ABSTRACT
Not until the Marcomannic Wars are there indications of a more proactive approach to frontier defence. Attacks by various Germanic tribes first on the Rhine and then on the Danube frontiers in the 160s provoked major imperial campaigns to drive out the invaders and then repair the damage caused. It could not yet have been known to the Romans that these invasions were but the first ripples of a tidal wave of pressure from the East which would continue intermittently over the next three centuries and would eventually sweep away the western part of the empire. Nevertheless, in the immediate aftermath of the incursions we begin to see structures put into place on the frontiers which would allow earlier and better warning of any threat which might materialize. The new structures first appeared on the German and the Danube frontiers, as one might expect, but they were soon imitated in other parts of the empire, though by no means all of them.
