Explanation: In the Late Republic and the entire period of the Empire, the obsession of choice for Roman men of ambition was to finally defeat Persia. It never really worked out. Persia is fucking big for one, and the only paths Rome had to it were through hostile land - through the arid regions of Mesopotamia up through the south, or through the Armenian Highlands going west-to-east. Add to that that the Roman military never quite figured out a decisive response to Persian cataphracts (heavy cavalry) and mounted archers, made of (respectively) nobility and nomads who had spent their entire lives in the saddle, and you have the recipe for hundreds of years of pointless tug-of-war between Persia and Rome.

Emperor Trajan came closest, conquering most of Mesopotamia and leaving Persia as a rump state - but those conquests were abandoned by Hadrian, his successor, considering them to be unsustainable - Rome’s military was stretched thin across its borders, it didn’t need a new salient to deal with!