I have a great deal of experience in Late Antiquity but this question has always confounded me. The “Germanization” of the Late Roman army is hotly debated, but when reading about the disasters of Honorius’ reign I always come back to the central issue — where was the supposedly vast army of the Notitia Dignitatum? Even as late as Valentinian I’s reign and indeed the Battle of the Frigidus River in the 390s, the western military establishment seems robust and numerous.
So why didn’t Honorius’ regime just recruit and train Italians to bolster their forces? Why were they so utterly reliant on foreign manpower? What had changed to make recruiting Italian soldiery unfeasible?
The regimes of the 4th century seem to have utilized native recruits, if to a lesser degree than in the Principate. What was stopping the failing West from following this successful path?
For two reasons. First, Maxentius had only one security threat to focus on while Honorius had two, and second, because most of the Roman army in the year and a half prior to the sack of Rome either deserted or defected to Alaric.
Honorius in reality was not in control until 408. After the death of his father Theodosius, his father's main general, the half-German Stilicho, was placed in command of the forces in the West and ruled Honorius essentially as a regent. Stilicho's reign started well - he defeated Alaric and numerous outer barbarians in Illyria and the route to Italy, but in doing so had depleted the frontiers of Gaul. When the Rhine froze over in the winter of 406, a rode of Germans advanced on Gaul and found little opposition, causing chaos in the province. Capitalizing on the opportunity, Constantine III, a Roman commander in Britain, proclaimed himself Emperor and marched the British legions to Gaul. Bringing fresh troops won him the support of many of the Gallic people and Jovinus, the commander on the Rhine, and he quickly established control. Stilicho dispatched Sarus the Goth, one of his generals, to put down the uprising, but Sarus failed despite initial progress. Stilicho decided to seal the alps to prevent Constantine from marching on Ravenna, and at this point his credibility as the "defender of Rome" had taken a serious hit.
Stilicho in the mean time had been conspiring whit his old enemy Alaric to make mutual war against the Eastern Roman Empire, ruled in theory by Honorius's brother Arcadius but in reality controlled by military regent Anthemius just like the West. Anthemius came to power in the early 400s after the deposition and death of two of Arcadius's prior regents and most sources conclude that he and Stilicho did not get along at all. Stilicho believed that Illyria and Dalmatia were his according to Theodosius's partition of the Empire between "Arcadius" (Rufinus) and "Honorius" (Stilicho). However, Anthemius rose to power during the reign of Arcadius and cared nothing for the word of Theodosius or the agreements of the late general Rufinus. In exchange for a cash payment, Stilicho arranged for Alaric to attack Anthemius. However, he had trouble passing this agreement by the Senate, and "persuaded" (most likely bullied) them into agreeing.
The Western army at this point was essentially dominated by barbarians. After the rising of Constantine III, an estimated 45,000 men were still under the control of Stilicho in Italy. Of these, at least 30,000 were barbarian foederati or ethnic German citizens of Rome, who were over-represented in the Roman army due to a comparative lack of agricultural land (at least at this point) which made the army one of their only routes of employment and social advancement. Stilicho also probably had something to do with this, promoting Germanic officers like Sarus and likely stacking the Roman army with Germans in order to increase the power of his own "military faction". This faction was strongly opposed by the Italian aristocracy who had contempt for all Rome's many ethnic minorities from the Greeks to the Gauls to the Britons, but felt threatened by none more than the Germans. Stilicho's bullying of the Senate to accept Alaric's bribe payment was for many the last straw.
In 408, a rumor circulated with the death of Arcadius that Stilicho intended to crown his son Emperor of the East. This was a nonsense allegation, first because Stilicho was already fighting for his own survival against Constantine III and second because Anthemius had made peace with the Persians and had plenty of soldiers to enforce his own candidate, the young Theodosius II. However, it was used as political propaganda to recruit officers into a coup against Stilicho. The Italian faction arranged for the murder of many of Rome's senior officers before organizing pogroms in Italian cities against ethnic Germans. Honorius was either part of the plot from the beginning or welcomed an escape from the domination of his father's right hand man, and approved of the pogroms before ordering Stilicho's death.
After the death of Stilicho and assaults on their families, nearly all the ethnic Germans in the Roman army and the foederati either defected to Alaric or deserted. This was some 30,000 troops, leaving Honorius with less than 20,000 troops to see to the defense of the entirety of Italy. At the same time, Honorius's subordinate commanders were still campaigning against Constantine III, leaving yet another distraction.
Sources:
Kennon, Drakeford. Settling the Wandering Kingdom: The Establishment of the Visigothic Kingship under Ataulf.
Wijnendaele, Jeroen. Alaric, Constantine III, and the downfall of Stilicho.
Winjendaele, Jeroen. Generalissimos and Warlords in the Late Roman West.
Winjendaele, Jeroen. Warlordism and the Disintegration of the Western Roman Army.
Winjendaele, Jeroen. Enemy within: Barbarian Recruits in the Late Roman Army
This is interesting and I would like to investigate this further. The decline of the Italian army is fascinating, because even in 392 the Roman senators had to petition Arbogast and Valentinian II to send troops to Italy from Gaul just to deal with some raiders in the Alps, because supposedly they had no garrison at all. Do you know what sources say that Maxentius had 100,000 men?