"A great scholar of Gaul and of the "barbarian" side of the Rhine frontier - John Drinkwater - has recently provided a cogent answer. He argues that emperor, military, and civilian populations alike needed the idea of a "barbarian threat" to justify their own existence. The threat of invasion justified high rates of taxation. It justified the splendid palaces and cities ringed with high walls which overlooked the Rhine and the Danube, from the North Sea to the Black Sea. It gave a raison d'etre to a powerful and well-paid military class."