Under the Roman Republic

Up until the 2nd century BC, the auxiliary forces were composed of the Italian allies of Rome. These allies provided light infantry - called velites - and cavalry. The heavy infantry - the legions - was composed exclusively of Roman citizens, with noble Romans making up the Roman cavalry. The standard deployment of forces for battle was like this: the main battle line consisted of the Roman heavy infantry, with a screen of allied light infantry in front of it. Roman cavalry was positioned at the right flank and allied cavalry (under a Roman commander) on the left flank. As the battle commenced, the light infantry would harass the enemy before withdrawing behind the heavy infantry. Meanwhile the cavalry would secure the flanks and attempt to threaten the opposing army's flanks. The main action would usually be by the Roman heavy infantry.

Once Rome's hegemony over central Italy had been established, the Italian allies were pretty reliable. Up until the war against Pyrrhus (280-275 BC) there were regular defections, but after "the local rulers seem to have concluded that more was to be gained by loyalty than by opposition" (Erdkamp, p.101). Even during the Second Punic War, after Rome's reputation had been crushed by a series of defeats by Hannibal's forces, only some of the allies defected to Hannibal.

During the 2nd century BC Rome began to include non-Italian units in its armies. There were Numidian and Iberian horsemen, Ligurian and Thracian light forces, Balearic slingers and Cretan archers. At this time, these forces were provided by client kingdoms and such. They generally were not very reliable and Roman generals distrusted their contributions to the battlefield (Cagniart, p.88). At the start of the 1st century BC, after the Social War, the Italian allies of Rome gained citizenship. From then on the Italian cities provided heavy infantry and the auxiliary units were exlusively non-Italian.

Under the Principate

Under the early Roman Empire, service in the auxiliary units became a path to citizenship. Excellent performance in battle might be rewarded with citizenship, granted to an individual or even entire units. Auxiliary units were initially raised from the provinces and identified by their geographic origin. They were then moved to where ever they were needed, after which their ethnic origin would dilute due to local recruitment. Apparently these forces were more reliable: auxiliary units should be "considered as complementary to the legions: well-trained and reliable troops who could fight in the line of battle along with legionaries, as well as providing the diversity of forces vital to Rome's military success, in the form of cavalry, camel riders, slingers, archers, and the skirmishing troops that had been lost to the legions with te manipular system." (Gilliver p. 193)

So why did the Romans trust them in battle?

From the above we can gather that Roman trust in their auxiliaries depended on where they were from. Foreign troops weren't very reliable as they had no real reason to fight for Rome other than that their ruler told them to. The Italian allies on the other hand had their fate tied to Rome. If Rome did well, so did they. (Ok, in the end they had to fight a war against Rome to get more recognition, but all in all they were better off with Rome than against it.) Under the Principate, auxiliaries were recruited from provinces within the empire. They were sent away to fight away far from home. If they stuck with Rome, they could earn pay, honors and even citizenship; if they defected they'd be at the wrong end of the empire stuck between a hostile Roman army and likely hostile locals.

Note I have described how things went up until the Principate. In the later period battle tactics changed, equipment changed, and Rome would ofter hire foreign tribes to fight for them; I imagine this would have involved some risk of defection but unfortunately this is not a period I'm very familiar with.

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Sources used:

All sources from P. Erdkamp (ed.), A Companion to the Roman Army, Oxford 2007

• P. Cagniart, The Late Republican Army (146-30 BC), p. 80-95
• P. Erdkamp, War and State Formation in the Roman Republic, p.96-113
• K. Gilliver, The Augustan Reform and the Structure of the Imperial Army, p. 183-200

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