..... It was the Goths and Huns who really did in the West, and both groups initially dealt with the East before moving West...showing that the East was at least able to handle the barbarians better than the West was.
The East did a poor job dealing with both. The crux of the problem was that the new christian population was not enthusiastic about fighting for the Empire, so the latter lacked the strength to either expel the goths or really get them under control. The East was slaughtered by the Huns in the 440s and was very lucky their onslaught was limited to the balkans. At one point an earthquake toppled part of the walls of Constantinople; had it not been fixed just in time the East would've been battered far worse than it was. About the only halfway effective way to "handle the barbarians" was to pay them protection money. Compare that with what the still pagan Empire did in the late third century.
As for the Barbarians who entered the West, I'll admit that there was opportunism more than anything involved. However, they were able to get as far as they did in because the West was too busy handling or recovering from the Goths and Huns in Italy, and couldn't spare the resources to adequately defend North Africa. So, yeah, it was still the actions of the primary groups that enabled the Vandals, Alans, etc. to do as well as they did.
Again the real crux of the problem was military weakness, due to the abandonment of the Empire by its citizens. Stilicho had to fight with a mostly barbarian force and when it was stupidly alienated in 408, the WRE was practically defenseless.
The goths and Huns had nothing to do with the failure to defend North Africa. In 429-39 CE, when North Africa was going down the drain, both the goths and Huns were quasi allies of the WRE. Aetius made good use of the Huns in Gaul, while just about the only soldiers Bonafatius had were gothic mercenaries.
The Goths, in fact, had crushed some of the Vandals in Spain at the behest of the WRE c 416 CE.
As for the WRE needing time to recover from the setbacks of 407-10, it had had well over a decade by the start of the Vandal invasion of Africa in 429. The problem was that the citizens generally would no longer fight and the barbarians, after 408, were reluctant to serve in the regular Roman army (many now did so only as mercenaries or "federates" under their own leaders). Ergo, the WRE tended to be chronically weak--even before the key revenue of Africa was lost.
Again compare that with the admirable comeback of the Empire after the worse setbacks of c 250-60.
Do not forget that, though it was long and painful, the East was able to survive the Slavic invasions later on, and that without Egypt and the Levant, though it was painful. Thus, the East was decidedly better than the West at dealing with barbarians.
Oh, and if Christianity was what caused the fall of the Empire because Christian societies can't fight as well, why was it the Christian Goths rather than the Tengriist Huns who took control and set up successor states?
Lol, it took a while for the christians to abandon pacifism--too late for the WRE. Christians could always fight, in fact, for example against fourth/fifth century heretics. Generally, though, they were pacifistic---at least down to the fifth century (Augustine introduced the concept of a just war, but it didn't seem to make any difference at first). And more was involved than just the pacifism of many early christians. There was also historical baggage. Christians could hardly be expected to fight hard for their old persecutor, and the killer of christ. At least one of the early church fathers made this attitude clear.
Now the East, being originally intended to be a christian thing from the start, since Constantine, was a bit better off in this regard. But luck played an important role in its survival in the fifth century. The Persians were mostly quiet and barbarians didn't spread far and wide on its territory.
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