The Arabs were already beginning to rove on the far side of the Euphrates; they plundered Baghdad, then a village, while a fair was being held there, as well as other places on the right bank of the Tigris. But Khalid presently received orders (the commencement of the summer of 634) to start for Syria, the conquest of which was at the time a matter of greater consequence to the caliph. His successor, Abu Obaid of Taif, brought some reinforcements with him; but when at length a regular Persian army came on the scene, the Moslems, in spite of their heroic valour, were completely defeated in the “battle of the Bridge,” on the Euphrates, November 26th, 634. After their leader had fallen Muthanna had great difficulty in extricating the remains of his army. Most of the Moslem conquests were lost without further ado. After some hesitation Omar (caliph since August 23rd, 634) resolved to send more troops to Irak. He appealed simultaneously to the greed and piety of the Arabs, urging them in the same breath to win the treasures of Chosroes and the joys of paradise. A larger Persian army was now defeated for the first time (at Buwaib, 635 or 636); the commander, a member of the house of Mihran, was among the slain. The Arabs were once more masters of the country west of the Euphrates. They found an energetic and cautious leader in Saad, son of Abu Wakkas, one of the first followers of the prophet. The lords at Ctesiphon now realised the great danger that impended over the empire. The news of the battle on the Yarmuk (August 20th, 636) which cost Heraclius, the conqueror of Persia, the whole of Syria, probably contributed to their fears. Rustem, therefore, took the head of a great army in person. As a token of the gravity of the struggle he bore with him the sacred banner of the empire (_dirafshi Kaviyan_), which was supposed to have come down from time immemorial. He also took with him a number of elephants, according to the Persian usage in war. At the approach of the advanced guard of the Persian army Saad evacuated his position and retreated to Kadisiya, on the verge of the desert (south or southwest of Hira). For months the armies confronted one another, with only a little space between. The Arabic force was certainly much the smaller of the two; they could not have fed a large army in that place, for they were dependent on the produce of their raids and such provisions as the caliph sent after them from Medina.