Constant warfare drained Roman treasuries and led to the continuous devaluation of their coins. For example, the silver content of the denarius coin dropped from more than 80% in the early days of Roman Empire to as low as 8% during the third century. The result was rising inflation as more coinage needed to be exchanged for the same or even fewer goods.
Heavy Taxation:
To generate enough revenues for both its defense and internal governance, the state resorted to heavier taxation of civilians and businesses. Excessive financial burdens weakened productive industries.
Disruption of Trade due to Barbarians Incursions:
Constant attacks by tribes such as the Sarmatians, Vandals, Goths, Franks, and Saxons severely restricted long-distance trade that was so vital to sustaining economic activity both for Roman cities as well as rural areas that largely produced agricultural surpluses for exports.
Disrupting the agricultural base of the Empire's wealth.
Large swathes of farmlands were damaged in the Germanic incursions as barbarian troops repeatedly crossed through these productive lands to gain territory within the Empire as well as loot.