By the beginning of the seventh century the two great powers of the ancient world — Christian Byzantium and the Zoroastrian Iran of the Sasanians — had utterly exhausted each other in an endless war. The idol was of their own making: war become the mode of existence of both powers — twenty years of mutual annihilation (602–628) burned through treasuries and armies, while taxation and the persecution of «heretics» estranged the provinces from the center, and their hold grew ever weaker. The two capitals regarded each other as the only possible rival — and never noticed that from the deserts of Arabia there had risen a force that had never before appeared on the map.
The catastrophe broke with astonishing speed. After the death of the Prophet Muhammad in 632, Arab armies under the banner of a new faith poured out beyond Arabia. The fateful year was 636: at Yarmouk they crushed the Byzantines, and at Qadisiyyah, the Persians. Within a few years Byzantium lost Syria, Palestine, and Egypt, while the Sasanian realm — with the fall of its capital Ctesiphon in 637 — simply vanished from the face of the earth. A single generation witnessed the collapse of a world that had seemed eternal.
The captivity of the ancient Christian East began. Jerusalem, Antioch, Alexandria — the cities where Christianity had been born and come of age — fell under the rule of the caliphate. Christians were not exterminated: they became "people of the Book," a tolerated but subordinate population — the dhimmi, paying a special tax and living as second-class inhabitants in lands that had once been their own.
But where Iran vanished altogether, Byzantium managed to return — shrunken and transformed. Having lost its rich southern provinces, it reassembled itself into a compact, austere defensive state whose whole life was now given over to survival. And it held: in 717–718 an enormous Arab army besieged Constantinople — and shattered against its walls, suffering vast losses. That siege marked the crest of the wave, beyond which it rolled no farther north. So the law of history was fulfilled: the pride of invincibility ended in collapse, but out of the collapse was born a state that had learned to live not by might but by endurance — and therefore outlived its conquerors by many centuries.