Arabian peninsula. There were still tribes which had not accepted his political and religious leadership, but they were too weak to form an effective opposition.
Mohammed had given the Arabs their first effective political organization, and his immediate successors profited more from this than they did from his promulgation of a new faith. Like all new religions, Mohammedanism was slow to sink into the minds and hearts of the people. The Arabs and their neighbors did not become fanatical Mohammedans overnight, and the great Arab conquests of the seventh century were the result of political, not religious, imperialism. Mohammed's successors, the Caliphs, could not claim to be prophets, and the only way in which they could maintain their position of leadership was by military success. They sent out raiding parties against the nearest imperial provinces and were amazed to find little resistance. Almost without planning it, they became involved in a conquest of Syria and Egypt. The native populations were not alarmed by the change of rulers; in fact, they often preferred the tolerant Arabs to the Greeks who had been accusing them of heresy. The old Persian kingdom, even weaker than the remnants of the Roman Empire, was also overrun by the Arabs, and the Caliphs soon found themselves masters of the whole Middle East. With this solid block of territory at their disposal, it was easy for them to push along the North African coast, and in 711 to cross into Spain. By 720 the Arab Empire stretched from the borders of India to the Pyrenees and Arab raiders were plunging deep into the heart of Gaul.
As a result of the Arab conquests, the last remnants of Mediterranean unity were destroyed, and three sharply contrasted civilizations arose within the old Graeco-Roman sphere of influence. The growth of Mohammedan sea power soon made it difficult, though not impossible, for Christians to use the Mediterranean. Land travel between East and West had always been slow and expensive. Religious differences emphasized the physical difficulties of communication. The Arab Empire gradually became thoroughly