"Peter Heather offers an alternate theory of the decline of the Roman Empire in the work The Fall of the Roman Empire (2005). Heather maintains the Roman imperial system with its sometimes violent imperial transitions and problematic communications notwithstanding, was in fairly good shape during the first, second, and part of the third centuries A.D. According to Heather, the first real indication of trouble was the emergence in Iran of the Sassanid Persian empire (226-651)." Heather says:
"The Sassanids were sufficiently powerful and internally cohesive to push back Roman legions from the Euphrates and from much of Armenia and southeast Turkey. Much as modern readers tend to think of the "Huns" as the nemesis of the Roman Empire, for the entire period under discussion it was the Persians who held the attention and concern of Rome and Constantinople. Indeed, 20-25% of the military might of the Roman Army was addressing the Persian threat from the late third century onward ... and upwards of 40% of the troops under the Eastern Emperors."
Heather goes on to state — and he is confirmed by Gibbon and Bury — that it took the Roman Empire about half a century to cope with the Sassanid threat, which it did by stripping the western provincial towns and cities of their regional taxation income. The resulting expansion of military forces in the Middle East was finally successful in stabilizing the frontiers with the Sassanids, but the reduction of real income in the provinces of the Empire led to two trends which, Heather says, had a negative long term impact. Firstly, the incentive for local officials to spend their time and money in the development of local infrastructure disappeared. Public buildings from the 4th century onward tended to be much more modest and funded from central budgets, as the regional taxes had dried up. Secondly, Heather says "the landowning provincial literati now shifted their attention to where the money was ... away from provincial and local politics to the imperial bureaucracies."
"Having set the scene of an Empire stretched militarily by the Sassanid threat, Heather then suggests, using archaeological evidence, that the Germanic tribes on the Empire's northern border had altered in nature since the 1st century AD. Contact with the Empire had increased their material wealth, and that in turn had led to disparities of wealth sufficient to create a ruling class capable of maintaining control over far larger groupings than had previously been possible. Essentially they had become significantly more formidable foes."
"Heather then posits what amounts to a domino theory — namely that pressure on peoples very far away from the Empire could result in sufficient pressure on peoples on the Empire's borders to make them contemplate the risk of full scale immigration to the empire. Thus he links the Gothic invasion of 376 directly to Hunnic movements around the Black Sea in the decade before. In the same way he sees the invasions across the Rhine in 406 as a direct consequence of further Hunnic incursions in Germania; as such he sees the Huns as deeply significant in the fall of the Western Empire long before they themselves became a military threat to the Empire."
"An empire at maximum stretch due to the Sassanids, then, encountered, due to the Hunnic expansion, unprecedented immigration in 376 and 406 by barbarian groupings who had become significantly more politically and militarily capable than in previous eras. Essentially he argues that the external pressures of 376-470 could have brought the Western Empire down at any point in its history."
"His theory is both modern and relevant in that he disputes Gibbon's contention that Christianity and moral decay led to the decline. He also rejects the political infighting of the Empire as a reason, considering it was a systemic recurring factor throughout the Empire's history which, while it might have contributed to an inability to respond to the circumstances of the 5th century, it consequently cannot be blamed for them. Instead he places its origin squarely on outside military factors, starting with the Great Sassanids. Like Bury, he does not believe the fall was inevitable, but rather a series of events which came together to shatter the Empire. He differs from Bury, however, in placing the onset of those events far earlier in the Empire's timeline, with the Sassanid rise."