Don't forget the Crisis of the Third Century when it pretty much went to hell. A lot of the damage that was done was economic Rome never really recovered from that period despite the legions managing to hold their own and eventually putting out all those fires everywhere. A good deal of the later Emperors basically did damage control to try and hold the empire together. But by the last days of the Western Empire, even if the Romans were better trained(Vegetius has a lot of say on this topic), there just weren't enough willing men left.
Despite what Vegetius spouts about 'Germanization,' the reequipping of the legions and new tactics were probably not the primary reasons for the total disintegration of the Empire. Heck, the East used similar tactics but managed to stay together wand wage continuous war against the Parthians for several centuries more.
@Christopher Burgoyne
I've never really liked the Roman strategy of defense in depth. Western Europe wasn't all that big and most of it was worth something. Combined with the fact that the 'barbarians' were fairly self-contained and had fewer logistical restraints, the strategy seems to be ill suited for the situation at hand. I'm sure there was reason to change from the tactics of the Early Imperial Period, but I never really saw it as a good idea...
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