[Jimmy Valentine]

Was this after the Teutoberg Forest disaster?

[pierrem15]

Yes. The Roman response to that disaster was to use quite a bit of money to set up client kingdoms (foederati) among the German tribes to provide some control over the frontier.

The Romans' ability to "manage" the Germans already began to break down during the reign of Marcus Aureleus at the end of the second century as attacks on the Empire by the Persians drained both money and troops from the Rhine and Danube frontiers.

As civil wars and wars with Sassanid Persia ravaged the Empire in the third century, this system began to break down completely and large barbarian raids deep into the Empire took place, which resulted in huge "punishment" raids into Germany when the distracted Romans had the opportunity.