Where was the Roman Army in AD408?

[Robert Vermaat]

So what happened to the Roman field army of Italy?

Technically the Goths were also a Roman army, albeit in opposition to Ravenna. That may have neutralised other armed forces as well - the mutiny may have been under control, but that did not mean Ravenna was 'in control' of those forces. Commanders who took over (such as Sarus) may have been very careful about whom to support. It wasn't a clear cut situation like 'we ousted Stilicho, now we can command his forces'. Olympius may have thought so (which is why he refuses Alaric's demands). he is shortly afterwards himself replaced by the praefectus praetorio Jovinus.

Even though many of the germanic troops who were loyal to Stilicho were either neutralized or went over to Alaraic (which I see more as a changing of sides by Roman units that as barbarians defecting to a barbaric army), the units weren't gone - we see plenty of military action shortly afterwards to against Contantine III, by Constantius III and between Boniface and Aetius. But having fought Alaric in the years before that, as well as beating Radagais and suffering a (no doubt not unanimous) mutiny so shortly before the siege of Rome, maybe the troop couldn't be sent anywhere. Maybe that was one of the grounds for the mutiny anyway?