This is coolbert:
Here with extracts and comments from an article by Ugo Bardi through Lew Rockwell:
HOW the Romans lost their Empire. At least from the perspective of the author.
"PEAK GOLD: How The Romans Lost Their Empire"
By Ugo Bardi
"In this post, I [Ugo] argue that precious metal currency was a fundamental factor that kept together the Roman empire and gave to the Romans their military power. But the Roman mines producing gold and silver peaked in the first century CE and the Romans gradually lost the capability of controlling their resources. In a way, they were doomed by 'peak gold.'”
Currency, coinage of the realm as used to pay soldiers became debased. Gold and silver that tender of the period adulterated, the intrinsic worth and purchasing power much devalued.
"The Roman Empire was so large and so successful because it was, possibly, the mightiest military force of ancient times. The Romans had been so successful at that not because of special military innovations. The recipe for their success was simple: they paid their fighters with precious metal currency. The combined technology of gold mining and coin minting had allowed the Romans to create one of the first standing armies in history."
The number and size of the Roman armies not necessarily declining but the quality and fighting power of the troops markedly less than had been the case in previous centuries.
"With the decline of the precious metal mines, it became more and more difficult for emperors to recruit troops . . . the number of troops was not reduced, but their quality strongly declined."
My understanding has always been that the heavy infantry of the Roman armies no longer were able and fit enough to fight the mounted horsemen and light cavalry of the barbarians.
Adrianople is the type specimen of Roman defeat in this regard. Roman troops when confronted by agile and fast moving barbarian horsemen, enemy light cavalry maneuvering quickly, Roman infantry unable to respond in a dynamic manner. Defeat inevitable and often in a catastrophic manner.
Those contingents of the Roman armies during the late period the percentage of troops citizens of the Empire but actual residents of Rome very small indeed. Loyalty too and devotion to the Eternal City not what it used to be! This of course over and period of many centuries.