The Germanic tribes had a long tradition of warfare as a way of life. The Romans were able to turn this to their own advantage, enlisting the support of conquered barbarians against those still threatening.
Areas of the empire were becoming depopulated and the Romans allowed barbarians to settle as 'foederati' (federates) in these regions, self-governing but obliged by treaty to fight for the Romans. In addition, ambitious young Germanic warriors were encouraged to enlist in the Roman auxiliary forces.
By the 4th century, such barbarian mercenaries made up the bulk of the Roman army, some of them rising to positions of great power, like the Vandal Stilicho who was commander-in-chief in the Western Empire in the late 4th century.
Some attempt has been made to track these mercenaries through their grave goods, as at Lankhills. Often the activities of distinct groups, such as those in Britain of the 5th century king Hengest and his followers, have passed from actual deed into legend.
