Where was the Roman Army in AD408?

[Nathan Ross]

It seems (as discussed in a previous thread) that the city of Rome was undefended by troops when Alaric besieged it in AD408. In fact he seems to have been pretty much unopposed by the Roman army in Italy: the five 'tagmata' under Valens that he defeated in 409 had been summoned from Illyricum; Honorius otherwise relied on Huns to defend him, and six more units from Constantinople to defend Ravenna in 410.

So what happened to the Roman field army of Italy? There was a big troop concentration at Ticinum earlier in 408, stationed to guard against/attack Constantine III in Gaul. They mutinied, but Honorius and Olympius seem to have brought them back under control - why could they do nothing against the Goths? After massacring Stilicho's people they seem to vanish from the scene!