Here`s the introduction covering the use of barbarian armies:
The common perception of the so-called, "barbarian" nations in history is that they were tactically inept and uncivilised.
Of course, the history was recorded by the winners, but some clues do come down to us that although the Gauls, Germans and others were governed by their social structures, unable to change the organisation of their armies, some did possess an understanding as to how the Romans fought and were able to adapt in some way in fighting them. Some barbarians fared better than others in this respect.
Some simply made the best of their own environment, using ambush (the Romans could be vulnerable on the march), or they resorted to small scale guerrilla warfare in mountainous and rough terrain.
Using surprise and ambush methods, Arminius, Varus` German auxiliary commander, was famously successful against the Romans at the Teutoberg Forest in the Clades Variana (the Varian Disaster).
In the Illyrian & Batonian Wars, Rome's trained auxiliaries turned against them and they too used their home advantage of terrain when in 9 AD the Batos chose to attack Aulus Caecina Severus in the Volcaean Marshes; in a terrain in which a full deployment of the Roman legions might have been problematic.
Attacking the Romans when they were vulnerable; in making their camps was tried by the Nervii against Caesar at the battle of the River Sabis. However, delivering a sustained attack was problematic to barbarian commanders, but if by weight of numbers, warband attacks could be renewed; there may be a chance that a pinned force of Romans could be totally beaten and this almost happened at the Sabis.
Spartacus was perhaps the best and the most obvious example of an auxiliary commander who had learned from the Romans both their strengths and weaknesses and used them to his advantage beating three armies; two consuls (Lentulus` and Gellius`) and Longinus, the Governor of Cisalpine Gaul and defeating in all, a total of six legions in just a matter of weeks in 72 BC.
The Gauls must have had understood how the Roman military machine worked too. They understood that attacking a Roman army's left flank was a tactic worth trying.
Generals of classical, regulated armies like the Romans led their armies positioned on the right of their battle line and their sub-generals, commanders of the supporting Roman lines were also traditionally placed on the right. In general then, when advancing, the left of a Roman army had no controlling officers present and according to the rule of seniority, it was most likely to have the least experienced legions placed there. This was a Roman army's weakest point and where any responses to a counter-attack would have been limited to local, sub-unit commanders.
Both Divico and Vercingetorix used this tactic against Caesar.
Almost anyone serving as an auxiliary, whose position in the Roman line of battle would have been as a left flank-guard would have well understood this potential weakness.
This must have been known in the mid-seventies BC that the Romans were fairly predictable, in that in a given space to deploy, they will tend to array their legions in the same order. Barbarians understood their enemy's standard operating procedures and knowing and anticipating what an enemy may do is perhaps the first part of military planning; essential for any general.
Barbarian rebel ex-auxiliary generals would know that a Roman army would find difficulty in wheeling to face its left flank and as warbands` movements are more fluid and rapid; this is a tactic to be expected by the most cautious of Roman generals.
A barbarian general with the advantage of choosing the ground upon which he fights, may find that by using his knowledge of the characteristics and limitations of his enemy, opportunities may arise where he gains the initiative by wrong-footing the Romans.
paints a legend from the Trojan Wars.
