Pirenne, Henri: The Expansion of Islam in the Mediterranean Basin 
From H.Pirenne’s «Mohammed & Charlemagne», Dover Publications, INC. Mineola, New York.
1. The Islamic Invasion
Nothing could be more suggestive, nothing could better enable us to comprehend the expansion of Islam in the 7th century, than to compare its effect upon the Roman Empire with that of the Germanic invasions. These latter invasions were the climax of a situation which was as old as the Empire, and indeed even older, and which had weighed upon it more or less heavily throughout its history. When the Empire, its frontiers penetrated, abandoned the struggle, the invaders promptly allowed themselves to become absorbed in it, and as far as possible they maintained its civilization, and entered into the community upon which this civilization was based.
On the other hand, before the Mohammedan epoch the Empire had had practically no dealings with the Arabian peninsula.(1) It contented itself with building a wall to protect Syria against the nomadic bands of the desert, much as it had built a wall in the north of Britain in order to check the invasions of the Picts; but this Syrian limes, some remains of which may still be seen on crossing the desert, was in no way comparable to that of the Rhine or the Danube.(2)
The Empire had never regarded this as one of its vulnerable points, nor had it ever massed there any large proportion of its military forces. It was a frontier of inspection, which was crossed by the caravans that brought perfumes and spices. The Persian Empire, another of Arabia’s neighbours, had taken the same precaution. After all, there was nothing to fear from the nomadic Bedouins of the Peninsula, whose civilization was still in the tribal stage, whose religious beliefs were hardly better than fetichism, and who spent their time in making war upon one another, or pillaging the caravans that travelled from south to north, from Yemen to Palestine, Syria and the Peninsula of Sinai, passing through Mecca and Yathreb (the future Medina).
Preoccupied by their secular conflict, neither the Roman nor the Persian Empire seems to have had any suspicion of the propaganda by which Mohammed, amidst the confused conflicts of the tribes, was on the point of giving his own people a religion which it would presently cast upon the world, while imposing its own dominion. The Empire was already in deadly danger when John of Damascus was still regarding Islam as a sort of schism, of much the same character as previous heresies.(3)
When Mohammed died, in 632, there was as yet no sign of the peril which was to manifest itself in so overwhelming a fashion a couple of years later. No measures had been taken to defend the frontier. It is evident that whereas the Germanic menace had always attracted the attention of the Emperors, the Arab onslaught took them by surprise. In a certain sense, the expansion of Islam was due to chance, if we can give this name to the unpredictable consequence of a combination of causes. The success of the attack is explained by the exhaustion of the two Empires which marched with Arabia, the Roman and the Persian, at the end of the long struggle between them, which had at last culminated in the victory of Heraclius over Chosroes (d. 627).(4)
Byzantium had just reconquered its prestige, and its future seemed assured by the fall of the secular enemy and the restoration to the Empire of Syria, Palestine and Egypt. The Holy Cross, which had long ago been carried off, was now triumphantly restored to Constantinople by the conqueror. The sovereign of India sent his felicitations, and the king of the Franks, Dagobert, concluded a perpetual peace with him. After this it was natural to expect that Heraclius would continue the Occidental policy of Justinian. It was true that the Lombards had occupied a portion of Italy, and the Visigoths, in 624, recaptured from Byzantium its last outposts in Spain; but what was that compared with the tremendous recovery which had just been accomplished in the Orient?