In talking about the military organization of Rome's enemies, I would like to talk about Rome's chief enemy during the Late Republic/Early Empire period, the Parthian Empire. Despite holding the region known today as Persia, the Parthians were in fact an Iranic Nomadic people who invaded and conquered the eastern half of the Seleucid Empire over the 3rd and 2nd centuries BCE.

As you are probably aware of, the Roman army after the Marian reforms were essentially a standing army that fought for a wage (including in salt and where the word salary comes from) and were granted land and Roman citizenship after retiring. This stands in contrast with the Parthians, whom Plutarch denotes as 'servants' or 'slaves', though this is likely a misunderstanding of the Parthians' system of organization. A more adept comparison could be made with the European feudal system, where in warriors by birth would pledge their loyalty to a sovereign. Another major feature of the armies of nomads is that they are in general warriors who gather around a charismatic leader (Attila and Genghis Khan being prime examples of this) and it is fair to say that the Parthians likely retained that to some degree.

In short, Rome emphasized the idea of a standing army in which anyone could join (Rome over the years had many peoples in their armies) for a wage and citizenship where as in Parthian society, the army acted more of a caste who gathered around the central authority of the king and were viewed as 'slaves' to some Romans.

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Thanks.

I'm actually writing my dissertation on the wars of Trajan at the moment, and in my readings (and obviously included in it) I've noted that the Dacians are said to have had a very strong military, albeit one that was modeled on the Romans.

The Dacians, after defeating Domitian's armies and eventually being bought off by him to make peace, were given military aid by Rome, and when Trajan later defeated them in a war the peace terms included the return of any and all Roman deserters and the promise to no longer recruit Roman soldiers and harbour deserters them at court. I can only take this, and all the works I've read have agreed, that the Dacians were taking in Roman aid/deserters to train their army - indeed, one outright states that the Dacians were the single biggest threat to Rome, and that besides them they had the strongest army in Europe.

As well as the issue of Domitian's military aid and Roman recruits/deserters, Dacia had a system of forts that posed an obstacle to the Romans when they invaded the country. There are also references to the Dacians possessing siege weapons, something not unbelievable considering the aforementioned military aid and the presence of Roman deserters at the royal court.

Finally, to top it all off I've read several historians give the number of men that Trajan took with him to conquer Dacia as 100,000. That's about 1/3 of Rome's entire military might at the time - obviously Dacia was a threat, and well organised. It may have only taken Trajan a year to defeat the Dacians, in both of the two wars he fought against them, but it's a relatively small country, the fighting was hard, and no one brings 1/3 of an empire's army to bear on one section of the frontier if the people there are a pushover.

So while we don't have many (or, really, any) specifics on just what the Dacian army of the early 2nd century was like, it seems to have been a large, strong force modeled on Rome's own military (likely with regards its discpline, not weapons, as I've never seen or read anything to suggest they were armed and armoured like Romans) with strategically placed forts throughout the country.

An argument can be made for the Early Republican legion formation as a development to counter the classical Greek phalanx. These early legions were itself a group of three formations: the Hastatii, the Principes and the Triarii. Hastatii were generally younger of the Roman citizens lightly armed, crucially with pila (sn. pilum), a spear with an elongated iron tip, designed to bend after penetrating a bronze shield (thus, making the shield even more cumbersome to maintain). The second line were the Principes, usually consisting of prime-aged Roman citizens, armed very similarly to what would later become the standard legionaire. Lastly the Triarii; which resembled the hoplites in armament and in tactics.

The legion formed thus would ideally work by bringing in the skirmishers (the hastatii) to disarm the front line of the phallanx and avoid the brunt of its force thanks to mobility. The principes, then, would hack and slash at the ranks of the phalanx. Lastly, if the battle went against the legion, the Triarii, would hold the line giving a chance for the rest to regroup.

Bearing in mind that the Roman military evolved in opposition to Greek or Greek-influenced armies (such as Etruscan and actual Greek colonies in Southern Italy), such natural evolution of Roman tactics makes sense to me.

The Ottoman Army(who destroyed the Roman Empire) is a highly organized army at that time. For more info: http://en.wikipedia.org/wiki/Military_of_the_Ottoman_Empire#Classical_Army_.281451.E2.80.931606.29