I have long ago stopped attributing the success of a people or nation in war as due to their greatness as individuals. In other words, that the Persians repeatedly defeated Romans throughout history is not due to an inherent superiority as a people, just as Caesar’s success in Gaul is not due to the inherent greatness of his legionaries in being Roman. Cue the ideas of social Darwinism and nationalism. I am not saying you are doing this, I just want to be careful.
My opinion (and it is nothing more than that) is that the Parthians were able to inflict such crushing defeats on the Romans due to their respective methods of warfare. The "Parthian shot" was virtually impossible for an army based entirely around heavy infantry to counter. The testudo was worthless as the horse archers could ride right up to the legionaries and target their vulnerable limbs and faces. Even worse for the legionaries, they would quickly grow exhausted. Then the Parthians’ could pick them off almost at will (with the assistance of heavy cavalry of course). Cue Saladin at the Battle of Hattin--you don't need to inflict casualties on a heavily armored foe in the desert. Just forcing them to wear their full armor (or, in our case, remain in the immobile testudo) is enough, and horse archers are of course a most valuable tool for this purpose. Once the heavily armored troops are tired and broken the real slaughter may commence.
That is not to discount the genius of Surena. I merely wanted to point out the tactical superiority of the Persian armament on their home terrain as equally consequential in their repeated victories of Roman legions. So I agree with you, I just wanted to extend your argument further--Carrhae was not solely the result of a fleeting moment of Roman incompetence or Persian genius, but a culmination of many factors (including third party variables we haven’t discussed such as traitorous guides).
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