In just over a hundred years--from the death of Muhammad in 632 to the beginning of the Abbasid Caliphate in 750--the followers of the Prophet swept across the whole of the Middle East, North Africa, and Spain. Their armies threatened states as far flung as the Franks in Western Europe and the Tang Empire in China. The conquered territory was larger than the Roman Empire at its greatest expansion, and it was claimed for the Arabs in roughly half the time. How this collection of Arabian tribes was able to engulf so many empires, states, and armies in such a short period has perplexed...