Troublesome enemies as the Arab tribes had often proved to the subjects of the Roman and Persian empires, no one had ever dreamed that they could constitute a menace to either. It is true that when the Moslem inroads began, the districts first affected were in a sorry plight. The frequent wars between the Romans and Persians had sorely enfeebled both empires, and this was more particularly the case with the last great war, which had lasted from 607 to 628. Large areas of Roman territory, especially in Palestine, Syria, and Egypt, had been frightfully ravaged and occupied for years by the Persians. The valiant and wily emperor Heraclius, however, succeeded in turning the tide of fortune, and ultimately dictated terms of peace to the Persians on their own soil. After that the Persian empire had been torn asunder by quarrels over the succession. Both empires had lost the Arab outpost they once possessed. The Persians had annihilated the Roman vassal kingdom of the Ghassanids, and their own subject dynasty in Hira (which had latterly adopted the Christian faith) had been dethroned by King Chosroes II. The folly of this was soon apparent. The Bedouins of the Shaiban tribe utterly routed the royal armies of Persia at Ibu Kar on the frontiers of Babylonia, probably at the very time when the king’s forces were pursuing their victorious progress through the distant west. It was not a great battle, and probably its only direct consequence was that the unwarlike peasants of neighbouring districts were pillaged by the Bedouins; but a victory over an army composed in part of regular troops gave the Arabs confidence. This very Shaiban tribe distinguished itself in the first Moslem advance into Persian territory.
Nevertheless there is much that remains enigmatical in the immense success that attended the Moslems. Their armies were not very large. The emperor Heraclius was an able man, with all the prestige of victory behind him. When the great struggle of Moslem and Persian began, the civil wars of the empire were over, and it had a powerful leader—not indeed in Yezdegerd, its youthful monarch, but in the mighty prince Rustem, who had procured the crown for him. The great financial straits to which both empires were unquestionably reduced must have had its effect upon the number and efficiency of their troops, but that they were still good for something is clear from the fact that both the decisive battle on the river Yarmuk (August, 636) in which the Romans were defeated, and that of Kadisiya (end of 636 or beginning of 637) in which a like fate waited on the Persian arms, lasted for several days. The resistance offered must have been very obstinate. The Roman and Persian armies may have included irregular troops of various kinds, but they certainly consisted largely of disciplined soldiers under experienced officers. The Persians brought elephants into the field, as well as their dreaded mounted cuirassiers. Among the Arabs there was no purely military order of battle; they fought in the order of their clans and tribes. This, though it probably insured a strong feeling of comradeship, was by no means an adequate equivalent for regular military units. Freiherr von Kremer[20] rightly sees in the salat a substitute, to some extent, for military drill. In that ceremony the Arabs, hitherto wholly unaccustomed to discipline, were obliged