It became normal practice in the 4th century AD for the Roman authorities to raise mercenary troops from the Germanic tribes based outside the Empire the to help fight off attacks from other Germanic tribes, and even to invite certain tribes to settle within the borders of the Empire at strategic locations. These mercenary troops were named foederati by the Romans, as they were hired by means of a treaty or foedus, which is roughly translated into the English as federates.

It was a practice essentially forced on the Romans as a result of the difficulties experienced in raising troops within the Empire. A difficulty that became more acute after the disaster at the Battle of Adrianople.

Of course the Imperial Roman Army had always used barbarian mercenary troops but previously only in the form of auxiliary troops that where attached to specific legions and under the control of Roman commanders. (And often limited to certain specific tasks, like the Batavians who specialised in river crossings.)

The foederati were much larger groups that remained under the command of their own leaders and retained their own customs and laws and often resembled nothing less than mobile kingdoms.

The use of such foederati encouraged the barbarian penetration of the Roman Empire and ultimately was one of the factors that led to the fall of Rome. When the Empire still retained a measure of authority the foederati could be controlled, but once the centre weakened they were well placed to turn on their former masters.

On the other hand, it also had the effect of Romanising these very foederati, who developed a knowledge of and appreciation for the Roman system of administration. Hence the end of the Western Roman Empire became not so much a matter of its conquest by external forces, but rather more a process of transition and fragmentation, and for most of its inhabitants became simply an exchange of one set of rulers for another.

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