When looked at through modern eyes, the various social, technical and/or economic idea's; such as that the Roman Empire in the West fell due to the loss of the tax revenues from the North African provinces, which in turn reduced the size of the army Rome could pay for seems logical. My problem with all these 'sophisticated' theories is simple:- If the barbarians were at the gates, ordinary people's homes, wives, children etc... were threatened as never before! Why didn't the millions of young men available within the Empire rally to defend their homes, even without pay? It has been suggested recently that ancient Rome was "a cross between Stalin's Russia and Mao's China"; but when the USSR faced a very similar threat in the 1940's, finding the money to pay his soldier's wages was the least of Stalin's problems. Did the warriors of Attilla quit because they hadn't been paid? of course they didn't, they merely looted another Roman province!!!
Despite having become unpopular with the politically correct brigade recently, the true cause of Rome's defeat was a centuries long decline in the calibre and unity of it's people, exactly as the ancient writers say. While things obviously moved at a slower pace two millenia ago; people's motivations and loyalties were basically the same then as they are today. For centuries Rome had imported conquered enemies by the million as slaves IE:- cheap labour. Rome also had a soup kitchen system where the urban poor could eat for free, thus preventing starvation. In times of hardship this system acted as a magnet to refugees from the less prosperous provinces, many of whom were none to happy with Roman rule in the first place. These twin immigration routes operated on the Empire's social and genetic base for generations, with radical effect on both Rome and Italy.
By the time Alaric's Gothic armies violated her frontiers, the empire's 'citizens' numbered possibly eighty million; but loyal citizens of genuine Roman blood, prepared to fight and if need be die; to ensure Roman victory, were far fewer. Threatened on a world war scale; for the first time in more than a century the Legions needed to recruit every fighting man the Empire had. The muster was a diverse horde of Gauls, Carthaginians, Syrians, Germanics, Sardinians, Africans, Spaniards, Jews, Dacians Lybians etc... none of whom felt genuinely Roman, many of whom had a close allegiance to the enemy, all of whom had a heritage of defeat at Roman hands, and most of whom were quite prepared to dump their weapons and run at the first sight of a barbarian warrior, killing any Roman officer who tried to stop them. Similarly, once conquered, these multi-racial masses genuinely saw no difference between barbarian German or civilized Roman landlords.
That's the reason; the Ancients tell us, caused Rome to fall. There were simply not enough loyal Romans left to defend her!!!
What do you do? and the lights went out; all over Europe, for a thousand years.............