Ancient Roman Auxiliaries

Auxiliaries (from Latin: auxilia = "supports") formed the standing non-citizen corps of the Roman army of the Principate (30 BC–284 AD), alongside the citizen legions. By the 2nd century, the auxilia contained the same number of infantry as the legions and in addition provided almost all the Roman army's cavalry and more specialised troops (especially light cavalry and archers). The auxilia thus represented three-fifths of Rome's regular land forces at that time. Like their legionary counterparts, auxiliary recruits were mostly volunteers, not conscripts.

Auxiliary troops were mainly recruited from the peregrini, i.e. free provincial subjects of the Roman Empire who did not hold Roman citizenship and constituted the vast majority of the empire's population in the 1st and 2nd centuries (ca. 90% in the early 1st century). Auxiliaries also included some Roman citizens and probably barbarians (barbari, as the Romans called peoples located outside the Empire's borders). This was in contrast to the legions, which admitted Roman citizens only.

In the Republican period, the standard trio of specialised auxilia were Balearic slingers, Cretan archers and Numidian light cavalry. These functions continued in the 2nd century auxilia, plus a few new ones:

Archers
Sagittarii ("archers", from sagitta = "arrow") units recorded in the 2nd century are: eight alae sagittariorum (mounted archers), eighteen cohortes sagittariorum (foot archers) and six cohortes sagittariorum equitatae (mixed foot/mounted archers). These 32 units (of which four were milliary) would have comprised officially 17,600 archers. They were now predominantly of Syrian origin, just one unit, cohors I Cretum sagitt. eq., bearing the name of the Cretan archers who had traditionally served the Republic. Of the 32 sagittarii units attested in mid 2nd century, thirteen have Syrian names, seven Thracian, five from Asia Minor, one from Crete and the remaining six of other or uncertain origin.
Three distinct types of archers are shown on Trajan's Column: (a) with scalar cuirass, conical steel helmet and cloak; (b) without armour, cloth conical cap, and long vest; (c) equipped in the same way as general auxiliary foot soldiers. The first two types were probably Syrian units; the third type probably Thracian. The standard bow used by Roman auxilia was the recurved composite bow, a sophisticated, compact and powerful weapon.
It is unclear from the evidence if all sagittarii units contained only archers. Some sagittarii units were equipped in the same way as ordinary alae and cohortes, apart from carrying bows. Also, it would be surprising if ordinary units completely lacked archers, since that would limit their capacity for independent operations. Indeed, some non-sagittarii units are shown employing bows.

Light cavalry
From the Second Punic War until the 3rd century AD, the bulk of Rome's light cavalry (apart from mounted archers from Syria) was provided by the inhabitants of the northwest African provinces of Africa proconsularis and Mauretania, the Numidae or Mauri (from whom derives the English term "Moors"), who were the ancestors of the Berber people of modern Algeria and Morocco. They were known as the the equites Maurorum or Numidarum ("Moorish or Numidian cavalry"). On Trajan's Column, Mauri horsemen, depicted with long hair in dreadlocks, are shown riding their small but resilient horses bare-back and unbridled, with a simple braided rope round their mount's neck for control. They wear no body or head armour, carrying only a small, round leather shield. Their weaponry cannot be discerned due to stone erosion, but is known from Livy to have consisted of several short javelins.Exceptionally fast and maneouvrable, Numidian cavalry would harass the enemy by hit-and-run attacks, riding up and loosing volleys of javelins, then scattering faster than any opposing cavalry could pursue. They were superbly suited to scouting, harassment, ambush and pursuit. It is unclear what proportion of the Numidian cavalry were regular auxilia units as opposed to irregular foederati units.
In the 3rd century, new formations of light cavalry appear, apparently recruited from the Danubian provinces: the equites Dalmatae ("Dalmatian cavalry"). Little is known about these, but they were prominent in the 4th century, with several units listed in the Notitia Dignitatum.

Slingers
Funditores, ("slingers", from funda = "sling") units do not appear in the epigraphic record.However, slingers are portrayed on Trajan's Column. They are shown unarmoured, wearing a short tunic. They carry a cloth bag, slung in front, to hold their shot (glandes). The late Roman army of the 4th century contained at least one independent funditores unit.

Heavily-armoured lancers
Contarii ("lancers", from contus = a heavy long lance) were a type of special heavy cavalry, covered from head to toe in scalar armour. Their numbers were probably considerably expanded in the 3rd century. Based on Parthian models, they were called contarii, cataphractarii and clibanarii. Together with new units of light mounted archers, they were designed to counter Parthian (and, in Pannonia, Sarmatian) battle tactics. Parthian armies consisted largely of cavalry. Their standard tactic was to use light mounted archers to weaken and break up the Roman infantry line, and then to rout them with a charge by the cataphractarii. The only special heavy cavalry units to appear in the 2nd century record are: ala I Ulpia contariorum and ala I Gallorum et Pannoniorum cataphractaria stationed in Pannonia and Moesia Inferior respectively in the 2nd century.

Imperial horse guards
Equites singulares Augusti ("personal cavalry of the emperor", from singuli = "attached to one individual"), was a unit made up of the best cavalry the auxilia could offer. Their core component were crack Batavi horsemen. This was the only unit of the Praetorian Guard which recruited non-citizens. Starting as a milliary ala (720-strong), it grew steadily in size, reaching 2,000 men at end 2nd century. They would always accompany the emperor on tours of the provinces and campaigns. In addition to the ala at Rome, it appears that after some campaigns, detachments of singulares were permanently left behind in the provinces, becoming regular alae, but retaining the prestigious singulares title and crack reputation e.g. Ala I Flavia singularium stationed in Raetia in mid 2nd century. On Trajan's Column, singulares are easily identifiable because they always accompany the Emperor himself, and are not dressed for battle: they do not wear a cuirass, but marching clothes (tunics and cloaks). Their standards carry the same lightning-and-thunderbolt motif (with or without wings) as the legions.

Camel troops
A unit of dromedarii ("camel-mounted troops") is attested from the 2nd century, the ala I Ulpia dromedariorum milliaria in Syria.

Scouts
Exploratores ("reconnaissance troops", from explorare = "to scout"): Examples include two numeri exploratorum attested in the 3rd century in Britain: Habitanco and Bremenio (both names of forts). Little is known about such units.