Before he died, Mohammed had begun to take measures for the spread of his religion by the sword beyond the limits of Arabia. In 629, the year after the end of the Persian war, the troops of Heraclius who garrisoned the fortresses on the desert frontier of Palestine, had been attacked by wandering bands of Arab zealots. But it was not till the Prophet himself was dead that the full storm of invasion fell upon the Roman empire and its Persian neighbours. It was Abu Bekr, the first ‘caliph’ or ‘successor’ of Mohammed, who sent forth in 633 the two armies which were bidden respectively to convert Syria and Chaldaea to Islam by the edge of the sword. Neither the Roman nor the Persian empire was well fitted for resistance at the moment. The twenty years of war brought about by the ambition of Chosroes had reduced each of them to the extreme of exhaustion. Since the end of the war Persia had been a prey to incessant civil strife and revolution: nine princes had mounted the throne in little more than four years. In the Roman empire Heraclius had been doing his best to repair the calamities of the war: his first care had been to repay, by means of the war indemnity paid by Siroes and the imposition of new taxes, the great loan which the Church had made him, in order to equip his troops for the struggle. [Sidenote: Exhaustion of the Roman Empire.] He had disbanded much of his victorious army in pursuit of the policy of retrenchment for which the ruined state of his empire called. But he could not repair the losses which Syria and Asia Minor had suffered in spending ten years beneath the Persian yoke. The very foundations of society seemed to have been sapped in the provinces of the East by the prolonged Persian occupation. The numerous heretical sects which swarmed in the valleys of the Nile and the Orontes had raised their heads during the Persian rule, and bore with ill-concealed reluctance the restoration of the imperial authority. The Jews, who had often sided with the Persians, were restless and discontented. It was said that half the population of Syria and Egypt wished ill to the empire. It would have required two generations of peace and wise administration to restore to their old condition those Oriental dioceses which had for the last three centuries been the stay and support of the East-Roman Empire; but less than four years after Heraclius had solemnly restored the ‘True Cross’ to the custody of the Patriarch of Jerusalem the Arabs burst into the land.