A very stimulating essay Scipio; I am awed by your scholarship and the background research that must have gone into it.

A few comments:

1. I have always had my doubts about the view that the Empire’s frontier defences were ever a sort of hard shell that was capable of providing enough resistance to stop or decisively disrupt a serious attack. Maybe this is a result of living only a few miles from Hadrian’s Wall. The Wall is indeed impressive where it follows the line of cliffs or escarpments, but there was a great deal of it that simply crossed flatish open country. No doubt it prevented cavalry and wheeled traffic from getting through anywhere but at the few gates, but the forts and watchtowers were just too spread out to stop infantry. I don’t believe that the Picts had very much in the way of cavalry; they could have scaled the Wall any night they chose and been established in strength on the southern side by morning, or been deep into the Province had they so wished. I think the Romans must always have depended upon strength in depth and that the reforms of Constantine merely put an added emphasis on this.

2. I tend to favour the view that it was not so much any decline in the army’s efficiency or tactics as such that lead to the irreversible problems of the 5th century, but rather adverse demographic change and the resulting loss of the supporting infrastructure. Population levels in the West fell far more seriously than in the East. It is true that both parts of the Empire were affected by plague, taxes and invasions but the peasant base always remained larger in the East. I think it must have been easier for the populations in Asia Minor to recover since they were largely shielded by the Bosphorus from the cumulatively crippling effects of constant minor raids across the Danube and Rhine. The West’s problems would have been exacerbated by the tendency for the barbarians to turn west to Italy and beyond once the Balkans were devastated. They didn’t go East until much later.

3. Over time, and despite the Army’s best efforts, the potential recruitment pool in the West thus got lower and lower. The tax burden then became greater and greater as it began to fall on fewer and fewer people. This must have had a depressing effect on civilian morale, leading to popular revolts like the Bacaudae and a general willingness to view life under the barbarians as not much different from life under the Empire. However brave and efficient the Army might have been it could not exist in a vacuum. Once native recruitment and popular support died away the later Emperors had little alternative but to recruit barbarian tribes still lead by their own leaders, an ultimately fatal move.

4. Once all this is coupled with the increased pressures from beyond the frontiers as a result of large-scale tribal movements in the far East of Europe, I can’t see that the Western Empire had much of a chance in the long run whatever the organization or tactics of the Army might have been, and whatever defensive strategy may have been adopted.