How do civilizations fall? Islamic thinkers had an image for it. Consider a civilization based upon a court in a thriving city— Baghdad, for example. Arts and the intellect flourish. But over several generations, as the great Islamic philosopher of the fourteenth century Ibn Khaldun put it, the civilized become decadent with luxury. They lose their sharpness and think only of the good and the beautiful. And then some tribe of fierce Bedouin, smelling out weakness, come thundering in from the desert and storm the city. As barbarians, they do not understand the usages of civilization. They stable their horses in the libraries and use sculptures as doorstops, pictures for target practice. Given a pillow, Ibn Khaldun tells us contemptuously, they suppose it to be a bundle of rags. In time, however, the power of a superior culture is felt, and these people adopt and sometimes extend the ways of civilization, until they too are overthrown in their turn.
This is the way the world goes. Sometimes it happens in one lifetime, as with those barbarian soldiers who rose to become Roman emperors, sometimes in slow motion, as with the fall of the Roman Empire, in which many centuries were to elapse before the new civilization emerged from the disorders of the barbarian invasions. With us, the decor is quite different, but the realities may be closer than we suppose.
One reason we may not realize this is that the very distinction between barbarism and civilization has been suppressed by the current relativism, which asserts the equality of all cultures. Nobody, of course, seriously believes this. Quite apart from technology, the moral inequality of cultures is conspicuous in the position of women in different cultures. It was only the West that abolished slavery. But it is a mark of current decorum—perhaps avoidance of the dreaded "triumphalism"—that we should not proclaim any superiority in European civilization, even though it is the one place the millions want to get into. Instead, we must make do with a rationalist doctrine which transposes the usages of European civilization into a rather unsatisfactory set of abstractions about universal rights. By this device, all cultures (including the Western) can equally be found guilty of moral violation in one way or another.
http://ontology.buffalo.edu/smith/courses01/rrtw/Minogue.htm