The Romans did not see the boundaries of the Empire as delineated solely by border posts and walls. What was important were the limits of Roman culture and influence, and this extended far beyond their obvious borders. Roman military defensive doctrine also relied on extending their effectiveness beyond their military fortifications into the hinterland beyond. Almost every border had a client state beyond to act as a buffer between Rome and their real enemies. These client states varied in their degree of Romanisation and in their liking for Rome, but all were heavily influenced and, to a degree, controlled by Rome through trade, bribery and threat.
You may also be interested in our Enemies of Rome.
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