Isdigerd, on learning the result of the battle of Jalula, quitted Holwan, and retired to Rei, a large town near the Caspian sea, at a short distance from the modern Teheran, thus placing the entire Zagros range between himself and his irresistible foes.  A general named Khosru-sum was left behind with a large body of troops, and was bidden to defend Holwan to the last extremity.  Instead of remaining, however, within the walls of the stronghold, Khosru-sum rashly led his force to meet that of El Kakaa, who defeated him at Kasr-i-Shirin and entirely dispersed his army.  Holwan, being left without protection, surrendered; the conquest of Shirwan, Mahsabadan, and Tekrit followed; and by the close of the year A.D. 637 the banner of the Prophet waved over the whole tract west of Zagros, from Nineveh almost to Susa, or from the Kurnib to the Kuran river.

Another short pause in the Arabian aggressions upon Persia now occurred; but in the year A.D. 639 their attacks were resumed, and the Persians had to submit to further losses.  Otba, governor of Busrah, sent an expedition across the Shat-el-Arab into.  Susiana, and, supported by the Arab population of the province, which deserted the Persian side, engaged Horrmuzan, the satrap, in two battles, defeated him, and forced him to cede a portion of his territory, including the important city of Ahwaz.  Soon afterwards, Ala, governor of Bahrein, conducted in person an expedition into Persia Proper, crossing the Gulf in the rude vessels of the time, and attacking Shehrek, the Persian satrap, who acknowledged the authority of Isdigerd.  Here, the Arabs were for once unsuccessful.  Shehrek collected a force which Ala was afraid to encounter; the Arab chief retreated to the coast, but found his fleet engulfed by the waves; and it was only with great difficulty that he made his escape by land from the country which he had ventured to invade.  He owed his escape to Otba, who sent troops from Busrah to his aid, defeated Shehrek, and rescued his fellow governor from the peril which threatened, him.

In the next year (A.D. 640) Hormuzan, incited by Isdigerd, made a desperate attempt to recover the territory which he had been compelled to cede.  Assisted by Shehrek, governor of Persia Proper, he attacked the Arabs unawares, but was speedily met, driven from Ram-Hormuz to Shuster, and there besieged for the space of six months.  As many as eighty engagements are said to have taken place before the walls, with no decided advantage to either side.  At length Al-Bera, son of Malik, one of the companions of the Prophet, and believed by many to possess the prophetic spirit, announced that victory was about to incline to the Moslems, but that he himself would be slain.  A chance arrow having fulfilled one-half of the prediction, the Arabs felt an assurance that the other half would follow, and fought with such fanatic ardor that their expectations were soon fulfilled.  The town was won; but Hormuzan retired into the citadel, and there