(02-01-2020, 02:20 AM)Lothia Wrote: stationed at Xanten, but under which Magister Militum would that area fall under?
I think there are two separate questions here, one about XXX UV and the other about the Magistri.
The legion was presumably still based at Xanten in the early 4th century, as the new smaller fortress was apparently named
Tricensima ('thirtieth'), and was still known by that name by Ammianus. But the fortress was apparently sacked and abandoned c.AD352, so the legion must have left at or before that time.
(*Ammianus also mentions the 'soldiers of the thirtieth' being at Amida in AD359, which suggests that the whole legion might have been sent east, perhaps as they were thought untrustworthy after supporting Magnentius in 350-53. This would explain why the Franks found it possible to capture and destroy their fortress!)
The earliest Magistri Militum probably date to the last years of Constantine, when he set up separate army commands under his sons in preparation for dividing the empire between them. Each Caesar - including Constantine II in Gaul - may only have had a
Magister Equitum, or both a
Magister Equitum and
Magister Peditum.
After Constans took over Gaul in 340, he may have maintained both Magistri in the western provinces, or only one. The earliest known Magistri date from the 340s, although the earliest from Gaul is I think Romulus,
Magister Equitum of Magnentius, who was killed at Mursa in 351. The title is only known from Greek literature.
The first attested Magister in Gaul in Latin would be Claudius Silvanus, who is addressed in a law of either 349 or (more likely) 352/3 (C.Th 7.1.2) as
Comes et Magister Equitum et Peditum. He was a usurper at Cologne in AD355, so this was probably the same command, over the Gallic provinces and Germanic frontier.
As far as I know, none of the earlier Magistri have the additional title
per Gallias, as suggested in the Notitia. Zosimus mentions that Chariobaudes, killed in AD408, was
Magister (Utriusque Militiae?) per Gallias. Nannienus and Quintinus, campaigning across the Rhine in AD388, are just described as
Militares Magistri; presumably one was
Equitum and the other
Peditum. An inscription to Aetius in Rome calls him
Magister Militum per Gallias.