In the reign of Augustus, we are told in the Gospel According to Mark, "there went out a decree from Caesar Augustus that all the world should be taxed." The author of the Gospel was presumably aware that the Principate did not literally cover the world. Augustus, his successors, their advisors and associates, and the Roman people of the time were certainly aware of it. Yet the famous Gospel expression aptly conveyed a sense that, nearly without exception, the Roman system included within its orbit all of the world that really mattered. Once Cleopatra clasped the asp to her bosom, the last of the powers that had previously contested the Mediterranean basin was subjected to Roman rule; all that remained were client states under Roman domination. On its various frontiers, the empire might trail off into the lands of forest tribes, or mountain tribes, or desert tribes that resisted Roman rule. But none of these was "a foreign power" in the modern sense, or in the comparable sense that Romans of an earlier period would have regarded Carthage, or Ptolemaic Egypt, or the other Hellenistic states.
Only one such organized, dynastic foreign power existed that lay outside the orbit of Roman domination, yet impinged on the Roman world and thus posed a potential threat: the Arsacid state of Parthia. While Parthia was by no means a power on a scale comparable to Rome itself, it was qualitatively different in order from any other independent power that existed on the edge of the Roman world. In 53 BC, the Parthians had nearly annihilated a Roman army of 44,000 men under Crassus. On the material level alone this was a disaster twice as great as that of Varus in the Teutoburger forest.
Moreover, while the Germanic tribal confederation that destroyed Varus and forced the Roman withdrawal to the Rhine was dissolved within a few years, the Parthian challenge was permanent. Mark Antony also attempted an invasion of Parthia, but wh...