IIRC, weren't the Parthians stopping the spread of the Roman Empire eastward from the Levant (i.e. weren't the Parthians were fighting a defensive war as opposed to taking the battle to Rome proper)?
Actually the Parthians weren't the barbarians. I was conflating the Parthhian wars with the barbarian migrations of the Goths and the rest.The civilizational antagonism was with the Parthians but the barbarians simply swarmed into the empire. The Mohammedans are both roles now.