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346 proceeding thence on his return journey to Medina. Abū 'Ubaida remained in the country as Omar's representative, but was not destined to remain in office much longer, for in the year 639, when many thousands from the ranks of the victors succumbed to a fearful epidemic of plague, Abū 'Ubaida was also carried off by it, as was also his successor in office, Yazīd, a short time later. Yazīd's brother, Mu'āwiya ibn Abī Sufyān, was then nominated to the succession by Omar, and in him the man appears at the head of Syria who was destined later in his own person to transfer the Caliphate to Damascus, a development which in its slow preparation is as clear as noonday.

The whole course of the Muslim expeditions in 'Irāḳ shews that the policy of the Caliphs was entirely determined by consideration for Syria. After the unfortunate battle of the Bridge not only the government but also the tribes were still more cautious towards 'Irāḳ expeditions. It was only the eager efforts of Muthannā, of the Bakr tribe, that finally succeeded in gaining the sanction of the Caliph to a new raid, and then only after the first conquest of Damascus. But there was a dearth of warriors; none cared much to proceed to 'Irāḳ, and it was only on the grant of special privileges that a few Yamanites consented to prepare for the march. In the meantime the Persians, who for over a year had not followed up their advantage in the battle of the Bridge, had crossed the Euphrates under Mihrān; but Muthannā, with his auxiliaries from Medina, succeeded in defeating them at Buwaib (Oct. or Nov. 635). With his weak forces he could not however think of following up this small victory, and Omar at that time required all available troops for Syria, where the great army of Heraclius was advancing towards the battle of the Yarmūk. It was not until after this latter decisive victory that the Caliph paid greater attention to the 'Irāḳ. Here also the first thing to be done was the despatch of a general representative, or vicegerent, for which position Sa'd ibn Abī Waḳḳāṣ was selected. To get the necessary troops however for an energetic attack was still attended with great difficulty. Sa'd took the whole of the winter 636-637 to assemble a few thousand men around him. Of the Arabian hordes, incited by religious enthusiasm, according to the customary European traditions, we can find but little trace.

In the meantime the Persians, alarmed by their own defeat at Buwaib, and still more by the terrible collapse of the Byzantine rule in Syria, decided to take energetic steps against the Arabs. The administrator of the kingdom, Rustam, assumed the command personally, and crossed the Euphrates. On the borders of the cultivated land, at Ḳādisīya, Sa'd and Rustam stood for a long time facing each other. Of the size of their respective armies we know nothing positive; the Arabs were certainly not more than 5-6000 strong, including Christians and heathens, and the numerical superiority of the Persians cannot have been considerable. More by chance than from any tactical initiative the two armies became