Excerpts from Books and Wikipedia
"Muhammad had left neither a will nor any other document to specify a successor. But he had mentioned orally that His cousin and son-in-law should succeed Him. . . . Thus it was that at the gathering assembled on the very day of the Prophet's death, whatever rights 'Ali did possess were, with no warrant of authority, entirely ignored. If voices were raised on his behalf they fell on deaf ears. . . . Within a year of the passing of the Prophet, Muslim armies were moving northwards. At first the principalities--those tributaries to the two great Empires--were engaged, but soon the Muslims came face to face with the Romans and the Persians themselves. Why did the Arabs, within such a short time, erupt from their homeland to challenge the might of the civilized world? . . . Given on the one hand the resurgent, vigorous faith of the Muslims, and on the other the overweening pride and arrogance of two war-weary, exhausted and effete Empires, the outcome was inevitable." [Balyuzi: Muhammad and the Course of Islam, p. 167, 169]