Where would the real disadvantage be...

[Maion Maroneios]

http://en.wikipedia.org/wiki/Battle_of_Carrhae
Somehting like this, maybe?

[Spartan198]

I kinda...forgot about that battle.

Though I think that maybe one disadvantage the Romans had was the fact that they were dedicated swordsmen, rather than spearmen.

And horse archers just...well, counter everything.

[Maion Maroneios]

That's why, after all, Legionaries changed to spearmen after the Empire split into East and West.

[Disciple of Tacitus]

There are more factors involved then the composition of the armies involved. Supply lines, command structures, communications. And of course, Location, location, location.

[beatoangelico]

That's why, after all, Legionaries changed to spearmen after the Empire split into East and West.

not exactly, the spatha continued to be the main weapon for all the IV century at least, but many carried also a short anti cavalry spear

[Africanvs]

At Carrhae the horse archers were resupplied with arrows by camel trains. The Romans were just in a hollow square formation completely surrounded. I believe the biggest problem for the Romans at Carrhae was Crassus. He was no general, just a guy with tons of illgotten money trying to make a name for himself against a strong enemy. Too strong it proved. The happiest day of Caesar's life was probably when Crassus went to Parthia to meet his death. That and knocking boots with Cleopatra. The key to horse archers, as discussed in the horse archer thread is stand off distance. If you have it such as at Carrhae and can just rain arrows on the infantry, they're doomed.
Check this out, great source going into detail about horse archer and swarming tactics used by steppe armies.

http://books.google.com/books?id=RTq...hl=en#PPA20,M1

According to this source the legionary was slowly replaced over time by the heavily armored cataphract.

[Aper]

Ok, horse archers are real badasses, but remember that after Charrae the Roman Empire got the initiative on the Parthians in all the war they fought... and the legions never lost a field battle against cavalry-based armies again until the collapse of the political system in the III century... that means strategies and tactics against the HA+Kats combination actually existed, and that the Romans adopted them (even if some cavalry-fans think different...).
Arrian vs. the Alans, Belisarius in the battle of Sura-Callinico and others Imperial commanders resisted or won using a defensive tactic and a "fortress-like" formation: armored infantry make a shield/spear wall to protect lots of good missile troops.
The problem for a western army is exactly to find lots of good missile troops..

[artaxerxes]

and the legions never lost a battle against cavalry-based armies again...

I seem to recall a certain Mark Anthony's unsuccesfull invasion of Parthia - which was so disastrouous that it actually sorta decided the coming war between Octavian and Mark Anthony as well, since his casulties were irreparable. And it did include battles, such as two Roman legions left with the siege weapons while Anthony moved ahead, whereafter Parthians crushed the legions and siege weapons. Except for that battle however, I'll prolly have to admit that the remainder of the Roman losses mostly came from being unable to take cities (having no siege weapons) and then having to retreat.

What of the Sassanid-Roman wars - the Shapurs defeated tons of Roman emperors, they must have won a battle now and then ;)