Overstretching led to a reliance on foreign troops

The argument

From the late second century onwards, Rome's politics were burdened by the need to maintain an enormous military to protect its borders. The burden was a consequence of success: an empire that had expanded continuously had acquired a frontier longer than the tax base behind it could comfortably defend. The military became an enormous tax burden many emperors struggled to pay for, due in part to Rome's worsening economic problems. The overexpansion of the Roman military led to the incorporation of barbarian peoples into the army, since the shortfall was in men as well as in money. Increasingly, foreign foederati were used to supplement the legions, and these Germanic recruits were an undisciplined shadow of the old Roman legion — recruited as units rather than trained into the legionary system that had made Roman armies effective in the first place. The policy was then extended to the very peoples it was meant to guard against. The Romans even tried to solve the problems of a mass invasion by adopting the Goths into their army, a solution that neither gave them control of the Goths nor protection against the armies that followed them. The reason the argument gives is that the Germanic tribes had their own social structures and no loyalty to the Roman state: a force held together by its own leadership and kinship could be paid by Rome without being answerable to it, and would ultimately set up its own kingdoms within the empire. So the massive expansion of the Roman empire led to an over-reliance on foreign troops to police its borders, and the attempt to convert invaders into defenders did not work. Rather than absorbing the threat, it settled armed and independent groups inside the frontier, and helped create many new kingdoms across Europe.

Premises

Counter-arguments

The Eastern Roman Empire would have had many of the same problems as the West in the 4th and 5th centuries. Their military was weak and overstretched, and the Gothic defeat of the Romans at Adrianople happened on their lands. The Eastern Empire did not fall however but became the medieval Byzantine Empire. The dangers presented by a weakened and overstretched military were not inevitable. The Byzantines would purge their armies of Germanic tribes under Leo I (457-474). The Greek East had a stronger economy and made better use of diplomacy to come to good terms with the invaders. These were important factors that could have prevented the fall of the West. Military problems did not prevent the survival of the Greek East. The problems in the Roman army were not the inevitable cause of the fall of Rome.

Rejecting the premises

[Rejecting P1] A long frontier had been a feature of the empire for centuries before the period in question, and the borders were at their most extensive when the army was most effective, so size alone does not identify what changed. [Rejecting P2] The fiscal strain is real but was as much a consequence of civil war, usurpation and the loss of tax-producing provinces as a cause of them — which makes the military burden part of a spiral rather than its origin. [Rejecting P3] The characterisation of the foederati as an undisciplined shadow of the legions is not supported: Germanic recruits had served in Roman armies for generations and rose to the highest commands, and the eastern empire faced the same recruitment pressures, met the Goths at Adrianople on its own soil, and did not fall — which shows the arrangement was not by itself fatal.