The question is, Whether Rome was conquered by the barbarians in the ordinary sense of the word, conquered.

We know that it was not, and Gibbon knew that it was not.
Yet perhaps most people rise from reading his book with an impression that the empire succumbed to the invasion of the barbarians, as Carthage, Gaul, and Greece had succumbed to the invasion of the Romans; that the struggle lay between classic Rome and outside uncivilised foes; and that after two centuries of hard fighting the latter were victorious.

The fact that the struggle lay between barbarians, who were within and friendly to the empire, and barbarians who were without it, and hostile rather to their more fortunate brethren, than to the empire which employed them, is implicitly involved in Gibbon's narrative, but it is not explicitly brought out.
Romanised Goths, Vandals, and Franks were the defenders, nearly the only defenders, of the empire against other tribes and nations who were not Romanised, and nothing can be more plain than that Gibbon saw this as well as any one since, but he has not set it forth with prominence and clearness.

With his complete mastery of the subject he would have done it admirably, if he had assumed the necessary point of view.
Similarly, with regard to the causes of the fall of the empire.

It is quite evident that he was not at all unconscious of the deep economic and social vices which undermined the great fabric.