So I was actually asked this a few months ago by Dr. Jeremy Swist in regards to a Metal album, lol.

So let's start with the first problem (one I routinely have to rectify): the concept of foederati and "barbarian mercenaries."

The problem is people have this idea that Germanic troops who served in the Roman army were simply paid money and expected to show up with their own equipment, were given no training, were only marginally under Roman authority, and were more akin to modern mercenary groups. None of that is fundamentally true.

Foederati were men who were enrolled under the foedus, which by the 6th century had definitively taken the form of the granting of the rights to collect pay via allotments of taxable income inside Roman territory in exchange for administrative or military service. This is best exemplified in the law codes of the Visigoths, Burgundians, and Ostrogoths. In fact the entire Ostrogothic military effectively worked on this premise, more or less, at least, although in the post-Roman successor kingdoms you see a lot of men granted the rights to this income (called hospitalitas) push to claim the land itself, not just the fraction of income they got from the taxable portions of the land. Cassiodorus' variae is a big source of evidence on this happening in Ostrogothic Italy. In fact, many of these men would have been Romans, since Romans composed the majority of the local population in the regions these barbarian groups were "settled."

There was also another type of foederati which stemmed from men enrolled via treaties with groups outside the empire, where these men simply received their pay the same way as any other Roman soldier, rather than through hospitalitas.

By the 6th century, the organization of the foederati seems to have been in professional regiments, and may have already been in the 5th century. These men were paid a different way, and were largely barbarus or semibarbarus in origin, but were recruited, trained, and supplied like other Roman military units. By the late 6th century, the term had been integrated into Roman military language to the point where simply referred to the middle formation of the battle line, which makes it clear that they were seen as part of the professional army.

The other forms of military service by "foreigners" was in the form of bucellarii or symmachi, the former of which can be thought of as mercenaries, and the latter of which can refer to mercenaries but usually means allies.

Bucellarii were men recruited into the private retainer of a high-ranking general, a practice which came in during the late 4th century, and their size had to be legally capped during the reign of Honorius since some prominent Roman generals (magistri militum) were recruiting large bodies of private soldiers. Bucellarii (meaning "hardtack-eaters") were supplied and paid by whomever they were loyal to, with men like Aspar or Belisarius (and probably Stilicho and Aetius too) having small armies of around 2,000 to 4,000 men personally on their payroll. They didn't have to be foreign though - Romans served as bucellarii just as often as Goths, Alans, or Huns. Eventually, in 7th century, these would all be banned and formalized as a guard unit, and eventually fell out of use entirely.

Symmachi are basically "allies." This can be seen as similar to the socii of the Roman Republican army. They can be Armenians, Ghassanids, Lakhmids, Tanukhs, Heruli, Huns, etc. etc. Sometimes they're small bands of mercenaries (like the Huns in Libya in Synesius writings), sometimes they're whole armies provided by a state allied to the Romans.

Now that that's sorted, let's move on to part 2: military equipment.

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