I've been reading a little about this lately, and something I've been wondering is just how the Romans were able to defeat enemies like Parthia.
Just because the Roman method of war revolved around their exceptional heavy infantry, with little regard given to cavalry or archers. This army structure seems like it would struggle to defeat, say, a Parthian army made up largely of mounted archers, but Rome did defeat Parthia on several occasions and hold onto the Eastern provinces for centuries.
So how was this possible? How did the Romans adapt to the situation?
Thanks for all responses!
You wrote like Parthia so I took the Sassanids as an example. In the year 530 A.D. Belisarius won the Battle of Dara despite being outnumbered by the Sassanids and their cavalry. He used huns and heruli to reinforce his own bad Roman cavalry (fight fire with fire). Roman cavalry, like the Cataphracts, were slower and so they had to use foreign soldiers. The "shooting arrows and running away" method were seen as cowardly in the eyes of the Romans, it was extremely effective againts their superiour infantry. Earlier Romans, like Crassus, had their armies annihilated by the Eastern enemies. Eventually they came to realize that it would be necessary to have the same weapon as the enemy. Beliusarius same cavalry were later used versus enemies in the west like the Vandals, very successfully.
A very interesting podcast of the East Roman army: http://thehistoryofbyzantium.com/2013/10/04/episode-37-the-army-of-the-strategikon/ Battle of Dara: http://thehistoryofbyzantium.com/2013/01/05/episode-17-527-532-part-1-the-battle-of-dara/
The Roman legions themselves did not adapt that much - what the Romans did adapt was to hire lots of auxilia out of the region. Generally, slingers had a longer range than archers in ancient times, and most often you could get more foot archers for the same dime than you could get horse archers.
So the answer is missile troops. Horse archers were seldomly heavily armoured, and a man and his horse are big targets, especially in any kind of formation. Foot missile troops, especially slingers, often had longer range too.
Rhodian and Macedonian slingers were renowned for their skill, as were Cretan and Syrian archers. Both were hired in large numbers as auxilia and/or mercenaries to reinforce the legionary army as it marched against any Persian (Parthian or Sassanid) threat.
The Roman method against the Parthians was basically to avoid all the mistakes at Carrhae - don't get caught days away from fortification, and don't let your cavalry get lured away. Fighting in relatively rugged terrain, superior Roman logistics and siegecraft and taking advantage of Parthian internal disunity also helped. Gnaeus Dominitius Cordubo fought no pitched battles in defeating the Parthians in 58-63, he simply ensured his army was never in a position to be surrounded and cut off while using superior siegecraft to capture Armenia's key points. That included fortifying his entire line of advance. (see for eg A. Goldsworthy "In the name of Rome" )
As an aside, the Roman method of war would evolve, especially over the life of the empire. The republic was limited in the number and quality of horsemen it could field - this was not a problem for the empire, and the cavalry arm would become increasingly important as the empire progressed (to the point, that it was the decisive arm in 530AD)