Omar was disconsolate at the death of Nómân; and he promoted his brother Nóeim ibn Mocarrin (one of the three heroes of Dzul Cassa) to high command. He had now embarked on an enterprise from which there was no returning. The proud Yezdegird refused to yield, and Omar no longer scrupled pursuing him to the bitter end. But a long series of campaigns was yet needful, effectually to reduce the empire. These it is not the object of this work to trace otherwise than in such brief and cursory way as shall enable the reader to estimate the expanding area and growing obligations of the Caliphate. The warlike races of the southern shores of the Caspian gathered under Isfandiar, brother of the ill-fated Rustem, for the defence of Rei, one of the royal cities of Persia. Assuming the offensive, they began to harry the Mussulman garrisons. Nóeim advanced to meet them; and another great battle and decisive victory placed the city at his mercy.[391] [Sidenote: Yezdegird flies to Merve, where Turks espouse his cause.]Isfandiar retired to Azerbâijân; where, again defeated, he was taken prisoner; and at last, without much compunction, he threw in his lot, and made common cause with the invading army. From Rei, Yezdegird fled south to Ispahan; finding no shelter there, he hurried on to Kermân; then he retired to Balkh: and at last he took refuge in Merve, whence he sought the aid of the Khâcân of the Turks, and of the Emperor of China. The Khâcân espoused his cause; and for several years the contest was waged with varying success in the vicinity of Merve. But in the end the Turks retired, and with them Yezdegird, across the Oxus. [Sidenote: Death of Yezdegird. A.H. XXXI. A.D. 651.] The conflict was subsequently renewed, and nine or ten years afterwards, in the reign of Othman, Yezdegird, bereft of his treasures and deserted by his followers, who in vain besought him to tender his submission, perished miserably in the hut of a miller, whither he had fled for refuge. [Sidenote: Reduction of the Persian empire.]