HOW do societies fail? Collapse can be precipitated by external invasion, but when an otherwise sound civilisation is overcome by the sheer military force of barbarians, it frequently succeeds in assimilating and converting the invaders, as the Chinese absorbed their Mongol conquerors, or the Persians civilised the Turks who ruled them for centuries.
This was even, in the very long run, the case with the Germanic barbarians who invaded the Roman Empire in the 5th century, although Europe had to endure five or so perilous centuries before emerging into the Middle Ages with a different ethnic composition and geographical balance.
The causes of this central event in Western history have preoccupied historians for centuries, but it seems clear that external invasion would not have succeeded without prior internal breakdown.
Gibbon thought the other-worldly concerns of Christianity distracted the Romans after their conversion, sapping their will to fight and eroding their belief in all the values that had sustained their power. But the decline was also manifest at every level in the failure to expend the resources needed to maintain complex infrastructure, failure to pay attention to the administration of temporal matters and the failure of education and training that resulted in a rapid loss of technical expertise in everything from engineering to art.