Roman Auxiliary Infantry of the Principate

After Augustus reforms the rather heterogeneous collection of auxiliary units serving Rome was completely reorganized, given regular status and trained to the same standards of discipline as the legions

The men of the Auxilia were freeborn non-citizens living on the periphery or the Empire with a heavy preponderance of Gauls, Thracians and Germans and were rewarded with the Roman citizenship on honourable discharge from the Army

The higher organization of auxiliary infantry was the cohors peditata that came in two flavours: quingenaria, modelled after the typical legionary cohors and the most common by far, and milliaria, modelled after the legion’s cohors prima

Cohors Peditata Quingenaria

The cohors peditata quingenaria was under command of a prefect and consisted of six centuriae of 10 contubernium totalling some 480 men. Around the second half of the 1st century appeared a new kind of unit named cohors peditata milliaria which included ten centuriae instead of just six, was under command of a tribune and had a theoretical strength of 800 men.

Cohors Peditata Milliaria

Besides these pure infantry units auxiliary were also organized into pure cavalry (ala) and mixed (cohors equitata) formations.

Roman Legions on the Principate

The Legions were composed of Roman citizens mainly drawn from Italy at first with a lot more coming from the Iberia provinces, Gallia and others by the end of the first century AD.

During approximately the first half of the Principate the legio was divided into ten cohortes plus a small number of mounted men known as the equites legionis. This cavalry unit was mainly devoted to communication, logistics and other non-combat activities and so the Roman Army of the Principate relied on auxiliary forces to provide mounted forces.

Early Principate Legion Organization

Each cohors was composed of six centuriae and each centuria comprised 10 contubernia of eight men each that fought, slept and trained together forming strong bonds within them.  The theoretical strength of a Roman legion was then 8 x 10 x 6 x 10 = 4.800 men

If you’re wondering why a unit of 80 men (10 contubernia of 8 men each) was called centuria (latin word for a hundred), the answer lies in two additional men allocated to each contubernium for non fighting duties, raising the total number to 100 (20 additional men per centuria)

At some time during the Principate, probably at the beginning of the Flavian era, the first cohors or cohors prima was reorganized to include five centuriae of 20 contubernia each and including the most veteran legionaries

Late Principate Roman Legion

In battle the Legio typically formed in what was called triplex acies with four cohortes in front and then two lines of three cohortes each in a kind of checker board pattern, even if it could use a two line deployment at times.

Legion Formed in Triplex Acies

Roman Army of the Principate

The Roman Principate comprises the period of time starting in 27 BC when Octavian was designated Augustus and Princeps by the Roman Senate and ending with the death of Trajan in AD 117. This period is also commonly known as the Early Roman Empire and was signed by a relative peace all across the Roman territory

Augustus decided to transform the Army into a fully professional force solely under the command of the Emperor even if he kept many of the names and terms of the dying Republic

The legion continued to be the core of the Army but its men were enlisted for a fixed period of time instead of the length of a campaign and there was a defined reward after honorable discharge from the service usually in money or land

The auxiliary forces were reorganized and given a regular status with training and service terms similar to that of legionaries, except that most auxilia were offered Roman Citizenship as the reward for their life at the Army

Finally the number of auxiliaries began to increase until they became a part as large of the Army than that of legionaries and often of similar fighting quality