Ziâd was also the bearer of a petition from the leaders in Irâc, who chafed at the limit placed on their progress, and now asked for leave to pursue the fugitives into Khorasan and to the Caspian shore. But Omar, content with what had been already gained, forbade the enterprise. ‘I desire,’ he wrote in reply, ‘that between Mesopotamia and the lands beyond, the hills shall form a barrier, so that the Persians shall not be able to get at us, nor we at them. The fruitful plains of Irâc suffice for all our wants; and I would rather the safety of my people than spoil and further conquest.’ The conviction of a world-wide mission for Islam was yet in embryo; and the obligation to enforce its claims by a universal crusade had not yet dawned upon the nation. And, in good truth, a dominion embracing, as Islam now did, Syria, Chaldæa, and Arabia, might have satisfied the ambition even of an Assyrian or Babylonian monarch. The sound and equal mind of Omar, far from being unsteadied by the flush and giddiness of victory, cared first to consolidate and secure the prize he had already gained. [Sidenote: Operations in Mesopotamia. A.H. XVI. Summer, A.D. 637.] The Persian frontier, for the time, was safe. A son of Hormuzân made an inroad from Masbazân, a fortress in the mountains, two days south of Holwân; but he was defeated and beheaded; and the place, strongly guarded, became one of an established line of frontier posts. There being no further attempt upon the peace of Medâin, the ambition of Sád and his generals, checked northwards by the Caliph’s interdict, was for the present confined to the reduction of Mesopotamia. For this end, troops were sent up the verdant banks of the Tigris as far as Tekrît--a stronghold on the river, about a hundred miles above Medâin, held by a mixed garrison of Roman troops and Christian Bedouins. These bravely resisted the attack. But after forty days the Romans thought to evacuate the place, and, deserting their native allies, escape by their boats. The Bedouins, on the other hand, were tampered with, and went secretly over to Islam; so that, when a final assault was delivered, they seized the water-gate, and the Romans, taken on both sides, were put to the sword.[300] The newly converted allies then joined the force, and pressed enthusiastically forward to Mosul, which, hearing of the fall of Tekrît, at once surrendered, and became tributary. On the Euphrates, the Moslem arms had already met with equal success. The Bedouin tribes in Upper Mesopotamia having been urged by the Byzantine Court to make an attack on the invaders, who were threatening Hims, Sád was charged by Omar to effect a diversion from Irâc. [Sidenote: Hît and Kirckesia taken. A.H. XV. A.D. 636.] The fortress of Hît on the Euphrates was accordingly besieged; but they found it too strong to carry by assault. Leaving, therefore, half of the force before the town, the commander marched rapidly up the river to Kirckesia,[301] at