Rome fell because it devalued its own currency

One of the key fiscal causes of Rome's decline was the systematic debasement of the denarius. Under Emperor Nero (54–68 AD), the silver content of the coin was around 90%; by 268 AD it had fallen to just 5%, with emperors melting down coins and re-striking them with less silver to pay their armies. The result was hyperinflation, loss of public trust, and a collapse of the money economy that forced a return to barter. Modern economists point to Rome as the ancient world's clearest example of inflation caused by monetary expansion divorced from productivity.

Source: Gibbon, The Decline and Fall of the Roman Empire (1776); Goodson, Monetary History of the Roman Empire (2010)