Where was the Roman Army in AD408?

[Nathan Ross]

we can't really determine (with Elton here) to whaich extent the standing army was manned by non-Romans

Yes, although I think Elton is maybe erring too much on the 'Roman' side in his analysis. His list of individual soldiers and their origins - particularly the ones from the Concordia inscriptions - appears to be largely determined by their names. I mentioned 'Gunthia' above - if Flavius Silvimarus hadn't helpfully noted that he also had a Germanic name, we would have assumed he was entirely Roman! - likewise with the earlier Silvanus, son of the Frank Bonitus, and the same might be true of many of the other 'Roman' names on Elton's list.

In fact, Elton's 'probable Roman' soldiers might more accurately be called 'indeterminate' - which would alter the balance of 'Roman' to 'Non-Roman' considerably...

So indeed, the palatine regiments may have been filled with non-Roman warriors, loyal to a man and less to a state

Which sounds rather like the situation in the late Republic, with Caesar's legions...

I notice that even those historians most keen to play down the differences between 'Roman' and 'Germanic' soldiers in this era still have to fall back on phrases like 'the regular Roman army' and 'Roman troops' when discussing, for example, the anti-Stilicho mutiny at Ticinum in 408, or the massacre of the Gothic families which followed. This 'regular Roman army' seems to be a bit of a chimera, appearing at one moment and vanishing the next!

namely the Notitia. The West still had tens of thousands of men available as field forces.

Bearing all of the above in mind, I find it hard to believe that the Notitia can possibly have been an accurate record of the military situation in the west at the time it was supposedly composed (or last updated...) Unless the field army of Italy, for one, was a sort of paper tiger - all those gloriously-titled 4th century palatine and comitaneses units actually worn down to a few hundred new recruits, conscripts or 'barbarian' soldiers - it's difficult to see how such a massive force could melt away for several years during Alaric's sojourn in Italy, only to reappear again some time later.

While we do know that at least some of the palatine auxilia units from the ND - the Cornuti Seniores, and the units of Batavi, Bracchiati and Heruli etc from Concordia - existed during this period, we have no way of knowing quite what sort of state they might have been in, or what kind of men might have served in them.

So 'tens of thousands' might be a stretch, when we consider that marching 6000 men down from Dalmatia provided a stronger and more disciplined force than relying on units much closer at hand.

the suppression of Magnus Maximus... the border forces "raided" for their manpower to replenish the field forces, which probably happened again after Eugenius was defeated...

Yes, the defeat of Maximus, and then the very gruelling two-day battle at the Frigidus, must have severely depleted the field armies of both Italy and Gaul. They're both presented as victories - for Theodosius, of course - but could equally be seen as massive defeats for the military manpower of the Roman west, which may never again have recovered.