Upvote:-1
Nobody knows for sure since there was no annual budget or even centralized government accounting.
However, it seems that the necessity to pay soldiers in the context of usurpation and civil wars was the deciding factor. During the late IInd century, it had become standard practice to pay a bonus to (loyal) troops on the accession of a new emperor; the so-called donativum. Initially, these payments were relatively modest and also quite infrequent as the regins were relatively long. But as the pace of usurpation started to pick up, these bonuses became nearly yearly events and the amounts also went up as usurpers tried to outbid each other.
In my view, the practice of the donativum and the endless civil wars were part of the process which nearly drove the Roman Empire into the ground: the transformation of the Principate into a Military Dictatorship gone mad. With every general in the Empire being a potential emperor, but also at the same time a potential victim of summary execution within the next 6 months if he chose the wrong side, it is a small wonder that financial discipline collapsed entirely.
Upvote:1
Look into the "Crisis of the Third Century." You'll note that the date given in the Perden lecture is towards the end of this period.
The best answer is probably "both" and "neither."
During the ~ 50 years of the Crisis Rome had 26 Emperors: most of them were generals who established their rule through force. Thus your answer is civil war.
But while the various Emperors killed each other, they neglected the borders, and raids from Sassinads, Goths, Vandals, etc were frequent. The Emperors then needed even more troops to defend the borders. Thus your answer is war for empire.
But the reality is the two are related to each other. Foreign powers raided the Empire because they believed they could get away with it: the Empire appeared divided and distracted. But if an Emperor cannot protect his people from outside forces, he loses the respect of his people and his soldiers, and the likelihood of a coup increases.
So the two effects re-enforce each other - internal discord leads to external attacks leads to internal discord. A simple answer - such as '[Civil or Foreign] Wars forced the Emperors to devalue the currency' is almost certain to be incomplete.