Hmm, I was able to conquer (using my army of 14 ballistas and Legio Consularis) several Persian cities - Susa (1 presidium), Siraf (2), Ectabana (2), Rhagae (2) and Thara (2). Persians offered me Herat, Sarmias and Hamadan for a Peace Treaty. So I reduced their possesions very much.
I tried...