The Roman auxilia consisted of non-citizens. In the first century CE, around 90% of the population of the Roman Empire were not citizens of Rome. This meant the auxilia were critical in expanding and defending Rome's territories. The auxilia were established by Augustus when the principate was founded, in roughly 30 BCE. By the end of the second century CE, the auxilia outnumbered the legionaries.

24 CE 130 CE 210 CE
Legions 125,000 155,000 182,000
Auxiliaries 125,000 218,000 250,000
Praetorian Guard 5,000 8,000 15,000
Total Soldiers 255,000 381,000 447,000

If a soldier joined the auxilia, he would not be stationed in the province of his birth. This meant that if there was a rebellion in a province, the soldier would not sympathize with the rebellers and would not defect.

The Roman auxilia was an attractive career choice for many people in Rome's provinces. It provided a steady income of 750 sestertii (150 sestertii less than legionaries), for people coming from the poorer parts of the Roman Empire, this was a massive boost to their income. On top of this income, on retirement, he would receive a pension as well as full Roman citizenship after his twenty-five years of service.

Regiments of auxilia were structured the same as a Roman cohort (480 men). However, they operated on their own and not part of a larger collection of cohorts like the legions. There were two main types of regiments: cavalry known as the 'alae' and the infantry referred to as the 'cohortes'.

Types of Roman Auxiliary Units

Unit Name Type No. of Soldiers
Ala quingenaria Cavalry 480
Ala milliaria Cavalry 720
Cohors quingenaria Infantry 480
Cohors milliaria Infantry 800
Cohors equitata quingenaria Infantry & Cavalry 600 (480/120)
Cohors equitata milliaria Infantry & Cavalry 1040 (800/240)

However, it is not so simple to divide the auxilia into predetermined units. The Roman auxilia were specialist units diverse in their nature.

Ranged Auxilia Units

A large proportion of the Roman army's ranged units belonged to the auxilia. By the end of the second century CE, archers made up 18,000 of the 180,000 auxilia forces (10%). The Greek island of Crete boasted some of the best archers in the ancient world and contributed a substantial number of soldiers to the Roman auxilia. Thrace and Syria also provided a significant number of ranged units to the Roman military. In the second century CE, there were thirty-two units of archers. Thirteen of these came from Syria, five from Anatolia, one from Crete and seven from other provinces.

Just as there were provinces prolific for their archers, it was the Balearic Islands which provided the best slingers.

Auxilia Infantry (Cohortes)

The auxilia infantry was very similar to the legionary infantry. Split into six centuries of eighty men each; they also received almost identical weapons, armor, and training. The infantry was equipped with helmets, mail armor and a sword (gladius). The cohortes would have been deployed on the flanks of the Roman legionary infantry.

Auxilia Cavalry (Alae)

The cavalry of the auxilia, the alae, were elite units. These units were organized into squadrons of thirty cavalrymen. They received extensive training in complicated maneuvers and also received around 20% more pay than the auxilia infantry. The auxilia cavalry was very important to the Roman army as the legions had limited cavalry capacity. These cavalry units were usually heavily armored in mail armor and would either carry a long Roman sword (spathe) or a spear (hasta).

In addition to the alae the 'equites cataphractarii' were extremely heavy cavalry. These units were based on the Parthian cataphracts. These units were capable of smashing into enemy infantry and completely destroying their formation often causing them to flee.

There would have also been a large contingent of light cavalry. These units were often recruited from northern Africa who possessed a large base of talented riders. They would only be equipped with a small round shield, and a spear (hasta). These units were incredibly quick and agile, allowing them to move around the battlefield easily. They would be useful in gathering intelligence, pre-battle skirmishes and chasing down the fleeing enemy.

Beyond conventional cavalry units, during the second and third centuries CE, there is evidence of camels being ridden into battle.

Auxilia Deployment (130 CE)

Province Modern Day Equivalent Auxilia Infantry Auxilia Cavalry Total Auxilia
Britannia England and Wales 25,500 10,500 36,000
Germania Germany 18,000 7,500 25,500
Raetia and Noricum Switzerland and Austria 11,000 5,000 16,000
Pannonia Hungary, Slovenia and Croatia 11,000 8,000 19,000
Moesia Serbia, Bulgaria 11,000 5,500 16,500
Dacia Romania 18,000 7,500 25,500
Cappadocia Turkey 8,000 3,500 11,500
Syria Syria 21,500 10,000 31,500
Aegyptus Egypt 5,000 3,000 8,000
Maurentania Algeria and Tunisia 15,000 8,000 23,000
Total 144,000 68,500 212,000