Galla (Gratian or John) surviving. But perhaps that would be too late. Maybe a POD with Valentinian I, Valens and their sons?
 
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Toraach

Banned
Constantine I had three sons, who roughly divided the Empire in this style. Well, they might survive and this division might last longer.
 
Constantine I had three sons, who roughly divided the Empire in this style. Well, they might survive and this division might last longer.

So (at risk of derailing the thread), in that situation, might we see a Catholic Christian West and an Arian Christian East?
 
This was my last take on a 3way split:
A little history.

Despite the alternate Christianity - Soterism or Salvationism as some call it - being accepted by the Roman emperors earlier than OTL the same crises hit and similar Palmyrene and Gallic empires separate before being readded in.
The Emperor who does so combines the talents of Aurelian and Dicoletian, he recognises that the Imperium is too large for one man and a measure of devolution needed so he sets up the Triarchy as a unified workable triumvirate: Asian and Egyptian provinces go under the Eastern Emperor; Albion, Gallic, and Hispanic provinces come under the Western Emperor; the remainder comes under the Central Emperor who is also senior and labelled Augustus. A West to East to Central succession is set up and despite the usual shenanigans is durable enough to become tradition. Unfortunately the Western Emperors while getting excellent training in Generalship against Germanic raids have to delegate a lot of their authority just to maintain status quo; they make good Eastern Emperors though.
Then the Huns hit.
As OTL the current Augustus utilises the Goths as foederates; he sets up a "Wall of Goths" along the Danube - the Visigoths aren't hustled out of Thrace TTL - that proves successful enough for a Germanic one along the Rhine. The Germanic Wall proves less successful and is particularly porous in the north. Stuff hits the revolving thing and the "Germanic Wall" crumbles; a group of mostly Burgundians "rampage" through Italy before settling in Africa; the Western Emperor's authority is shattered and it's left to the Augustus to clear things up. The Vandals, among minor others, get set up as a new improved wall along the Loire; the Burgundian King is accepted as Dux of Africa . Albion is left more isolated and the Dux there is effectively autonomous. Appointment of Western Emperors is no longer continuous and the Western Duces become highly autonomous and largely offices held by barbarian Kings as "protectorates".
The Imperial Succession is no longer effective and begins to strain the relationship between East and Central. The Lombard penetration of the Gothic Wall into Greece followed by Slav incursions during an ineffective Augustus leads to brief civil war and the Eastern Emperor taking over as Augustus leaving a succession of junior Emperors in Italy.
 
I've made a map - here, with frontier regions colored brighter than less vulnerable regions - illustrating what I personally view as the ideal division of the Roman Empire, which is technically a tripartite split though with different borders than OP's version. I wouldn't have it be divided into de facto separate empires as IRL, with different Emperors ruling over culturally distinct regions, since that's just asking for them to split and become rivals. Instead, the Emperor rules the Central part directly while delegating the West and the East to military prefects. If some sort of formal succession-by-appointment can be introduced, then these prefects should compete for the position of heir, ensuring at least one - the likely heir - would (probably) be loyal to the Emperor if the other revolts. I would not make either prefect formally superior to the other, however, since that would lessen the competition for imperial favor. It also allows for a Western prefect to be the preferred heir when experience against barbarians is more necessary, or an Eastern prefect when the Persians are the bigger threat.

I basically agree on the Western portion; Gaul, Hispania, Mauretania, and Britain. I don't view Britain as a worthwhile conquest, so I'd only want to keep the minimum necessary to have a staging area to disrupt any unification of the rest. Even that's not all too important, as long as the tin islands are secure. The Western prefect is tasked with guarding the Rhine and neutralizing any threats in Britain.

The Eastern portion I would shrink heavily to basically Aegyptus and the Diocese of the Orient. Possibly add the rest of Anatolia here instead of in the Center, if the defenses of the Orient needs more funding, but I don't think that would be necessary. The Eastern prefect is tasked with defending against Persian and Arab raids and holding the line in a full-fledged war.

The Central portion would consist of the rest, dedicated to the Danube but also to reinforcing the other two in the event of a major incursion. This also ensures Illyria is not split and so is more easily defended.

The pink indicates areas of strategic importance that are nevertheless outside the ideal defensive borders of the Roman Empire. In the north, this could lead to the establishment of an also-effective Elbe-Carpathians-Dneister border, while in the east it's mostly just denial of Mesopotamia to enemy empires based out of Persia. Whether that denial involves conquering Mesopotamia directly or simply keeping it apart from Persia is not as important, though obviously the former is preferable.

The Empire would still need a lot of other reforms beyond territorial division to survive, of course, and I wouldn't have high hopes of it succeeding with those reforms but I still like these borders nevertheless.
 
Crisis of the Third Century did something close to this, but the "West" (the Brittanic Empire) was only limited to Brittania, Gaul, and northern Hispania before "central" took back Gaul for the first 1/4 of the split and then finally Brittania itself came back into the fold.
 
But perhaps that would be too late. Maybe a POD with Valentinian I, Valens and their sons?
Here are three suggestions with your POD:
1) Valentinianus Galates doesn't die, while his father manages better the gothic threat. Both Valentinianus I and Valens live until their sons are able to rule without any regent like Arbogastes. Gratianus rules over the Praefecture of Italy and the Diocesi of Illyricum, Valentinianus II rules over the Praefecture of Gaul while their cousin rules the East and what is left of Illyricum.
2) Magnus Maximus doesn't invade Italy and Theodosius is to busy to face him, Gaul is ruled by Maximus, Valentinianus rules in Italy, the Theodosians remain in the East.
3) Eugenius wins at Frigidus and remains sole ruler in the west while Theodosius dies either in battle or shortly after (historically he died while celebrating his victory in Milan a few months later). Both Arcadius and Honorius ascend to the throne, one in the Balkans and the other in Asia. A different alternative to this could be the rise of a new usurper in the East, revolting against the young Arcadius.
 
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