had brilliant traditions, for it was these people who had won the first and much celebrated victory of the Arabs over Persian regular troops at Dhu Ḳār before the rise of Islām (between 604 and 611). This tribe of the Banū Shaibān and their leader Muthannā ibn Ḥāritha, whose example was followed by the others, induced Khālid and his Muslims to cross the Persian boundary for the first time. That was not a matter of chance, but shews the deep inner connexion of the Saracen expansion with the migration already in being before the rise of Islām. The Shaibān, like all the other components of the Bakr ibn Wä'il, were wholly independent of Medina, and had no intention of becoming Muslims. But when Medina suddenly extended its dominion beyond Yamāma, and all Arabia echoed with the fame of Khālid in warfare, the Bakr found themselves in a dilemma between the rising Arabian great power and their old hereditary enemy, Persia. What could be more obvious than that, simply because they needed a screen for their rear, they should draw the related Muslims into their alliance and with their assistance continue their raids into the cultivated country? Khālid, reckless plunger that he was, seized with avidity this opportunity for fresh deeds of valour. Tradition reports that the chiefs of the Bakr tribes, and of them Muthannā first and foremost, paid a visit to the Caliph Abū Bakr at Medina, professed Islām, and received from Abū Bakr the command to conquer 'Irāḳ in conjunction with Khālid. In reality it is doubtful whether the Caliph even so much as knew of any connexion between Khālid and the Bakr tribes. At the same time it is not improbable that he gave his consent for Khālid to participate in one of the customary raids of the Bakr ibn Wā'il, but the conversion of the head of the tribes was no part of his plan, much less the conversion of the tribes themselves. They certainly from this time onward were in touch with Medina, and regarded themselves as in political alliance with the Muslims; and in the rapid developments of the next few years they were merged in the Caliph's dominions. Abū Bakr did not at first contemplate any systematic occupation of 'Irāḳ, for he was at that time considering an expedition against Syria, which from the point of view of Medina was of infinitely greater importance. Even at that time they desired to have Khālid in Syria; but he had in any case already taken part in the raid. the Banū Shaibān, either with or without the knowledge of the Caliph. How little any conquest of Persia was contemplated is shewn by the fact that the main body of Khālid's troops was ordered home to recruit, and he undertook his first invasion of Persian territory with only about 500 men, certainly well-selected troops, and then continued his march further with the same contingent into Syria.
Khālid attracted volunteers of all kinds from Central Arabia, and marched with them westward of the Euphrates to avoid the marshes; at Khaffān he effected a junction with the Bakr under Muthannā; their combined forces amounted in all to only two to three thousand men, but
