Questions on the deployment of auxiliaries and the praefectus castrorum's role

[Keeper of the Sacred Chickens]

Hello,

A recent Netflix show has just reignited my interest in Roman military history and so I come bearing questions. Rome's use of auxiliaries has always fascinated me and I was wondering, in the Principate, who was in charge of deciding which auxiliaries would be used in a specific campaign? And how would they decide whether or not to bring more units of archers or more units of cavalry etc.? Was there a general rule as to how many (and what kind of) auxiliary units should accompany a legion on campaign or on garrison duties?

I also have questions about the role of the praefectus castrorum. My understanding is that he was a lot like the modern quartermaster or logistics officer of the legion. But at the same time, he was a highly experienced centurion as well. So what would he be doing during a battle? Also, supposing his seniors were incapacitated, would he be in command of the legion plus the auxiliaries or just the legion?

Thanks!

[Nathan Ross]

Was there a general rule as to how many (and what kind of) auxiliary units should accompany a legion on campaign or on garrison duties?

(I've only just seen this post - after nearly 2 months!)

There's a common conception that Roman campaign armies tended to be split approx 50/50 between legionaries and auxiliaries - I'm not sure where the idea comes from, or how accurate or universal it might be (I think Arrian's 'Array against the Alans' follows more or less this rule?) but it seems to be a good general measure.

Many auxiliary units were based in close proximity to legions, either in the same fortresses or in neighbouring forts, so it would have been a simple matter to put together a campaign force by combining detachments from the various forts in the province(s) concerned.

Commanders could draw forces from further afield, and there is inscriptional evidence of (for example) a force apparently made up of the mounted components of several auxiliary cohortes equitata brigaded together and sent on campaign in (I think) Syria. For large scale campaigns this sort of long-distance troop movement would presumably need the emperor's own authorisation.

So what would he be doing during a battle? Also, supposing his seniors were incapacitated, would he be in command of the legion plus the auxiliaries or just the legion?

The Praefectus was in charge of the legion camp or fortress, as you say, so in normal circumstances he would not be found on a battlefield (unless things were going really badly!)

There were occasions when the Praefectus, as the highest ranking soldier present, may have taken command of his legion or of a mixed force of detachments (as he would have outranked any auxiliary commanders). Poenius Postumus, the Praef Cast of Legion II Augusta, was ordered to march his men from their base in (probably) Exeter to join Suetonius Paulinus against Boudica - he refused for some reason, and later killed himself in shame.

It's possible that Postumus was commanding a detachment of veterans of II Aug at Exeter while the main body of the legion was away on campaign in Wales. If so, the Praef Cast of Legion XX appears to has answered the call, as a veteran detachment of that legion was present at the final battle, presumably under his command. Alternatively, Postumus could have been command of the entire legion, with the legatus and senior tribunes strangely absent.

[Steven James]

Nathan wrote:

There's a common conception that Roman campaign armies tended to be split approx 50/50 between legionaries and auxiliaries - I'm not sure where the idea comes from…...

 

Apparently it comes from Tacitus. Sorry I don’t have the reference, and the last time I examined this reference, I did not find it open to doubt. However, from this opaque reference many academics have based the concept of the auxiliary and the legionaries as being a 50/50 split.

[Nathan Ross]

Apparently it comes from Tacitus.

Ah yes, that does sound familiar, now you come to mention it - thanks!

[Steven James]

I wrote:
 Apparently it comes from Tacitus. Sorry I don’t have the reference, and the last time I examined this reference, I did not find it open to doubt. 

That should read: I did find (the reference) open to doubt. In other words, Tacitus does not exactly state a 50/50 split. The Tacitus reference I think is associate with his description of the postings of 25 legions within the empire.

Found it. Tacitus (Annals 4 5 21) “Again, at suitable points of the provinces, there were the federate warships, cavalry divisions and auxiliary cohorts in not much inferior strength: but to trace them was dubious, as they shifted from station to station, and, according to the exigency of the moment, increased in number or were occasionally diminished.”

 

From the comment “auxiliary cohorts in not much inferior strength,” historians have come to the conclusion legionnaires and auxiliaries were around 50/50.

[Colonel Chabert]

John Spaul's book "Cohors" has a table entitle "Summary of the Distribution of units among the Provinces c.AD155". Adding up the totals, he has 28 legions @ 5,000 each giving 140,000 and then for the Auxiliaries 56,160 cavalry and 124,680 infantry giving a total of 180,840. So more men in the auxiliary cohorts and alae than in the legions.
The distribution between different provinces is very uneven with some provinces (eg Britannia) having a lot more auxiliaries than legionaries whilst in others (eg Moesia Superior) the situation is the opposite. This of course refers to where units were stationned and not where they might have gone on campaign but it does give an idea of the manpower that was available to choose from.

[Nathan Ross]

distribution between different provinces is very uneven with some provinces (eg Britannia) having a lot more auxiliaries than legionaries whilst in others (eg Moesia Superior) the situation is the opposite.

In other words, Tacitus does not exactly state a 50/50 split.

Thanks - interesting points.

Looking back at Arrian's list, meanwhile, his force comprises ten auxiliary cohorts, three of them milliaria, four cavalry alae and a 'numerus exploratorum', with two legions - one of these legions appears to be at full strength while the other appears to be a vexillation as it is led by tribunes.

If we assume an approx 500 men for a basic cohort or ala, and for the numerus, and double that for milliaria, and give the legions 6000 and 3000 men respectively, we do indeed come up with 9000 auxiliaries and 9000 legionaries. This is very rough, and 'paper strength' too, but might suggest that something like a 50/50 split was intended in at least this case.

[Steven James]

Nathan wrote:

Looking back at Arrian's list, meanwhile, his force comprises ten auxiliary cohorts, three of them milliaria, four cavalry alae and a 'numerus exploratorum', with two legions - one of these legions appears to be at full strength while the other appears to be a vexillation as it is led by tribunes. If we assume an approx 500 men for a basic cohort or ala, and for the numerus, and double that for milliaria, and give the legions 6000 and 3000 men respectively, we do indeed come up with 9000 auxiliaries and 9000 legionaries. This is very rough, and 'paper strength' too, but might suggest that something like a 50/50 split was intended in at least this case.

 

I’ve undertaken an analysis of Arrian. The method I used was to compare the march order with the battle order. Arrian makes no mention of differing depths between the legions, which for me is a clue to the size of the legions as the 15th legion is made up of vexillations and the 12th legion is at full strength. The mention of the “numerus exploratum” is another clue and for me this indicates they belong to the 15th legion, as the numerus exploratum is following the same doctrine of the allied extraordinarii of the early and mid-republic.

 

For the infantry I have calculated 8,000 legionaries to 6,000 auxiliary infantry, which excludes any light infantry (archers and slingers). However, for the battle of Mons Graupius in 83 AD, I do get a 50/50 split between legionaries and auxiliary infantry. It is possible with Arrian, that with the inclusion of the archers and slingers, it could be a 50/50 split.

 

Tacitus (Histories 1 59) has eight cohorts of Batavians belonging to the fourteenth legion. Then there is Tacitus (Histories 2 89) “The eagles of four legions were at the head of the line, while the colours of four other legions were to be seen on either side; then came the standards of twelve troops of cavalry, and after them foot and horse; next marched thirty-four cohorts distinguished by the names of their countries or by their arms.”

 

I am still undecided about a 50/50 split. A lot more research is required on my part. However, I feel a debate of a 50/50 split by historians has been poorly investigated.

[Robert Vermaat]

I also have questions about the role of the praefectus castrorum. My understanding is that he was a lot like the modern quartermaster or logistics officer of the legion. But at the same time, he was a highly experienced centurion as well. So what would he be doing during a battle? Also, supposing his seniors were incapacitated, would he be in command of the legion plus the auxiliaries or just the legion?

Does this have to do with the rather unsubstantiated theory that a certain praefectus castrorum of the VIth legion supposedly suddenly commanded all the three legions of Britain?
Or with the equally unsubstantiated theory that around the 180s, the function of the praefectus castrorum was 'evolving into' the function of praefectus legionum?

[Mark Hygate]

Whilst I have indeed done it myself on occasion, it is not easy to equate modern ranks and roles with those of the past - especially the far past of the, arguably, first really professional army.

Unless I have missed something, the Praefectus Castrorum rank first appears at the time (most likely) or not long after the apocryphal 'Reforms of Augustus'.  For my part I see it as a direct result of a 'tweak' to what more modern crafters of military establishments would call 'the Sergeant's Mess' (and you mess with that at your peril!).  More seriously a 'tweak' that took the 60 Centurion structure of the Legion and promoted the most senior to an actual 'Officer' (Equestrian) position, leaving the 59 Centurion structure of the Imperial-era Legion.  This was done because that provided an extremely experienced and professional soldier to back up the otherwise political and short term appointments of the Legate and Senior Tribune.

I strongly suspect that the Praefectus Castrorum, therefore, was responsible for all the regular day-to-day aspects of running the Legion (and any attached Auxiliaries); which, of course and then leading to the title, would mostly occur whilst 'in camp'.  The higher class Senators and Patricii filling the Legate and Senior Tribune would most certainly have not wished to be as much involved...

If you're after modern titles, then I'd suggest that the Praefectus Castrorum be considered as a 'Second in Command' (taking charge in the absence of the Legate) or perhaps more succinctly 'Chief of Staff'.

As an aside - given my interpretation as a 'second in command'; I would not find it impossible to believe that a Praefectus Castrorum could possibly have been the senior 'officer' in Britain - but only if all 3 Legates were somehow either: absent; on leave; incapactiated; or dead...

[Robert Vermaat]

As an aside - given my interpretation as a 'second in command'; I would not find it impossible to believe that a Praefectus Castrorum could possibly have been the senior 'officer' in Britain - but only if all 3 Legates were somehow either: absent; on leave; incapactiated; or dead...

Not to mention the two other praefecti castrorum

[Mark Hygate]

As an aside - given my interpretation as a 'second in command'; I would not find it impossible to believe that a Praefectus Castrorum could possibly have been the senior 'officer' in Britain - but only if all 3 Legates were somehow either: absent; on leave; incapactiated; or dead...

Not to mention the two other praefecti castrorum

Well, being sensible and experienced old soldiers the other two probably ran for cover first!