On the death of Hārūn er-Rashīd, A.H. 193 (809), a serious dispute arose between his two sons, Amīn and Ma´mūn. The former, probably on the advice of his vezīr, Fadhl ibn Rabī`a,[205] ordered the army, which was at Tūs, to return to Baghdād. This act was not only unfriendly towards his brother, but was also in direct contravention of his father’s will. Ma´mūn, in retaliation, put a stop to all postal communication between Baghdād and the East, and assumed the title of Caliph over a kingdom which extended from Hamadān to Tibet, and from the Caspian to the Persian Gulf. With the help of his able vezīr, Fadhl ibn Sahl,[206] he succeeded in establishing order throughout his realms. Meanwhile Harthama took Samarkand after a protracted siege; whereupon Rāfi` threw himself on Ma´mūn’s mercy and was pardoned, and thus peace was restored throughout Khorāsān. But the elements of civil disorder still held sway. While Amīn, on the one hand, struck Ma´mūn from the succession, the latter ordered the omission of his brother’s name from the public prayers. Amīn, angered at his rival’s attitude, resolved on reducing him by force of arms. To this end he despatched `Alī ibn `Isā against him at the head of 50,000 men. On reaching Ray, A.H. 195 (810), he encountered Tāhir, who had been posted there by Ma´mūn to watch the frontier. In the battle that ensued `Alī was slain in single combat by Tāhir, and his army was put to flight. Tāhir, in obedience to Ma´mūn’s orders, now marched on Baghdād, and with reinforcements brought by Harthama defeated all the armies sent to stop his progress. Having secured the submission of Arabia and Mesopotamia, he laid siege to Baghdād, and took the city by storm in A.H. 198 (813), after twelve months’ investment. Amīn made a vain attempt to escape, and was finally slain by a party of Persian soldiers.