You make some good points, Volusian but I am totally convinced that the Roman Empire of the early 2nd century depended on its walls and the natural features to keep them barbaroi out. I think the sheer fact that Hadrian's Wall (although living in Ireland we didn't see much of the Romans) was built, maintained and garrisoned is evidence that some sort of preclusive strategy was in place.

Having said that I would would accept the idea that the wall acted much like the Great Wall of China in that its main raison d'etre was to make it difficult, if not impossible, for the invaders to get out should they get in.

As for the Danube/Rhine frontier, I think that the number of legions stationed on the borders throughout the 2nd and 3rd centuries suggests that the Romans were trying to keep the Germanics out instead of beating them when they came in. By allowing the legions to settle permanently on the frontiers, Severus started a dangerous precident. While the men may have been now fighting for their own land, they were not living in a barracks full time and being a legionary was no longer their full time occupation, suggesting that discipline slipped.

It is not like the Western army collapsed over night and for the remainder of the 4th century it was superior to the Eastern army, which suffered the defeats in Persia and at Adrainople. True, the east did win at Frigidus but that was due to a freak wind and a barbarian 'human shield' attack.

I tend to think that it was not a population shortage as in the population was dropping but in the way that between 360 and 410, the number of wars and battles fought throughout the empire had seriously depleted the number of available men to serve. These were then replaced by the only ready source of manpower - barbarians. Arther Ferrill cites the Russian archduke who said "I hate wars, they spoil the armies"

As for the West not having a chance against the Wandering of the tribes, with the calibre of leaders and the situation the Empire was in I also find it difficult to see how the Western Empire could have survived. She did not get the luck she had had in the past with finding a suitably great leader at the right time. Valentinian I was a great leader but died before he could make any long term impact. What the empire really needed was a logistical genius like Julius Caesar or Trajan be alas that was not to be