The Roman Army in Britain
based on material from Peter Green's website:
Britannia
Part I: The Early Empire

Armies in the Ancient World had a range of functions. Fighting wars was only one, and for many soldiers it would have been a rare experience. The Roman army in Britain: The frontier systems should also not be thought of as modern frontiers. They were not the equivalent of Iron Curtains between Empire and the barbarians. They were zones that the army patrolled in front of, to ensure that allied tribes lived in peace, and that lawlessness did not encroach on the province. That lawlessness might be the modern equivalent of a single criminal or, rarely, an alliance of unfriendly tribes.
click The army of the first and second centuries had around 30 legions; each approximately 6,000 men strong, and recruited from citizens throughout the Empire. Legions consisted of ten cohorts, with six centuries of 80 men in each cohort, apart from the first cohort which, from around AD70, was double strength, ie six centuries of 160 men. There were also 120 mounted troops to act as messengers and scouts. The legion's commanding officer was the legate; appointed from the senatorial class by the Emperor. The other senior officers were six tribunes and 60 centurions. The legions fought as heavy infantry but also provided engineers, cadres of officers and men to assist the civil government, and officers for the noncitizen troops - the auxilia. The auxilia provided the army's cavalry and light infantry; the soldiers received citizenship when they retired. Auxiliary regiments were attached to a legion, and normally there would be the same number of auxiliaries as legionaries in the resulting group. The legion's commanding officer was in control of the whole group.
Detachments from legions, or occasionally from auxiliary regiments, operating on their own, or with other detachments, were known as vexillations (from vexillatio - the flag that identified them), and were the way of providing temporary reinforcements to provincial armies during major campaigns.
Britain's garrison fluctuated between three and four legions during the first century, depending on the demands of other provinces, but from the mid AD80s the number remained at three, though not always the same three. Assuming an equal number of auxiliaries this suggests a total army strength of between 36,000 and 48,000 men, making the British army one of the largest in the Empire. Legions, like all army units, were identified by numbers, but were not numbered sequentially or exclusively, and had titles that reflected their history and helped distinguish them. The British legions were: Other legions are recorded as serving in Britain. These are evidence either of vexillations to reinforce the garrison or the 'home unit' of officers commanding auxiliary regiments. A vexillation of 1,000 men each from the VII Gemina, VIII Augusta and XXII Primigenia for example is recorded on the tombstone of the commanding officer.
click Auxiliary regiments were either around 500 strong (called quingenaria), or, from the second half of the 1st century, could be around a 1,000 strong (milliaria). Milliaria units were rare - Britain had only one milliaria cavalry unit in its garrison. Cavalry units known as ala (ala - wing) are thought to have consisted of 16 turmae (troops) of 30 troopers, if they were quingenaria, or 32 turmae if they were milliaria. Infantry units, known as cohors peditata, had clicksix centuries with 80 soldiers to each; milliaria cohorts had ten centuries (like a legion's first cohort). There were also mixed infantry/cavalry regiments - cohors equitata. Their organisation is less clear, but are usually assumed to have had six centuries of 80 men, and four turmae of 30 troopers; a total strength of 600 men. Cohors equitata milliaria had ten centuries of 80 men and eight turmae of 30 troopers, making an establishment size of 1040, plus officers.
Like the legions, auxiliary units had numbers and names. The most usual part of the name showed where the unit was originally raised. There were, for example, five cohorts raised in Gaul, Cohors I - V Gallorum. Like the names of legions, their names became embroidered with the regiment's history.
Ala Gallorum et Thracum Classiana invicta bis torquata civium Romanorum was raised in Gaul during the reign of Tiberius; it took the title Classiana from the name of its first commander; the addition of a contingent of Thracians gave it the et Thracum; it gained the title invicta (invincible), and the honour of a torque twice - hence bis (double) torquata - and the grant of citizenship to all of its serving men, civium Romanorum, through its achievements in battle. The first torque was possibly gained during the reigns of the Flavians (Vespasian, Titus and Domitian) or Trajan, and the second during Trajan or Hadrian's reign; on both occasions in Britain.
Legions and auxiliary units would rarely be at full strength. Sickness and the provision of men to assist the civil administration could reduce units to around half their theoretical strength. Full complements might have been only achieved at the beginning of campaigns. Writing tablets, excavated at Chesterholm, listing unit rosters, show that units could often be considerably under strength, and that regiments might also be divided between several forts, sometimes mixed with troops from other units. Despite their unit titles, regiments recruited locally. Ala Gallorum, although initially raised in Gaul, would have started to recruit locally once in Britain, and would have been made up of Britons by the time it moved to Germany in the AD120s.
Fleets (classis) provided the Empire with both sea power and logistical support. In Britain, until the late third century, the Classis Britannica appears to have operated almost entirely in the logistics role: transporting the invasion army; provisioning the units in northern Britain; and managing the iron industry in the Weald of what is now Kent and Sussex.
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