It is customary to regard the Barbarian invasions as the main cause of the fall of Rome, just as a thermonuclear war might one day mark the end of our industrial society. However, though the invasions undoubtedly contributed to the plight of Roman society, they were probably but a minor cause of its collapse.
Let us not forget that the Roman armies had been successfully fighting migrant German tribes since the days of Augustus. Why should they suddenly be overcome by those they came up against in the sixth century?
In fact, Samuel Dill considers the invasions of the third and fourth centuries to have been considerably more formidable, but,
“The invaders, however numerous, are invariably driven back and in a short time there are few traces of their ravages. The truth seems to be that, however terrible the plundering bands might be to the unarmed population, yet in regular battle, the Germans were immensely inferior to the Roman troops.” 
Ammianus, who had borne a part in many of these engagements, also points out that, in spite of the courage of the Germans, their impetuous fury was no match for the steady discipline and coolness of troops under Roman officers. The result of this moral superiority, founded on long tradition, was that the Roman soldier in the third and fourth centuries was ready to face any odds.
It would thus appear that if the invasions of the fifth and sixth centuries were more successful than the previous ones, it was not because of the increased strength of the invaders.