Focusing on Britain, the historian Michael Wood writes: Rome in the 4th Century had been a great power defended by a huge army. A century later the power and the army had gone.
Instead the West was ruled by new barbarian elites, Angles and Saxons, Visigoths and Franks. And nowhere were these changes more dramatic than on the very fringe of the Roman world in Britain.
Edward Gibbon, in his great book Decline and Fall, famously blamed the collapse not only on the barbarians, but on Christianity. He thought it had undermined society with its focus on another, better world.
Modern historians, though, see it differently, and some of their ideas seem startlingly relevant to us now.
First was the widening gulf between the social classes, rich and poor. When rich and poor start to live completely different lives this leads (then as now) to the poor opting out of the state. All studies today show that society is happier when the gap between rich and poor is reduced.
Widen it and you affect the group ethos of society, and also the ability to get things done through tax.
In the Roman West real wealth lay more in land and property than in finance (though there were banks) – but in the 300s the big land-owning aristocrats who often had fantastic wealth, contributed much less money than they had in the past to defence and government.
That in turn led as it has today to a “credibility gap” between ordinary people and the bureaucrats and rich people at the top.
Not surprisingly then, many people – especially religious groups – tried to opt out altogether.
Other strands in the collapse of the Roman West are more difficult to quantify, but they centre on “group feeling”, the glue that keeps society working together towards common goals. Lose that and you get a kind of nervous breakdown in the social order, which leads to what archaeologists call “systems collapse”.
The British historian Gildas (c 500-570) in his diatribe against contemporary rulers in the early 500s, looking back over the story of the Fall of Roman Britain, lists the military failures, but behind them he speaks bitterly of a loss of nerve and direction, a failure of “group feeling”.
Gildas talks about right-wing politicians advocating glibly attractive solutions that appealed to the populace while “any leader who seemed more soft, or who was more inclined to actually tell things as they are, was painted as ruinous to the country and everyone directed their contempt towards him”.