WI: Western and Eastern Achaemenid Empires

In a scenario where Alexander never conquered the Achaemenid Empire and they survived for at least 300 more years, could this surviving Achaemenid Empire go through a Rome-like development, where the core territory falls but the wealthier, new territory becomes the center of the empire? In other words, what if Persia had its own "Byzantium", comprising roughly the same territory as the Eastern Roman Empire, but Persianized instead of Romanized?

Considering Zoroastrian influence (or non-influence) and the prevalence of Greek traders and mercenaries, what would Persia's "Byzantium" be like? Would they end up becoming Egyptian, Aramaic, or Greek, or remain Persian? If they remained Persian, would they reconquer Persia proper from the Parthians (or whoever ends up taking the area)?
 
In a scenario where Alexander never conquered the Achaemenid Empire and they survived for at least 300 more years, could this surviving Achaemenid Empire go through a Rome-like development, where the core territory falls but the wealthier, new territory becomes the center of the empire? In other words, what if Persia had its own "Byzantium", comprising roughly the same territory as the Eastern Roman Empire, but Persianized instead of Romanized?

Considering Zoroastrian influence (or non-influence) and the prevalence of Greek traders and mercenaries, what would Persia's "Byzantium" be like? Would they end up becoming Egyptian, Aramaic, or Greek, or remain Persian? If they remained Persian, would they reconquer Persia proper from the Parthians (or whoever ends up taking the area)?

Considering how important Greek mercenaries had become in the late Achaemenid empire, the Western successor state could well be Greek dominated (but still think of itself as Persian).

Another possibility would be for the Armenians to form the cultural core of such a successor state. The Armenians were highly Persianized during the Achaemenid period (indeed, Armenia was ruled by the Achaemenid dynasty for far longer than Persia itself).

I don't think Egypt would form the core of a Western successor state - more likely a state centered on Egypt would claim to be inheritors of the Pharaohs.

fasquardon
 
The Sassanid Empire was sort of an "Eastern" Achaeamenid Empire.

However, there was no reason at all for an unconquered Achaeminid Empire to ever split.

The Late Roman split was made to deal with the problem of legions on remote frontiers proclaiming new empires. Diocletian's solution was essentially to give every frontier its own Emperor, and the Emperors and Caesars would act in concert as colleagues. This followed from a period where separate Emperors did in fact function on the Rhine and Mesopotamian frontiers, though as break-away movements from the central empire.

The Roman Empire had this problem because constitutionally, it remained a Republic, and the Emperor's constitutional role technically was as commander in chief of the army (that is where the word "Emperor" comes from). So any general could become Emperor if he had the backing of his troops, many did, and more tried.

The Persian Empire was a hereditary monarchy that simply didn't have this problem. When Shahs were deposed, it was via palace coups.

The geography of the Persian empire also favored centralization. It was a land based empire, and armies based in the central Babylon-Susa-Ectbatana triangle could reach any frontier without much difficulty. The ships at the time were just not up to reliably transporting large armies over large distances, so the Roman legions had to be stationed close to the frontiers, even after a more defense in depth strategy was adopted.

In some sense the successor states to the Achaeminid Empire were the Ummayyad and Abbasid Caliphates. The Abbasid Caliphate was split apart just once, in an attempt to resolve a dynastic problem, and it didn't last long. There was sort of a second split at the end of the tenth century when the Fatimids conquered Egypt, Syria, and the Hejaz, but this was obviously not done deliberately and marked the end of the Caliphate as a functioning polity.
 
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