IT was about 620 that a merchant of Mecca, Mohammed by name, claiming divine revelation and the succession to the Hebrew prophets and to the Christ, worked out a new and simple monotheism, founded on the debased Judaism and Christianity which he met among his heathen people. That people would have none of him, and in 622, the year in which the Mohammedan era of the Hegira (" the Flight ") begins, he departed from Mecca to the more receptive Medina. The new creed found few followers, but many persecutors ; yet it survived and, reacting vigorously, helped to defeat the Meccans in battle and swept through Arabia like a flame. Backed by the warrior legions of the desert, who burned to propagate their faith and win a physical paradise by dying for it, Mohammed sent a proud challenge to the kings of the earth. He himself died before he could do more than begin the work of conquest, but under the Caliphs, " the Successors of the Prophet," the Moslem storm burst out of its desert fastnesses. The Byzantine Empire had just been restored to greatness by the victories over the Persians of the greatest soldier of his time, the Emperor Heraclius, yet it was in no condition to withstand this new and more formidable enemy. Jerusalem, Aleppo, Antioch and Alexandria in turn fell prey to the invaders, who, internal dissensions notwithstanding, swept like a flood over Central and Western Asia and North Africa. Eastward they wiped out the Persian Empire, reached the Caucasus in the north, and, taking Samarcand in their stride, swamped Asia as far as the Indus and the Hindu Kush. Westward they pushed through North Africa, crossed the straits of Gibraltar that still bear the name of their leader Tarik, wiped out the Gothic kingdom of Spain and flung its remnants into the hill fastnesses of the north. By the end of the seventh century they were at once battering at the gates of Constantinople and beginning to cross the Pyrenees; the Mediterranean was almost a Moslem lake. Greek fire (a predecessor of gunpowder, which produced flame, smoke, and an explosion), beat them back from the Bosphorus (718), and in the West the wave of invasion, pushed beyond its effective limits, was rolled back by Charles Martel and his Franks at Tours in 732. Tours is one of the outstanding battles in history, not because history would have been very different had Charles lost it, but because it marks the ebb of the invasion. It occurred at the turning-point in the history of the great attack on Christian civilisation. In the West the forces of that civilisation were able to assume the offensive, the offensive which after twelve centuries has not succeeded in expelling the Moslem from Europe.