> of fighting age men
https://www.history.com/news/8-reasons-why-rome-fell
The Praetorian Guard—the emperor’s personal bodyguards—assassinated and installed new sovereigns at will, and once even auctioned the spot off to the highest bidder. The political rot also extended to the Roman Senate, which failed to temper the excesses of the emperors due to its own widespread corruption and incompetence. As the situation worsened, civic pride waned and many Roman citizens lost trust in their leadership.
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Weakening of the Roman legions
For most of its history, Rome’s military was the envy of the ancient world. But during the decline, the makeup of the once mighty legions began to change. Unable to recruit enough soldiers from the Roman citizenry, emperors like Diocletian and Constantine began hiring foreign mercenaries to prop up their armies. The ranks of the legions eventually swelled with Germanic Goths and other barbarians, so much so that Romans began using the Latin word “barbarus” in place of “soldier.”
While these Germanic soldiers of fortune proved to be fierce warriors, they also had little or no loyalty to the empire, and their power-hungry officers often turned against their Roman employers. In fact, many of the barbarians who sacked the city of Rome and brought down the Western Empire had earned their military stripes while serving in the Roman legions.