Late Roman Field Armies

[Protectores Donutici]

What field armies existed during the Late Roman Empire?
Also, did late Roman legions still send vexillationes?

[Flavivs Aetivs]

Hey there.

You should check out the Notitia Dignitatum, which lists all the field armies from 395 to 419 AD.

http://lukeuedasarson.com/NotitiaPatterns.html

[ValentinianVictrix]

I would add a caveat to Evan's answer in that the make up of the field armies may have differed after 378AD when the field army commanded by Valen's was largely destroyed. The Notitia only show's surviving field armies at the time it was drawn up, and even then its estimated that it shows field armies in the East from around 390AD and those in the West to around 420AD, lots of room for missing ones.

Yes, they still sent detachments, and these could vary considerably in size.

[Nathan Ross]

they still sent detachments, and these could vary considerably in size.

This would depend on the exact era as well. Tetrarchic field armies were mainly composed of detachments drawn from the legions on the frontiers, some of which were still called vexillations into the 320s. But the word more commonly referred to a cavalry unit by this period.

The Notitia provides a sort of snapshot of the military organisation at an uncertain period (perhaps several periods combined). The field armies would be the comitatenses and palatinae, while the frontier armies (limitanei) comprised the units under the command of the dux of each province or area. The 'pseudocomitatenses' were probably former frontier units added to the field army at some point in the late 4th or early 5th century; other than that, I don't think we have any direct evidence of detachments augmenting the field army, although it almost certainly happened. The various units listed in the ND would have been assembled at different times, and with differing origins, but the process behind it all (if there was one) is still rather obscure!

[nikgaukroger]

they still sent detachments, and these could vary considerably in size.

This would depend on the exact era as well. Tetrarchic field armies were mainly composed of detachments drawn from the legions on the frontiers, some of which were still called vexillations into the 320s. But the word more commonly referred to a cavalry unit by this period.

The Notitia provides a sort of snapshot of the military organisation at an uncertain period (perhaps several periods combined). The field armies would be the comitatenses and palatinae, while the frontier armies (limitanei) comprised the units under the command of the dux of each province or area. The 'pseudocomitatenses' were probably former frontier units added to the field army at some point in the late 4th or early 5th century; other than that, I don't think we have any direct evidence of detachments augmenting the field army, although it almost certainly happened. The various units listed in the ND would have been assembled at different times, and with differing origins, but the process behind it all (if there was one) is still rather obscure!

IIRC troops under the command of one of the border duces are mentioned as accompanying Julian's Persian expedition - which might be a detachment augmenting the field army (unless they were upgraded to pseudo-comitatenses for the duration). But even then that would probably not be detachments from individual units at this time I guess.

[Frank]

If we define a vexillatio as units detached from an exercitus provinciae to an campaigning exercitus led by the emperor or another highest grade officer, then every unit sent by a dux to a field army campaigning near to the dux' province is a vexillatio in the classic (2nd 3rd century) sense.

And honestly, I cannot imagine, that e.g. the duces near the Rhine had not to contribute/detach units, if the Magister per Gallias started a campaign.

[Nathan Ross]

If we define a vexillatio as units detached from an exercitus provinciae to an campaigning exercitus led by the emperor or another highest grade officer, then every unit sent by a dux to a field army campaigning near to the dux' province is a vexillatio in the classic (2nd 3rd century) sense.

But surely a vexillatio in the older sense was a detachment drawn from a larger unit and operating under a vexillum, rather than their own standards (or, in the case of auxiliaries, several cohorts brigaded together under a vexillum)?

If so, the change in the use of the term might be because unit sizes had shrunk overall, meaning that entire units could be withdrawn from the frontiers without weakening the defences too gravely?

We do have the note in Ammianus about Constantius II ordering Julian to select several hundred men from some of his Gallic auxilia units and send them east, so there must still have been some mechanism for detaching smaller groups from larger ones. Seniores/iunores (again), perhaps?

[Frank]

From my understanding, the larger unit is always the exercitus. For example, if the Legatus Augusti pro Praetore Moesiae had to sent a detachement in order to support Trajans campaign starting in Syria he sent some legionary cohorts from his legions, plus some auxilia cohorts and alae. These are all full units of cohort size from my understanding. If all these units marched together to Syria under one vexillum or not is unknown as far as I know.

Well, there was also the case, that e.g. a detachement of the size of 2 cohorts from a legion was established via collecting 1000 men from all 10 cohorts of a legion. Or all alae of the exercitus contributed a few turmae, which were in total then a unit of ala size. At least there is a theory about it.

Size does not matter. Even if during late empire the units were smaller. A detachement is a detachment. Regardless if 5 men or 2000 men. Well, there was obviously a point in time, when the romans did not call a detachment vexillatio anymore. But that does not mean, that they stopped working with detachments. At least detachements from the duces directly onsite, where the campaign happens.

Of course the military strategy of the late empire with regional and central field armies was different than in the 2nd century. Limitanei were supposed to stay were they are and not move to the other end of the world. So I cannot believe, that a Dux Moesia had to contribute units to Julians campaign in Mesopotamia, like the Legatus Moesiae did in Trajans time. But the Duces of the eastern front (Syriae, Mesopotamia, Osroene, Palaestinae, ...) should have reenforced his campaigning army onsite.

[ValentinianVictrix]

Ammianus recounts several episodes where units from one field army were transferred to another. He also gave an account of the famous episode where Sebastianus was allowed to take 300 men from each legion, totalling some 2000 men, so that he could then lead a quick raid upon a large band of Goths camped by the Hebrus river north of Adrianople.
Julian combined at least one Western field army with several Eastern ones when he invaded Sassanid Persia.

[Robert Vermaat]

I would add a caveat to Evan's answer in that the make up of the field armies may have differed after 378AD when the field army commanded by Valen's was largely destroyed.

Which was, I think, what Evan said? The ND being drawn up c. 390, after Valens' defeat? 

The original question being about the late Roman field army, this very interesting discussion about vexillations and transfers is nonetheless digressing from that.

[Flavivs Aetivs]

The Notitia provides a sort of snapshot of the military organisation at an uncertain period (perhaps several periods combined).

It is. The Comes Dalmatae command isn't first attested until 402. The "Magister Militum" command structure changes under Constantius III (and again under Aetius). The Spanish army was probably destroyed between 409-414. Elements of the Spanish and British armies were moved into the Gallic army during the Notitia's timeframe. Finally, the Placidi Valentiniani Felices gives us the last date of edit, which is 419 or 425 according to AHM Jones (however I have seen suggestions of 428 by other authors).

[nikgaukroger]

Julian combined at least one Western field army with several Eastern ones when he invaded Sassanid Persia.

Adrian, do you base that on any actual information or is it supposition? I can't recall any specific enough information that would let us say anything further than he combined whatever troops he brought west in his usurpation with Constantius' army which he took over after the latter's death. I'd suggest that anything more is speculative.

[ValentinianVictrix]

Nik- We know that Julian formed three separate armies when he invaded Sassanid Persia and Ammianus recounted the fact that Julian had brought with him the very troops from Gaul that Constantius II had requested but who had refused to go East and who instead raised Julian to the Purple, causing the civil war between Constantius and Julian. Ammianus gave some information about units serving under Julian's command and these comprised of both Western and Eastern units. He would have had to combine field armies to create his invasion force and the fact that there were both Western and Eastern field army units in Julian's force indicates to me that to create the three armies for the invasion Julian must have combined field armies to achieve this.

[nikgaukroger]

Indeed and I said he did combine troops from east and west. However, what I was getting at was your statement about at least one western and several eastern field armies which I don't think the evidence supports such a definitive statement. How many field armies do we have evidence for in the reign of Constantius II? I think it may be 3 - which essentially are the armies of Constantine II, Constans, and Constantius II after their division of the empire.

[ValentinianVictrix]

There were at least three field armies in the East during Constantius' reign- under the commands of these magister's- Magister Militum per Orientem, Magister Militum per Thracias and the Magister Militum per Illyricum. Valen's himself commanded the Army of Thrace when he crossed the Danube in three campaigns against the Greuthungi Goths. Then there was the field army in Italy and at least one in Gaul.
Of course one of the great 'what if's' in the combining of field armies would have been if Gratian had reached Adrianople in time to combine his army with that of Valens (elements of Gratian's army were with Valens at the Battle of Adrianople as they had been despatched with Richomeres ahead of Gratian's main force).