The modern instinct is to seek a reason for everything, Sir John Glubb says, and to doubt the veracity of a statement for which a reason cannot be found:
So many examples can be given of the sudden eruption of an obscure race into a nation of conquerors that the truth of the phenomenon cannot be held to be doubtful. To assign a cause is more difficult. Perhaps the easiest explanation is to assume that the poor and obscure race is tempted by the wealth of the ancient civilisation, and there would undoubtedly appear to be an element of greed for loot in barbarian invasions.
Such a motivation may be divided into two classes. The first is mere loot, plunder and rape, as, for example, in the case of Attila and the Huns, who ravaged a great part of Europe from A.D. 450 to 453. However, when Attila died in the latter year, his empire fell apart and his tribes returned to Eastern Europe.
Many of the barbarians who founded dynasties in Western Europe on the ruins of the Roman Empire, however, did so out of admiration for Roman civilisation, and themselves aspired to become Romans.