Parthia did have a weakness in that the Parthians were seen as interlopers by the Persians whom they ruled over. The Sassanians who followed them were far more

Parthia did have a weakness in that the Parthians were seen as interlopers by the Persians whom they ruled over. The Sassanians who followed them were far more unified as they were actually Persian and able to claim the heritage of the Achaemenids(the Persian Empire of Cyrus, Darius, etc). In addition, the Parthian capitals were fairly far West. Trajan was able able to successfully invade Parthia. My guess would be that Julius was a good enough general to successfully invade Parthia, like Trajan and extract some concessions. The big question is if he could have actually conquered Persia. This is one instance where the centralization of Persia may have been able to play into his hands. If you look at Alexander’s conquest after a few decisive victories in pitched battles, he was able to conquer Persia. Same with the later Muslim conquest of Persia, where decisive pitched battles delivered large chunks of Persian territories to the Muslims. So while definitely not easy, I don’t think it was impossible, and Julius was one of the greatest generals of Antiquity, so it certainly would have been interesting to see if he could have done it.