Several answers may be suggested in explanation.
In the first place the process of disintegration was a slow one, for the whole tempo of life was slow and what might take decades in our own time took centuries then.

It is only because we can look back from the vantage point of a much later age that we can see the inexorable pattern which events are forming, so that we long to cry to these dead people down the corridor of the ages, warning them to make a stand before it is too late, hearing no answering echo, 'Physician, heal thyself!' They suffered from the fatal myopia of contemporaries.

It was the affairs of the moment that occupied them; for them it was the danger of the moment that must be averted and they did not recognize that each compromise and each defeat was a link in the chain dragging them over the abyss.
At what point did barbarism within become a wasting disease?
Yet from the first skinclad German taken into a legion to the great barbarian patricians of Italy, making and unmaking emperors, the chain is unbroken.

At what point in the assault from without did the attack become fatal?
Was it the withdrawal from Dacia in 270--allow the barbarians their sphere of influence in the east of Europe, fling them the last-won recruit to Romania and they will be satiated and leave the west alone?
Was it the settlement of the Goths as _foederati_ within the Empire in 382 and the beginning of that compromise between the Roman empire and the Germans which, as Bury says, masked the transition from the rule of one to the rule of the other, from federate states within the Empire to independent states replacing it?
Was this policy of appeasement the fatal error?
Was it the removal of the legions from Britain, a distant people (as a Roman senator might have said) of whom we know nothing?
Or was it that fatal combination of Spain and Africa, when the Vandals ensconced themselves in both provinces by 428 and the Vandal fleet (with Majorca and the islands for its bases) cut off Rome from her corn supplies and broke the backbone of ancient civilization, which was the Mediterranean sea?
Not once alone in the history of Europe has the triumph of a hostile rule in Africa and Spain spelt disaster to our civilization.
But if the gradualness of this process misled the Romans there were other and equally potent reasons for their blindness.

Most potent of all was the fact that they mistook entirely the very nature of civilization itself.