Why did the Parthian Empire often remain on the defensive against Rome?

The Parthian Empire’s tendency to remain on the defensive was largely due to its decentralized, feudal political structure. This internal organization, combined with the constant necessity of defending its borders against hostile forces in both the east and the west, prevented the empire from maintaining a sustained offensive strategy following the reign of Mithradates II.1

While the Parthians were wealthy and controlled vital trade routes between Asia and the Greco-Roman world, they were not an inherently expansionist power. Consequently, the conflicts between Parthia and Rome were typically initiated by Rome, which sought to inherit the territories of Alexander the Great and expand its influence as far as the Euphrates River.1

Although the Parthians successfully repelled Roman incursions—most notably at the Battle of Carrhae in 53 BCE, where their light cavalry and horse archers proved superior to Roman legions in open desert terrain—they rarely capitalized on these victories to launch their own large-scale offensives.123