Explanation: While the Romans rarely had trouble with the infantry rabble of the Parthian Empire, the Parthian cavalry was a formidable force on the battlefield, consisting primary of noble-born heavily-armored lancers, and nomadic horse archers. The horse archers would harass Roman infantry, staying out of javelin range, and loosing arrows as they rode away when pursued - the famous ‘Parthian shot’. Should the Romans break formation, or else try to form a testudo, the heavily armored lancers would ride them down.

Rome never really did form a full answer to this double threat, and Persia remained a dream for every conquest-oriented Emperor.

Idem, sed latine:

“It’s not like they have infinite ammo”

but they did