Where was the Roman Army in AD408?

[Nathan Ross]

Aetius' and Majorian's army at Vicus Helenae is explicitly stated as Roman

Maybe so - but can we be confident about the meaning of 'Roman' by that point?

Besides, they would surely have been able to take over the armies previously fielded by the successive Gallic usurpers Constantine III and Jovinus, which would presumably have comprised the nucleus of the old Gallic field army and whatever limitanei units they had drawn from the Rhine and Britain. But this force would not have been available to defend Italy in 408-10... (or subsequently, really)

To be fair, there's no effective way to interpret that.

I suppose we can't be certain. But it does seem a fairly good interpretation that the defense of Italy in June 440 was no longer in the hands of a regular central field army, if small-scale garrisons and armed civilians had to make up the lack! And the law about 'restoring' the army dates from March of the same year, which suggests the process of 'restoration' was quite a new thing at that point.

But that's impossible for it not to have existed.

I would think it more likely that the Italian field army did not exist, than that a force of thirty thousand trained and disciplined Roman regular troops stood idly by at some depot in northern Italy while Alaric and his Goths rampaged up and down the peninsula for nearly three years!

However, you're surely right that some Roman military forces existed - I found one inscription, CIL 06, 3296, from Rome: de numer]o cornutorum seniorum / dd(ominis) nn(ostris) Honorio A[ug(usto) et Theodosio co(n)s(ulibus) - which I make to be AD407 (Honorius cos. VII and Theodosius II)

So if the Cornuti Seniores were still around - perhaps they accompanied the emperor on his visit to Rome that year? - then presumably other units from the ND would have been in existence too. But I don't think we can extrapolate from this that the full army list was still current at that date, can we?

As you've pointed out, most numbers given for armies of this period seem to be surprisingly small. If Honorius could pull 6000 troops from Dalmatia to 'defend Rome', I would think Stilicho's original force at Ticinium could hardly have been much larger.