How ‘bout this? Within a generation of the Marian reforms, the Roman Army invaded Rome at least twice [I think three times]. The Roman Army, even with good emperors had to be bought. It both held the state together, and, increasingly, tore it apart, with various armies and legions proclaiming [for a good payday] their general as ‘Caesar’.
And by the late Empire, Romans were not stepping up to serve in the Roman Army. Increasingly, barbarians were, sometimes in whole tribal units [ Alaric was a Roman general, and Chief of the Visigoths]. This had three serious effects for the Western Roman Empire. First, the barbarians learned the Roman military system. Second, the same group that defended the Empire was best positioned to bring it down. Third, they had learned the Roman Army’s tradition of interfering in the government of Rome for personal benefit, and of being separate, both in allegiance and outlook, to the society they were to defend. And when the politicians didn’t pony up the lands and other ‘gifts’ on their lists, the tribes took matters into their own hands.
What you said!