Iranian military historians generally believe that the Parthians relied on armies fully composed of cavalry because of the ever-present possibility of a two-front war. You see, they might have been nomads originally and therefore naturally reliant on cavalry, but after their settlement they could have easily used more infantry -- see the Huns.

The thing is they didn't. Pressure from Greeks and Romans from the West, and from Greeks and nomads from the East forced them to make a mobile army. Apparently the infastructure of Iran wasn't that well-developed, because they decided to fully rely on cavalry, and not develop a (perhaps small) core of reliable infantry for sieges and perhaps battlefield uses -- like the Sassanids -- to get around Iran quickly to adequately respond to threats on both frontiers.



~Wiz