My impression from reading Gibbons is that the West Roman Army "disappeared" after its defeats in 406 after the Alemmani crossed the Rhine.
Ah. This is the wrong impression to get.
Teeninvestor said:
Was it destroyed in battles? or did it linger on, unable to fight? Because by the time Alaric's visigoths entered Italy, it seems like there was no longer an army to fight them. I mean, they besieged Rome three times without interference....
The reason that Rome was besieged repeatedly was because the Italian field army was sent into Gaul to deal with the revolt of 'Constantine III', who took the legions from Britain and moved into Gaul. (That's actually one of the reasons why the Alemanni, Vandali et al weren't met closer to the Rhine by the Roman military; it was off suppressing a usurper.) Basically, from 406 to about 413, when the victories of Constantius III reconquered Gaul for the Empire, a three-way war was going on: Constantine III and his usurper troops were fighting the Vandali, Alemanni, Alani, Suevi, and the rest, who were also fighting Constantius, who was in control of the legitimist Roman troops, in bad need of reinforcement.
To deal with the problem in Gaul, Stilicho, who had been
magister militum before then and who had dealt quite ably with the invasion of Radagaisus, was engaging in some diplomatic maneuvering with the Eastern Empire. He wanted to acquire Eastern Illyricum so as to get the benefit of Alaric's Visigothi, who were settled there, to fight off his various enemies. This actually ended up leading to war with the Eastern Empire, which sucked up more military resources as Alaric and Stilicho banded together to try to force Arcadius, the Eastern Emperor, to heel quickly and get the crisis over with. They got lucky when Arcadius died in 408, but that was, in a rather roundabout way, Stilicho's undoing. He'd spent basically all of his political capital trying to get the Senate and Emperor to fund an alliance with the Visigothi, and people were already starting to get angry; when he left for Constantinople in 408 to oversee the transition of power, one Olympius managed to persuade the Western Emperor, Honorius, that he was merely trying to gain the Eastern throne for his son Eucherius. Stilicho's supporters were duly purged in the army and he himself was recalled to face execution at Ravenna.
Without Stilicho, the Senate, in a fit of lunacy, declared war on the Visigothi, which then descended into Italy. A good portion of the remainder of the Roman field army in Italy (well, what parts hadn't been sent to Gaul to fight the currently-losing battle against Constantine III) was mauled then, and even further in the fighting that ensued when Alaric promoted a Senator, Priscus Attalus, to the purple as a direct rival to Honorius. After that, though, the Empire not only got lucky, but some of its innate strength shone through. When Alaric died in southern Italy in 410, his Visigothi were under the leadership of Athaulf, who quit Italy for southern France. The aforementioned Constantius was appointed
magister militum and in a brilliant 411 campaign around Arelate shredded Constantine III's troops. Another minor usurper, named Jovinus, popped up; he too was put down, and by 413 all Gaul was back in imperial hands. The Alani, Suevi, Vandali, and Alemanni had mostly passed into Spain by this time (and would end up fighting a series of campaigns with the Empire and the Visigothi during the late 410s and early 420s). Athaulf, after playing some power games with the Roman princess, Galla Placidia, was the victim of a coup in 415, which installed a man more amenable to alliance with Rome, Wallia. Wallia and Constantius would finish cleaning up Gaul in the next few years and then carry the war into Spain.
Teeninvestor said:
After Alaric's retreat from Rome, there was technically a "Roman" Army in the west but it was made up completely of visigoths and other Germanic tribes.
That's not true at all. The Visigothi after the 410s were
foederati; they were settled in Aquitania and had the obligation to serve Rome militarily as an adjunct to the main army. Anyway, the
Notitia Dignitatum for 420, the list of all Western Roman military forces after the momentous previous decades, shows that the Western Roman military was comprised of some 181
legiones (not equivalent to Republican legions, which had several times more men; a better comparison would be a modern 'regiment'), irregardless of the levy from the Romans' various
foederati. The army had suffered some 47% casualties since the last
Notitia (in 395, at the end of the reign of Theodosius) among its
comitatenses, the regular field army units. That had largely been recouped numerically, however, by promoting units of
limitanei (frontier guards, basically) into the field army; these were
legiones pseudocomitatenses. This was, of course, in addition to the ~100 legions recruited since 395.
So yeah: definitely not "all barbarian" by any stretch of the imagination.
In addition, Gibbon didn't have the benefit of the last three hundred years of scholarship on the barbarians. Even in the last fifty years our knowledge of virtually everything in Europe and the Mediterranean during the Later Roman Empire has been transformed.
