At length the great battle of Kadisiya (end of 636 or 637) was fought. It lasted for several days; Saad was ill, but nevertheless took the command. The Persians were, for the most part, much better armed than the Arabs, but the courage of the latter was wound up to the highest pitch. They were terrified by the elephants at first, but as they pressed on gallantly for all their fears, the animals appear to have got beyond control and to have become a source of confusion to the Persian ranks. The great majority of the Persians certainly behaved with cowardice, after their ancient fashion; but the Arabs had hard work before the foe was defeated, Rustem himself slain, and the banner of the Persian Empire taken. ARAB CONQUEST [Sidenote: [637-652 A.D.]] The battle of Kadisiya practically decided the fate of the provinces on the Tigris. There were a few other fights, some of them in the vast territory of ancient Babylon, but the Arabs soon afterwards reached Seleucia, took it after a protracted siege, crossed the rapid stream of the Tigris, and quickly forced their way into Ctesiphon. The young king Yezdegerd had already fled to Holwan (on the border between Babylonia and Media). On their way thither, at Jalula, the Arabs won another victory over the Persians under Khorrezadh, Rustem’s brother, and Yezdegerd fled further into the interior. Meanwhile other Arabs had conquered the delta of the stream and thence advanced into Susiana. A very able resolute commander might still have saved the actual land of Iran for the Persians. Omar, who was very cautious in spite of his energy, was apprehensive lest the Arabs should extend their forces too far, and at first would not give orders for an advance into the highlands. At length he did so. A great Persian army had been collected at Nehavend, a little to the south of the ancient highway from Babylon to Ecbatana. Here a great battle was fought (in 640, 641, or 642), in which the Arabs--first under the command of Nohman and, after he had fallen, under Hudhaifa the Meccan--won a brilliant victory. With good reason the Moslems called the triumph of Nehavend the “victory of victories.” It completely shattered the empire of Persia. The Arabs had a long contest before them, until they had really conquered all the provinces of the vast monarchy, but it consisted of isolated struggles in which there could be no doubt of the ultimate issue, as their enemies had lost all cohesion. Many towns and districts had to be subjugated again and again, because they were constantly rebelling. The most obstinate resistance appears to have been offered in Persia proper, especially about Istakhr, the cradle of the empire of the Sassanidæ and the centre of its religion. Many of the great provincial nobles and some of the lesser entered into friendly agreement with the Arabs. They one and all met them on the footing of independent sovereigns.