Foederati & auxiliary troops

The Roman army did not consist solely of Roman citizens. Besides the legionaries, there were also the auxiliae, troops composed of non-citizen allies, and later the foederati, barbarian peoples in the service of Rome. In this blog, we discuss the diversity of the Roman army and how these alliances became crucial during the turbulent Migration Period.

Foederati and auxilia troops were  ‘auxiliary forces’ of the Roman armies. Both types of warriors were recruited from non-ethnic Romans, with auxilia being residents within Roman borders and foederati being warriors from allied peoples. In this blog, you will discover the role of foederati and auxilia in the Roman world. 

Foederati were warriors recruited from peoples or cities allied with Rome. During the Roman Republic, the foederati mainly consisted of the socii , or allies. Later, in the Roman Empire, the term referred to foreign client kingdoms or allied barbarian tribes that provided military troops. Often, these groups were granted permission to settle within the borders of the empire in return.

Foederati were employed from the time of the Republic until the fall of the Roman Empire. After the third century AD, they became increasingly important and largely took over the role of the auxilia troops. Their contribution to the Roman army grew as the empire became more reliant on external forces to defend its borders.

A common image is that Roman rulers raided Germanic villages to demand children as tribute. However, this is a simplistic cliché. Foederati often came from Indo-European peoples with a culture that had many similarities to that of Rome. Like the Romans, they knew a society based on oath-bound reciprocity and a martial culture where individual achievements were central.

For the foederati, participation in the Roman army was an opportunity to gain fame, wealth, and status. For the aristocracy of the allied tribes, it also offered the opportunity to work more closely with Rome and benefit from technological and economic advantages.

For Rome, employing foederati was a practical solution: they provided specialized mercenaries who strengthened the army. At the same time, many ex-foederati learned from Roman military tactics and culture. They spread this knowledge in their own regions, leading to significant innovations such as the introduction of the rune script in the 3rd century AD and possibly the association with Wodan as the supreme god. Germanic peoples began building large wooden temples based on Roman temples. Likely, battle formations of later Germanic peoples were inspired by the Roman way of warfare. 

In the early Roman Republic, the foederati were allies of Rome who were obligated by treaty to defend Rome. They were not Roman colonies and did not have Roman citizenship. After 200 BC, the Roman Republic increasingly began to use non-Italian troops, especially cavalry. During the Julio-Claudian period (27 BC–68 AD), these auxilia were transformed from a loose group of volunteers into an organized standing army. They received a standardized structure, uniform equipment, and fixed service terms. The Latin tribes were considered blood relatives, but other allies were only bound by treaties. This unequal relationship caused tensions, which eventually led to the Social War (91-88 BC).

During this war, the Lex Julia in 90 BC offered Roman citizenship to allies who agreed to the terms. Nevertheless, some cities, such as Heraclea and Naples, refused to fully integrate into the Roman state. Even outside Italy, cities like Gades (modern-day Cádiz) and Massilia (modern-day Marseille) had a special status as foederati. Foederati would continue to play a central role until the fall of the Roman Empire. 

In the Roman Empire, the inhabitants were divided into different classes, with slaves as the lowest group. Roman citizens had special rights, such as the right to vote, hold political office, own land, and enjoy protection under Roman law. Only Roman citizens could serve in the legions, the main component of the army. However, outside Italy in the 1st and 2nd centuries AD, 90% of the population were non-citizens, the so-called peregrini, who had limited rights but no Roman citizenship.

Emperor Augustus introduced the auxilia in 27 BC, auxiliary troops composed of non-citizens serving in the Roman army. By the 2nd century AD, the auxilia were crucial to the army, both in numbers and specialization. They were as numerous as the legions and provided virtually all specialized units, such as light cavalry and archers. The auxilia were recruited from the peregrini, meaning Rome could attract soldiers from various provinces, even from areas outside the Roman sphere of influence. These non-citizens made up three-fifths of the land forces.

Although they did not have citizenship rights, many auxilia soldiers received Roman citizenship after their service, allowing them to benefit from the privileges that came with it. This aided both the military strength of Rome and the Romanization of the provinces. After the general grant of citizenship in 212 AD, the auxilia were largely incorporated into the citizen regiments.

The auxilia attracted volunteers, especially poor farmers who found the living conditions attractive. Soldiers received free housing, food, equipment, and were exempt from taxes. This allowed them to save money or support their families. The low pay was compensated by the benefits that came with the service.

Although auxilia soldiers did not enjoy the same benefits as legionnaires, such as the donativum (bonus upon the accession of a new emperor), they did receive Roman citizenship after 25 years of service. This brought significant advantages, such as tax exemptions. Historians suspect that the auxilia may have received a bonus starting with Emperor Hadrian, as military expenditures increased during this time.

By the 3rd century AD, more and more barbarian units, such as the Ala I Sarmatarum and Cuneus Frisiorum, joined the army. Native leaders and chieftains often also received command posts in the auxilia, where they possibly obtained knightly rank. The auxilia became increasingly important, not only as soldiers but also as part of the military aristocracy within the empire.

Initially, there were clear differences between legionaries and auxilia in training and combat capacity. By the end of the 2nd century, however, these differences had largely disappeared, and the auxilia were as well trained as the legions. Although the names of many auxilia units persisted until the 4th century, their size, structure, and quality changed significantly. By that time, the auxilia were heavily Romanized. As early as 212 AD, Caracalla, in his Edict Constitutio Antoniniana, granted full Roman citizenship to all free men in the Roman Empire. This changed the position of the auxilia. From the reform of Diocletian (284–305 AD), both types of armies were increasingly merged as comitatenses.

Auxilia units were often stationed in provinces other than where the recruits came from. This ensured security against uprisings and helped in the Romanization of the provinces. Through this dispersion, Roman culture was more widely shared and adopted.

During the reign of Emperor Claudius (41–54 AD), the organization of the auxilia underwent significant changes. Soldiers now had to serve 25 years to receive Roman citizenship, which also applied to their children. This was documented in the first Roman military diplomas s of that time: bronze folding tablets confirming the soldier's service time.

Claudius also decided that the prefects of the auxilia had to be of equestrian rank, excluding centurions from this position. This increased the prestige of the auxilia, as their commanders now held the same rank as the senior staff members of a legion. It is likely that the salary of the auxilia was standardized, although this is not certain.

By the end of the Julio-Claudian period, the weapons and equipment of the auxilia were standardized, similar to those of the legions. The main difference was that the auxilia had cavalry and specialized units, which the legions did not have.

Emperor Claudius expanded the Roman Empire with three regions s that became important sources of auxilia recruits: Britannia (43 AD) and the former client states Mauretania (44) and Thrace (46). Especially Thrace, like Illyria, provided a large number of cavalrymen and archers. By the mid-2nd century, Britannia housed the most auxilia regiments of all provinces.

Under the reign of Nero (54–68 AD), the number of auxilia soldiers was estimated to have risen to 200,000, divided into about 400 regiments.

In the 2nd century, the auxilia was significantly larger than the legions, which comprised about 155,000 men (28 legions of 5,500 soldiers each, including only 3,360 cavalrymen). During the second half of the century, the Roman army further expanded with five new legions (27,500 men), bringing the total to 33. Approximately 50 new auxilia regiments were likely added, reaching a pike of around 440 regiments and 250,000 men by the end of Emperor Septimius Severus's reign (211 AD).

In 175 AD, Emperor Marcus Aurelius stationed 5,500 surrendered Sarmatian cavalrymen at Hadrian's Wall. This was possibly an early example of a process where foederati were transformed into regular auxilia. This process increased in the 4th century, as evidenced by the Notitia Dignitatum, which lists many units with barbarian names.

Under Emperor Diocletian (284–305), the traditional legions, alae, and cohortes were divided into smaller units with new names. 

Under Constantine I (312–337), military units were classified into three ranks:

Palatini - Elite units in the imperial escort armies.

Comitatenses - High-quality mobile armies in border provinces.

Limitanei - Lower quality border guard units.

The old auxilia regiments formed the basic for units in all three ranks. Thus, about 70 alae and cohortes from the 2nd century retained their names and became border guard units (limitanei). Other regiments were incorporated into the palatini and comitatenses.

During the great revolt in Illyricum (6–9 AD), the Romans faced a severe guerrilla war in the mountains of Bosnia. The conflict lasted three years and resulted in enormous losses. The Roman historian Suetonius described this revolt, around 100 AD, as the most severe ordeal for Rome since the Punic Wars. In 9 AD, Tiberius eventually managed to suppress the revolt, but this coincided with another disaster: the destruction of three Roman legions by Arminius in Germania. The Roman army even suspected that Arminius had formed an alliance with the Illyrians.

Despite this uprising, the Illyrians later became, along with the Thracians, an indispensable force within the Roman army. By the 2nd century, nearly half of the army was stationed along the Danube frontier, where the legions and auxilia were largely composed of Illyrian recruits. In the 3rd century, the Illyrians even took over the higher officer ranks, a role previously mainly filled by Italians.

From 268 to 379 AD, almost all Roman emperors came from the Illyrian provinces of Dalmatia, Moesia Superior, and Pannonia. Notable emperors such as Diocletian and Constantine the Great belonged to this military aristocracy. The Illyrians were known as excellent soldiers and played a crucial role in saving the empire during the crisis of the late 3rd century.

The Batavians, a loyal ally of Rome, played a crucial role in Roman military operations. After the Batavian Rebellion, their regiments were rebuilt and sent to Britannia with General Petilius Cerialis. Here they served with particular distinction, both in Britannia and elsewhere in the empire, during the 1st century AD and beyond.

Even in 395 AD, long after the units no longer consisted solely of Batavians, they remained elite units within the army. Thus, there still existed equites Batavi seniores (cavalry) and auxilium Batavi seniores (infantry), classified as the best troops of the Roman Empire.

In 106 AD, Emperor Trajan defeated the Dacian kingdom under King Decebalus and made it the Roman province of Dacia Traiana. Around the middle of the 2nd century AD, there were 44 auxiliary regiments stationed in Dacia, about 10% of the total. In Britannia, there were 60 regiments. Together, these two provinces housed a quarter of all auxiliary units.

Archer units (sagittarii) were an important part of the auxilia. In the 2nd century, there were 32 sagittarii regiments, of which 13 were from Syria, 7 from Thrace, 5 from Anatolia, 1 from Crete, and the rest from other regionss. These units specialized in archery, but regular regiments probably also had some archers for independent operations. On Trajan's Column, three types of archers are depicted:

1. Syrian or Anatolian style: With scale armor, a steel conical helmet, and a cloak.
2. Unarmored archers: Dressed in a long tunic and a fabric pointed cap.
3. General auxiliary equipment: Resembling regular infantrymen but armed with bows instead of spears.

From 218 BC, sling specialists were mainly recruited from the Balearic Islands, known for their tradition in sling techniques. In the classical period, "Baleares" even became synonymous with slingers. Although independent sling units do not appear in inscriptions from the Principate, they are depicted on Trajan's Column. Slingers were unarmored, wore a short tunic, and a fabric bag for their ammunition (lead bullets).

Exploratores were reconnaissance units often active on the borders. Examples from 3rd-century Britain include units in Habitanco and Bremenio (fort locations). The term numeri refers to irregular units without a standard structure, often provided by Sarmatians and Germans. These units often served as border guards.

Besides the auxilia, the Foederati continued to play an important role during the Roman and Late Roman Empire. Under Trajan, their number was estimated to be 11,000 men, divided into about 40 units of 300 men each. They were valued for their specialized combat skills, such as Numidian light cavalry.

On Trajan's Column, foederati are depicted as long-haired, bearded men, barefoot and bare-chested, with long trousers and maces. This stereotyping distinguished them from the regular auxilia. In reality, their equipment varied by tribe. Their frequent appearance on the Column underscores their contribution to the Dacian wars.

A well-known example is the 5,500 Sarmatian cavalrymen deployed by Marcus Aurelius around 175 AD as a garrison at Hadrian's Wall, after their defeat in the Marcomannic wars.

In the 4th century AD, the foederati played an important role in the Roman army, especially after the auxilia were incorporated into the military reforms of Emperor Constantine I. Foederati were allies who fought for Rome in exchange for land and other benefits.

The Goths were recognized as foederati after a treaty in 332. In 358, the Franks became foederati when Emperor Julian allowed them to inhabit the plundered areas in Northern Gaul. They assisted the Romans in defending the Rhine, but in 406 and 407 the region was conquered by the Vandals and Alans, marking the end of the Roman presence along the Rhine.

In 376, the Goths asked Emperor Valens to settle on the southern side of the Danube, bringing them into the empire as foederati. However, they rebelled in 378 and defeated the Romans at the Battle of Adrianople. The heavy losses forced the Roman army to become even more reliant on foederati units.

The loyalty of these allies was not always guaranteed. In 395, the Visigoths, led by Alaric, rebelled again. General Stilicho, whose father also came from the foederati, defeated the Goths and Vandals in 406 with the help of other foederati.

By the 5th century, the Western Roman Empire was so weakened that it became almost entirely dependent on the foederati. In 451, the foederati defeated Attila the Hun. Among them were Visigoths, Franks, Alans, and Saxons. In 476, the Western Roman Empire was finally brought down when the foederati leader Odoacer deposed the emperor.

Despite the fall of the Western Roman Empire, foederati continued to play a role in the Eastern Roman army. The Ostrogoths, who settled as foederati in Pannonia, transitioned from allies to enemies of the Byzantines over the course of the 5th century. Ultimately, Theoderic conquered Italy.

In the 6th century, foederati remained part of the Eastern Roman army. They were often professional units composed of both Romans and non-Romans, such as Hunnic archers and Heruli mercenaries. Foederati were also formed from Arab tribes to defend against Persian allies, and these units persisted until the 9th century.