tribute. These terms, formulated by Muhammad himself, set the pattern followed by his successors in treating with conquered populations. The prevailing notion that Moslems offered Christians only a choice between conversion to Islam and death by the sword has no basis in fact ; they much pre- ferred to hold such peoples under their rule and collect tribute, which normally ceased once the conquered accepted Islam.
By 633, the year after Muhammad's death, the dissident Arabian tribes had been subdued and the whole peninsula north of the Empty Quarter had been consolidated and unified under the leadership of one man, the first caliph abu-Bakr (632-634). The momentum acquired in these internal wars had to seek new outlets, especially since the new religion had supposedly converted its adherents into one brotherhood. The martial spirit of the tribes, to whom raids had been a sort of national sport from time im- memorial, was not weakened by Islam; on the contrary it was redirected and intensified.
Viewed in its proper perspective, the Islamic expansion was the last in the long series of migrations which took the surplus Semitic population from the barren peninsula to the bordering fertile regions and the more abundant life they offered. The Islamic movement, however, did possess one distinctive feature — religious impulse. Combined with the economic, this made the movement irresistible and carried it far beyond the confines of any preceding one. Islam provided a battle-cry, serving as a cohesive agency cement- ing tribes never united before. But while the desire to spread the new faith or to attain paradise may have moti- vated some of the bedouin warriors, the desire for the comforts and luxuries of settled life in the Fertile Crescent was the driving force in the case of many more of them.
Several considerations directed this martial energy Syria- ward. The Arabian tribes domiciled there were expected to collaborate with their invading kinsmen, as the annual
