Rome was engaged in border skirmishes with the tribes north of the great European rivers
Strong emperors occasionally extended the empire over the rivers while weak emperors tended to lose those land
The largest organised rival of the Romans was the Persian Empire to the east, occupying modern Syria, Iran, Iraq, and Afghanistan. The Persians were the political descendants of the Parthians who had revolted away from Greek rule following Alexander's conquests and, thereafter, successfully resisted Roman invasions.