lwarmonger

I think your synthesis is a pretty good start, but it doesn't really address the root causes of the fall. From all the historians have been able to determine, the barbarian hordes that invaded the Empire in the 5th Century were very small (although somewhat larger than they had been in the past due to larger and larger tribal federations forming). As for the Sassanids, given the resources available to the respective Empires there really shouldn't have been too great a difficulty in holding them more or less indefinitely. And the borders had been more or less the same length for the past 300 years or so... meaning imperial overstretch was nothing new.

I think every one of your points is valid, but only in conjunction with certain elements of internal decay which many historians have tried to address but most of which you have already debunked.

Personally, I believe the cause for internal decay stemmed from three major sources.:

First, the shift in the 3rd and 4th centuries from an urban society of citizens to an agrarian society of peasants. That reduced the majority of the populations stake in society, and led to an aristocratic landed elite resting atop a mass of easily conquerable peasants... as well as reducing taxable revenues because the landed elite were better able to evade taxes than individual freeholders and urban merchants.

Second was the increasing separation of the military from the population as a whole. With the majority of the Empire's citizens turned into serfs it became increasingly difficult to recruit from people within the empire... which meant that the Empire turned to Germans from outside the Empire or from the frontier areas (Dalmatia, the empire's prime recruitment ground, was nearly the source of a war between the Western minister Stilcho and his Byzantine equivalent as late as 410). That meant that the military, while not suffering from having Germans in it per se(it wasn't until tribes were incorporated into the Roman army wholesale that reliability problems occurred with the Germans acting for themselves), became far less attached to the state. Without the prospect of land or citizenship at the end (since citizenship was automatic after the 3rd century and who wanted to be a serf?) the long service professional army looked only to itself and whomever it could put in charge in exchange for the money that came with coronations and plunder that came with campaigning in civil war.

Third, was the lack of a clear line of succession and the fact that no regime could be completely legitimized. In the Roman Empire might made right... if you commanded an army you could take a shot at the imperial purple, and if you won than you were legitimate... until someone stronger came along. Now granted that this problem pre-dated the Empire, however it became far more pronounced as the population became more separated from the military.