=Revolt of the Tribes.=—No sooner was the death of the Prophet bruited abroad than the tribes who had only recently adopted Islâm broke out in revolt. The discipline of Islâm and its rules and principles were too irksome to them. They repudiated their adhesion to the new religion and reverted to paganism. Medîna was hemmed in again by surging hordes of angry idolaters. Undaunted by his own danger, the aged Caliph sped on the expedition to Syria the Prophet had prepared to seek reparation for the murder of the Moslem envoy. Before sending them forth on their distant errand, in the spirit of the Master, he gave to the captain of his army the following injunction:— =His Injunction to the Troops.=—‘See that thou avoidest treachery, injustice, and oppression. Depart not in any wise from the right. Thou shalt mutilate none, neither shalt thou kill child or aged man, nor any woman. Destroy no palm-tree, nor burn any fields of corn. Cut not down any tree wherein is food for man or beast. Slay not the flocks or herds or camels, saving for needful sustenance. When thou makest a covenant, stand to it, and be as good as thy word. Ye may eat of the meat which the men of the land shall bring unto you in their vessels, making mention thereon of the name of the Lord. As you go on you will find some religious persons who live retired in monasteries, and propose to themselves to serve God that way: let them alone, and neither kill them nor destroy their monasteries. And the monks with shaven heads, if they submit, leave them unmolested. Now march forward in the name of the Lord, and may He protect you from sword and pestilence!’ How different this sounds to the command given to the ancient Jews: ‘Now go and smite Amalek, and utterly destroy all that they have and spare them not; but slay both man and woman, infant and suckling, ox and sheep, camel and ass.’[36] The Moslems believed in the righteousness of their cause, and in divine help in defence of their Faith; they were surrounded by formidable enemies; the very existence of their new life depended on their energy and self-sacrifice, but trust in God and enthusiasm led them to victory. The Syrians received a well-merited chastisement, whilst the revolted tribes were beaten back and gradually reduced to submission. Within the space of a few months the entire peninsula acknowledged once more the creed of Islâm. =Death of Abû Bakr and Election of Omar.=—Abû Bakr held the reins of office for less than two years. He died on the 22nd August 634 A.C., and the great Omar was elected to the vicegerency of the Prophet. =War with Persia—Its Cause.=—The pacification of the north-eastern corner of Arabia brought the Moslems into collision with the kingdom of Hira, a feudatory state subject to Persia. The raids from Hira led to an expedition into that country which ended in its annexation. The subjugation of Hira and Chaldæa brought the Persian forces into the field.