5 Auxiliary Units of the Roman Army

The Roman army utilized many esteemed auxiliary units with specialist fighting capabilities. Let’s have a look at five of the most famous.

Jan 11, 2023By Colin J Campbell, MLitt in Ancient History, BA Ancient History & Civilization

roman horsemen trajan roman glass fragment


Indigenous levies were recruited widely to augment the operational ability of the Roman legions. Although Rome did not lack heavy infantry, she was sometimes in need of cavalry, lighter infantry, and skirmishing capabilities. In nearly all periods of her history, Rome actively augmented these arms from other peoples. Though there were many types of allies and auxiliaries, some of the most elite offered the Roman army specialist capabilities.


5 Auxiliary Units of the Roman Army


1. How Balearic Slingers Helped the Roman Army

slingers on march
A Roman Army on the March – Marching Slingers, by Antionio Fantuzzi, 1540-45, via Met Museum


The Balearic slingers were the elite skirmishers in the ancient world. Light troops that could move fast, skirmish ahead of battle lines, and snipe opponents. They hailed from the current Spanish Balearic islands of the Mediterranean and were of Phoenician heritage. Operating a range of slings, they were deadly over a considerable distance.


The Roman army encountered the Balearic slingers via their enemy Carthage. Attested as early as the late 4th century BCE, Carthage employed slingers:


“By hurling a shower of great stones, [Hamilcar’s slingers] wounded many and even killed not a few of those who were attacking, and they shattered the defensive armour of most of them. For these men, who are accustomed to sling stones weighing a mina, contribute a great deal toward victory in battle, since from childhood they practise constantly with the sling.”
[Diodorus Siculus, Library, 19.109]

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A Greek, or Attic mina is estimated at c. 431 grams or 15 ounces. That is heavier than a modern can of Coke at 12 ounces! It was, according to Diodorus, much larger than the clay pellets or lead bullets used by other Greek and Near Eastern slingers.


The slingers perfected their skill as a national custom. Sources tell a popular story that Balearic children would not eat until they had first shown their mothers their skills with the sling. Slingers carried three slings for different ranges (or missile sizes) into battle. They were mobile, deadly, and accurate, capable of unleashing projectiles with the ‘force of a catapult’.


The Carthaginians who invaded Italy in 218 BCE used many Balearic slingers. They impressed the Roman army and were present at the fateful battle of Zama (202 BCE) where Hannibal was eventually defeated.


balaeric slingers
Depiction of Balearic Slingers, via thairath.co.th


By 121 BCE, Quintus Caecilius Metellus subdued the Balearic islands for Rome. When the Roman consul first landed on the islands, he:


“… stretched hides above the decks as a protection against the slings.”
 [Strabo, Geography, 3.5]


In the Jugurthine War (112 – 106 BCE) a body of slingers fought for the Roman army generals Marius and Sulla in North Africa.


Julius Caesar used slingers throughout the Gallic and Civil Wars. They were particularly useful in skirmishing and in siege scenarios. In the Civil Wars Pompey also drew on them and at Dyrrachium they gave Caesar trouble:


“Pompey resolved not to oppose [Caesar] with his whole force, or to come to a general engagement, yet he detached to particular places slingers and archers, with which his army abounded, and several of our men were wounded, and filled with great dread … .”
[Caesar, Civil Wars, 3.44]


Slingers continued to be mentioned in the Imperial period, but it is not clear if they hailed specifically from the Balearics or if the Roman army recruitment diversified.


It’s possible that the Roman army assimilated the skillset into their auxiliary and legionary units. Vegetius, writing later in the 4th Century CE, advocated that all Roman army troops should train with the sling.


2. Cretan Archers

archer ancient greek pottery
Archer, probably Odysseus, from an ancient Greek teracotta jug, attributed to the Disney painter, 430-420 BCE, via Met Museum


With equal reputation, Cretan archers were also recognized in the ancient world. They had long been famous in Greek warfare. Their skills also constituted part of their culture:


“… Cretans are runners, since this land of ours is rugged and more suitable for the practice of foot-running. Under these conditions we are obliged to have light armour for running and to avoid heavy equipment; so bows and arrows are adopted as suitable because of their lightness. Thus, all these customs of ours are adapted for war.”
[Plato, Laws, 1.625d]


The bow constituted their principal weapon, though they also carried small shields, light axes, or knives. They were useful light troops for a number of scenarios.


In 218 BCE, the Roman army sent for men to fight Hannibal from their regional ally, Hiero of Syracuse. Among other forces, he sent 500 Cretan archers.


Following expansion into the Hellenistic world in the late 3rd and 2nd century BCE, the ancient Roman army increased her recruitment of these famed archers.


trophy archer marble
Stylized Archers Trophy, 18th Century, via Met Museum


Mobile, accurate, and deadly, the Cretans augmented the Roman army missile capability in the field. Livy graphically recounts the impact that allied Cretan archers had against Galatians in 189 BCE. Among other missile troops, the archers advanced ahead of the legions, engaging at range:


“On all sides [Galatians] were being hit by the arrows and leaden bullets and javelins which they were powerless to ward off … when they are being wounded by missiles flung from a distance by an unseen foe … they dash recklessly against their own comrades like wild beasts that have been speared.  Their practice of always fighting naked makes their wounds more visible … Consequently, more blood flowed from them, the open gashes appeared more horrible, and the whiteness of their bodies showed up the stain of the dark blood.”
[Livy, History 38.21]


Devastating in close combat, the Galatians were ill equipped for ranged missiles. As their shields were not large, the damage was terrible.


Ancient Roman army use of Cretan archers continued in the late Republic. Archers were crucial at both Caesar’s sea-borne landing in Britain in 55 BCE and the siege of Alesia in 52 BCE. They also fought for various contingents in the Civil Wars.


Details are thin for the Imperial Period, but Cretan Archers continued to be attested in inscription and units designations of the 1st and 2nd Centuries CE. A Cretan unit fought in the Dacian Wars for Trajan.


As the empire developed, other archers from Africa, the Balkans and Near East started to dominate. From the 1st Century CE onward, Rome came into contact increasingly with horse archers like the Parthians. Correspondingly the Roman army archers of the later empire were increasingly mounted, though foot units endured.


3. Numidian Horsemen

numidian horseman print
Depiction of Numidian Horseman, via Stringfixer.com


Ancient Rome could draw its own cavalry from the Equites (knights) class. However, this relatively narrow band of wealthy citizens could not sustain the exponential demands of an expansionist empire. Cavalry auxiliaries also tended to be drawn from Latin and Italian allies. As even they grew stretched, Rome increasingly looked to foreign and indigenous fighters.


In the Second Punic War (218 – 201BCE) Rome was exposed to highly effective Numidian cavalry from North Africa. Lightly armed on relatively small mounts, these men were deadly and fast. They were not much to look at, but looks were deceiving:


“Both men and horses were of a small size and thin make, the riders un-accoutred and unarmed, excepting that they carried javelins in their hands; and the horses without bridles, and awkward in their gait, running with their necks stiff and their heads stretched out.”
[Livy, History, 35.11]


With no armor, they were incredibly nimble. Known for harassing tactics, they could advance and retreat while releasing javelins on the move. Operating in skirmishing roles, they were also experts at ambush and reconnoitre. Ancient Romans initially feared the deadly horsemen of North Africa.


It was not long until Rome drew these skilled horsemen to them through alliance. Via their regional ally Masinissa, the Romans used up to 6000 Numidian horsemen when they finally defeated Hannibal at the battle of Zama (202 BCE).


During the Jugurthine war (112 – 106 BCE) Rome subjugated Numidia and increased her influence over this warlike people. Caesar mentions Numidian units in service for his Gallic Wars, though these seem to have been javelin troops on foot.


By the time of the Civil Wars, Numidian cavalry were fighting with Pompeian forces in North Africa. Under king Juba I, and the Pompeiian commander Publius Attius Varus, Caesar’s lieutenant, Gaius Scribonius Curio faced Numidian horsemen at the battle of Utica (49 BCE). Initially victorious, Curio was later defeated at the battle of Bagradas (49 BCE). Cut off and decimated, the Numidian cavalry (with other contingents) proved their worth. Curio went down fighting like a true Roman, refusing to quit the field.


Caesar personally restored his position in North Africa. Ultimately, the Numidian horse could not stand up to the heavier cavalry he brought from Gaul and Germany. It was probably for this reason that Numidians did not endure as a distinct entity into the Imperial Period, though their skills were undoubtedly assimilated into auxiliary units of the later empire.


4. Gallic Horsemen

gallic long sword
Gallic Long Sword, c. 2nd Century BCE to 1st Century CE, via Met Museum


Before ancient Rome was a superpower, Celtic Gauls fought across the ancient world for Hellenistic states. Tough mercenaries, they had a reputation as fearsome warriors.


Although Gallic hosts are widely depicted as fighting on foot, Gauls were also great horsemen and many units that Rome utilized were mounted warriors.


Though ancient Romans held considerable prejudice towards the fiery temperament of ‘barbarian’ peoples, the Gauls did impress them. If not great soldiers, Gauls certainly made for great warriors. Because of their ‘barbarian’ reputation, they commanded a psychological dividend (at least in early periods), terrifying more ‘civilized’ enemies.


Romans came into contact with Gauls as they advanced Northward through the Italian peninsula. They also met them from periodic migrations and in the armies of their enemies like the Carthaginians and Hellenistic states.


Julius Caesar recruited heavily from among Gallic allies like the Allobroges, the Aedui, and others. In his initial Gallic campaigns, he may not have trusted these forces, as we know from his commentaries that he preferred to mount elements his own 10th legion on horse rather than rely on his tribal cavalry in 58 BCE. However, this did not last, and in Gaul Caesar developed a major Gallic cavalry arm that would change the Roman world.


The result was a period of high dispersion for Gallic cavalry, which featured heavily in the wars of that period. Several of Caesar’s lieutenants, like Publius Licinius Crassus (son of the famous triumvir: Marcus Licinius Crassus) would take Gallic horsemen with them all over the empire, including the deserts of the Near East. In 53 BCE, the young Crassus would die at Carrhae, surrounded by his Gallic horsemen on a hillside, showered by Parthian arrows.


head torso gaul
Head and Torso of A Gaul from a Sarcophagus, c. 160 CE, via Met Museum


Caesar himself utilized Gallic cavalry (along with Spanish and German horsemen) in the Civil Wars and we hear of Gaul’s serving on all sides of that war. In one significant passage, Caesar describes the two Gallic warrior brothers who led a component of his tribal cavalry:


“Among the cavalry in Caesar’s camp were two brothers, Allobrogians by birth, named Roscillus and Aegus, the sons of Adbucillus, who had long held the chief sway in his own state; men of singular bravery, and who had been of signal service to Caesar in all his Gallic wars. … These men were not only highly honoured by Caesar on account of their bravery, but in great esteem with the whole army.”
[Caesar, Civil Wars, 3.59]


Caesar fell out with the brothers, who were accused of pocketing the pay of their fellow countrymen. Something did not add up, and Caesar noted the fallout led to the brothers defecting to Pompey at Dyrrachium in 48 BCE. It clearly stung the Roman Dictator and his awkward effort to explain the unique incident chimes as a loss of face.


Gallic contingents continued to serve on all sides of the Civil Wars, but it may have been the loss of face for Caesar personally that ensured that a distinct Gallic horse guard never endured into the Ιmperial Εra. That and the fact the Caesar found an even more fearsome body of horsemen.


5. Germanic Horsemen 

mettenleiter caesar ariovistus print
Caesar Meets the Germanic War Chief Ariovistus in 58 BCE, by Johann Michael Mettenleiter, 1808, Via Rijksmuseum


One type of auxiliary troop utilized relatively late by the Roman army were the Germanic horsemen. Yet they made a significant impact. Ancient Romans had been directly aware of the fearsome Germans since the Cimbrian invasion of late 2nd Century BCE that threatened the Italian peninsula. Although Roman army discipline would regularly defeat the Germans in pitched battle, they had a fearsome reputation. Seen as even more barbarous and warlike than the Gauls, the Germanic tribes commanded real shock value.


Certain Germanic tribes (though not all) had a particularly strong cavalry tradition. They routinely carried spear and shield, though were only rarely armored. Native German mounts were typically small compared to their Southern counterparts. Caesar noted this, but he also marveled at their tenacious and aggressive qualities, often scattering much larger forces of opposing cavalry. He also admired a style of fighting the Germans practiced:


“The kind of fighting in which the Germans had trained themselves was as follows. There was 6000 horsemen and as many footmen, as swift as they were brave, who had been chosen out of the whole force, one for each horseman for his personal protection.”
[Caesar, Gallic Wars, 1.48.5]


Caesar went on to praise how the integrated infantry kept pace with their paired horsemen, running alongside and holding onto the manes of mounts. Years later, in the Civil Wars, Caesar copied this style of fighting while facing Pompey’s more numerous cavalry:


“… [Caesar’s cavalry] were very much inferior in numbers, [so] he ordered young light armed infantry from a picked corps of frontline men, especially selected for agility, to fight among the cavalry, and by daily practice acquire the technique of this kind of fighting also.”
[Caesar, Civil Wars, 3.84.5]


There is no greater praise than emulation, especially from a commander of Caesar’s caliber.


germanic horsemen drawing
Depiction of Later Period Germanic Horsemen, 1921, via Internet Archive


Like the Gauls, Germanic horsemen fought all over the Mediterranean and were in many battles of the Civil Wars. It was undoubtedly the Germans’ sustained reputation for fierce fighting and singular loyalty that saw them retained after Caesar’s assassination in 44 BCE.


Instituted by Augustus, the Germani Corporis Custodes or German Guards occupied a conspicuous position of eminence within the new Imperial system. They formed a personal (not military or state) bodyguard for the early Julio-Claudian emperors. In this sense, they outlasted the Gallic and Spanish contingents that had served Julius Caesar. They fell away into more routine military service within the auxiliaries and legions.


In the Imperial Period, allied Germanic tribes like the Batavians (from the Dutch Rhine region) continued to command the respect of the Roman army and were famed for their versatile fighting. These men were recruited heavily into the auxilia of the early empire. They are attested frequently in the 1st century CE and it is clear that the Romans valued them highly. It was Batavian cohorts that broke the last tribal defense of the Caledonian tribes at the battle of Mons Graupius in 83/84CE, ensuring that no Roman blood was spilled.


…victory would be vastly more glorious if it cost no Roman blood.
[Tacitus, Agricola, 35]


In the later 3rd and 4th centuries CE Roman army manpower issues would necessitate the wide-scale recruitment from Germanic as well as other tribal contingents from even beyond the fringes of empire. This was all part of a legacy that attested Rome’s grudging respect, at least in war, for their fierce northern neighbors.


Auxiliary Units of the Roman Army

roman horsemen trajans column
Speculum Romanae Magnificentiae: Roman Horsemen Crossing a Bridge (from Trajan’s Column), by Marco Dente and Antonio Salamanca, 16th century, via Met Museum


The ancient Roman army utilized the military services of many varied indigenous peoples. Those named here are just a small sample of the most famous. Perhaps we should not be surprised. Indigenous exploitation has been a feature of empires all through history. However, what is surprising, is how certain auxiliary units clearly fulfilled specialist roles for which the Roman army were themselves deficient. From the light skirmishing role of Balearic or Cretan missile troops, to the cavalry role of Numidian, Gallic, or Germanic horsemen, Rome needed these skills.


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Roman Glass Fragment Depicting Arms, 1st Century CE, via Met Museum


Foreign auxiliaries were present throughout the history of the Roman army. Usage seemed particularly prominent towards the end of the Republican era, but in truth it endured and adapted into the imperial periods also. It just became more standardized. Many distinct indigenous and allied units became uniformly assimilated into the auxiliaries of empire over time. Romanization also diminished the differences as indigenous people slowly attained status and citizenship.


The Romans were the ultimate imperial pragmatists. The Roman army was mighty, but it was — at least in part — due to its use of foreign manpower. Military and social assimilation was the true strength of ancient Rome.

Author Image

By Colin J CampbellMLitt in Ancient History, BA Ancient History & CivilizationColin J Campbell is a contributing writer and researcher, living in Melbourne, Australia. He currently writes across a wide range of creative non-fiction topics. He has strong interest in writing, visuals and sounds. Originally from Scotland, Colin studied Ancient History and Civilizations before completing an MLitt (distinction) in Roman history from the University of Newcastle. Focusing on ‘Slave, Bandit and Pirate Disorders’ within Roman Italy, he developed interests in the personal security habits of Romans. Colin also has expert knowledge in a wide range of topics that include military, politics, architecture, society and social issues.