I think two main ones are discoverable. The one physical, arising outwardly from the course of events; and the other moral, working inwardly from the mental state of man himself.
The physical cause was the prolongation of the invasion. We are not to conclude that the invasion of the barbarians was arrested at the fifth century, nor that because the Roman Empire had fallen, and barbaric kingdoms were founded on its ruins, the populations brought their movements to a close. On the contrary, they continued long after the fall of the Empire, of which we have all-sufficient proof.
We see the Frank kings, even of the first race, continually compelled to make war beyond the Rhine; we see Clotaire, Dagobert, incessantly engaged in expeditions into Germany, fighting against the Thuringians, the Danes, and the Saxons, who occupied the right bank of the Rhine. For what reason? Because those nations wished to cross the river, and gather their share of the spoils of the Empire. What caused, about the same period, those great invasions of Italy by the Franks established in Gaul, principally of the eastern or Austrasian Franks? Why did they precipitate themselves on Switzerland, pass the Alps, and enter Italy? Because they were pushed on the north-east by new populations; their expeditions were undertaken from necessity, and were not mere forays for pillage; their settlements were interfered with, and they went forth to seek others. Then a new German nation appeared upon the stage, and founded in Italy the kingdom of the Lombards. In Gaul, the first Frank dynasty was subverted: the Carlovingians succeeded the Merovingians. It is now acknowledged that this change of dynasty was in truth an accession of population which displaced the western for the eastern Franks. The change was effected, and the second race reigned. Charlemagne began against the Saxons what the Merovingians had directed against the Thuringians, and became involved in ceaseless wars with the nations beyond the Rhine. And these were urged onwards by the Obotrites, the Wiltzes, the Sorabes, the Bohemians, by the whole Slavonic race which pressed upon the Germanic, and from the sixth to the ninth century goaded it to advance towards the west. To the whole of the north-east, the invading movement continued and controlled events.
In the south a movement of the same nature occurred, occasioned by the Moslem Arabs. Whilst the Germanic and Slavonic populations crowded along the Rhine and the Danube, the Arabs began their career of conquest on all the coasts of the Mediterranean.
The invasion of the Arabs had a peculiar character. The spirit of conquest and that of proselytism were united; their invasion was made both to conquer territory and spread their faith. There was a great difference between this movement and that of the Germans. In the Christian world, the spiritual and temporal arms were disjoined. Zeal for the propagation of a faith was not felt by the same men who burned with the desire of conquest. The Germans on their conversion had preserved their manners, sentiments, and tastes; earthly interests and passions continued to sway them; and though they might be Christians, they were not missionaries. The Arabs, on the contrary, were conquerors and missionaries; with them the sword and the Word were wielded by the same hands. At a later date, this circumstance gave the unfortunate turn to the Mussulman civilisation; for it is from the unity of the temporal and spiritual powers, from the confused mixture of moral influence with material force, that the tyranny which seems inherent in that civilisation took its rise; and such is, as I believe, the principal cause of the stagnant state into which it has fallen. But this was far from appearing at the first outburst; on the contrary, a prodigious power was thereby imparted to the Arab invasion. Strengthened as it was by moral ideas and passions, it gained, upon the instant, a lustre and greatness which had been signally wanting to the German invasion; more energy and enthusiasm were displayed in it, and the minds of men were affected by it in a very different manner.
Such was the situation of Europe from the fifth to the ninth century; pressed on the south by the Mohammedans, on the north by the Germans and Slavi, the interior of the European region was inevitably kept in continual disorder by the reaction of this double invasion. Populations were incessantly displaced and hurled upon each other; no settlement could be established; the nomade life recommenced in every quarter. There was certainly some difference in this respect amongst the various countries; the turmoil was greater in Germany than in the rest of Europe, for it was the very furnace of agitation; and France was more convulsed than Italy. But nowhere could society get fixed or regulated; barbarism was prolonged on all sides, from the same cause which had given it a commencement.
So much for the material cause which sprang from the course of events. I now come to the moral cause, founded upon the internal state of mankind, which was not less powerful.
Whatever external events may be, it is, after all, man himself who makes his world; it is from the ideas and sentiments, the moral and intellectual dispositions of men, that the world is regulated in its progress; it is upon the inward state of men that the outward state of society depends.
What is needful to men in order to found a society at all durable and regular? It is evidently requisite that they have a certain number of ideas sufficiently expansive to suit that society, and to be applicable to its wants and relations. It is furthermore necessary that these ideas be common to the majority of the members of the society, and that they exercise some sway over their desires and actions.
It is clear that if men have no ideas extending beyond their own existence, if their intellectual horizon be limited to themselves, if they give unrestrained play to the fury of their passions and inclinations, if they have not amongst them a certain number of notions and sentiments held in common, around which they may be rallied, then it is clear, I repeat, that no society can possibly exist among them, and that each individual will be an element of disorder and dissolution in any society into which he enters.