The ERE can be badly beaten,but it can't be totally defeated or lose much lands because they can always rebuild their army.That's why whenever a barbarian group beats the ERE,they just ask for the money--because both the ERE and the barbarians knew that it's going to end up being much costly for both sides if they continued fighting.The army that these barbarian groups defeated also only a fraction of the ERE's army.The ERE still have armies in the Orient.
In theory the WRE had armies in Gaul, Spain and North Africa yet the barbarians incurred little resistance in those areas. The Huns (and perhaps others) could've overrun much of the ERE after destroying its (best) Balkan forces. But the barbarians mainly targeted the west.
The empire was definitely close to crumbling in the late third century,the fact that the empire survived had a lot to do with the fact that they haven't reached pitch bottom yet.
It was at its nadir then, due to massive defeats and raids added to plague, civil war, inflation...At no time before or since did the empire face such a combination of daunting problems occurring more or less simultaneously. From the point of view of manpower losses, the plague--killing a high percentage of the empire's population--and massive defeats around midcentury, were much worse than fourth century setbacks. Yet Roman armies not only remained effective in the years immediately after the mid century crises, they were still very large at least down to 363.
Apart from civil wars and enemy invasions which continued after the crisis of the third century,
It's noteworthy that after the civil wars of Constantine's time, Roman armies remained very big c 351 CE and even after the high losses in the latter period, they were still big in Julian's time.
there is evidence that there was some kind of climate change,with the weather becoming progressively colder,so that further contributed to economic decline.The climate got so bad that the Rhine froze over and allowed the barbarians to cross.Even before the collapse of the Rhine frontier,the WRE was often raided by the barbarians,so that didn't make it a safe environment for citizens to prosper.
But the Romans down to 407 must've revived after setbacks, as Gaul retained cities to then. I don't think the climate was great either throughout much of the medieval period but population increased much over what it had been in Roman times.
As mentioned before,massive civil wars continued after Constantine's temporary unification of the empire.
But loss of soldiers and population due to 4rth century civil wars were nowhere near as bad as third century losses. Why did the roman army do so well after 267, i.e. after
many millions of deaths from plague, massive defeats in the Near East and Europe AND economic ruin, AND civil wars?
I just can't believe the factors which did not preclude revival in the third century somehow proved so debilitating in the fourth. There had to have been another factor.
Even Julian struggled to scrap together an army to fight the Alamanni.The guy only managed to scrap together an army of fifteen thousand soldiers to fight against the Alamanni,that's how bad the situation was.
It couldn't have been that bad for just a few years later he led a very big army against Persia. And it was tactically proficient. Strategically the effort failed but that's beside the point. The Roman Empire, down to the real triumph of christianity in the later fourth century, retained ample military resources, and great recuperative powers.
Citizen unwillingness to serve is definitely there,but one of the reason why the empire hired these troops has to do with pragmatism as well.Since the early periods of the empire,the empire's prime source of recruits always came from the soldiers' children,given they are often trained from a young age. Once this source of manpower dries up(which is inevitable given the army got wiped out twice within a short space of time),the empire would have to recruit from the rest of the citizenry. These people do not form good troops.
I'd assume after the catastrophes of the mid third century the Romans must've conscripted lots of peasants with no prior training or experience--which wasn't such a problem once they are trained--and the Roman army performed very well as early as 268-69.
It's known that the quality of the East Roman army declined considerably after Adrianople for example due to the need to refill the legions through crash training courses.
The decline should've only been temporary--in the past it would've been (rebuilt Roman armies had won from before the time of hannibal to past the time of Shapur). Unfortunately by c 380 other factors were at play, sapping the fighting spirit of citizen troops....
As mentioned earlier,the Roman style of fighting has changed drastically from that of the third century and favored troops far better trained in individual combat.
That must've been after the disintegratian of regular roman forces.
The quality of barbarian troops has also steadily increased over the past few centuries due to increasing contact with Rome.
In the third century, Cniva's men seemed pretty proficient, but the Romans still ultimately beat the goths. Stilicho could also beat them.
Unlike citizen soldiers,this was already a trained source of manpower,so there's no need to spend money and time on training these troops...
Ferrill would certainly has disagreed with that! In his view rigorous formal training and discipline had always given Roman armies an edge over barbarians.
Nevertheless,the situation in 408 suggests however that citizens still formed a large part of the army. Olympus' coup against Stilicho was backed by military force. This force helped massacre the barbarian troops' families,suggesting that a sizeable Roman element was still in the army.
It doesn't take a very big or powerful force to massacre defenseless civilians.....Furthermore why didn't this "sizeable element" prevent the desertion of barbarians or break the sieges of Rome?? Why did the WRE seem so powerless after "the anti-barbiarian with hunt" as Ferrill put it?? Not just during the siege and sack of Rome but for years afterwards? It seems that by alienating barbarians, the WRE deprived itself of its key source of recruits, the implication being that
the bulk of its own citizens would no longer fight for it.
Barbarian troops in the service of the empire wasn't really a problem. What really was the problem was when these troops serve under their own chieftains rather than being under Roman officers after 408.
Of course, after the barbarians were treated like dirt in the regular army, they'd only fight under their own leaders--so called federates--or mercenaries who came without families.