The stunning defeat at Carrhae could easily be avoided, but I'm wondering if Crassus can actually defeat or drive off Surena's army from Mesopotamia entirely. After that, would Orodes II try to give battle to the Romans following their Armenian campaign, or stall them?
I quite like this PoD because of its double major effects: a successful, early invasion of the Parthian Empire and a vastly altered dynamic in the ending years of the First Triumvirate and the lead up to the civil war.
But, first things first. Crassus at 60 doesn't seem to be the brightest military bulb, so is it feasible that he could pull off such a campaign. Barring that, would it be in character of him to resign command to his son Publius (who was quite the rising star apparently) or another of his commanders, such as Longinius?