Lack of military recruits
"An empire acquired by the sword must be maintained by the sword. But even before the frontiers of the Roman empire had been pushed out to their greatest extent, the military spirit that animated the early Romans had become extinct, and all enthusiasm for the military life and the military virtues had been lost. Under the later empire, service in the army grew so unpopular and even odious that many cut off the fingers of the right hand in order to escape military duty. The government was forced to impose severe penalties for such acts. In some cases it even punished such conduct by the infliction of death by burning. Christianity .with its Quaker teachings coming in at just this time contributed also to render more general the disesteem in which the military life was coming to be held. In the earlier period of the empire, any Christian who voluntarily entered the army was cut off from the Church. If any were compelled to enter the legions, such were enjoined to "fight backwardly."6
"The result of this decline in the military spirit among the Romans was, as we have seen, that the recruiting ground of the legions became the barbarian lands outside the empire. The ranks of the army were filled with barbarians ; and able men from among them, like Stilicho and Ricimer,7 usurped as commanders the places once held by the Fabii and the Scipios."
"This loss of the military spirit in a military age, and this transformation in the armies of Rome could of course have no other outcome than such as we have seen to be the issue of it all— the entrance into the army of a non-Roman spirit, and the final overthrow of the imperial government by the revolt of the mutinous legions."
Rome, Its Rise and Fall By Philip Van Ness Myers 1901