In the 7th century A.D. the two powers bordering on the Arab area were the Byzantine and Persian empires. Both of these were, to all appearance, flourishing and stable, but both alike were in reality greatly weakened by external and internal causes which were closely parallel in the two. Externally, both had been severely shaken by some centuries of warfare in which they had disputed the supremacy of Western Asia, and both had suffered from rear attacks by more barbarous foes. Internally, both alike had a thoroughly unsatisfactory social structure, though the details differ: in the Byzantine Empire almost the whole burden of a very heavy taxation fell upon the middle classes, the _curiales_, and the armies were mainly composed of foreign mercenaries, whilst in the Persian Empire a rigid caste system stifled natural development. In both we see a state church engaged in active persecution and thereby alienating a large section of the subject population. The career of Muslim conquest came with great suddenness. Between the years 14 and 21 A.H. (A.D. 635-641) the Arabs obtained possession of Syria, Iraq, Egypt, and Persia. They owed to Islam the united action which made these conquests possible, but the older Muslims who had shared the ideals and labours of the Prophet, though put at the head, were carried forward reluctantly and yet irresistibly by the expanding force behind them. Many of them viewed these large accessions with very real anxiety. When the second Khalif Umar saw the large number of prisoners and captives from Jalûlâ (Persia) flocking into Arabia, he exclaimed, “O, God, I take refuge with thee from the children of these captives of Jalûlâ.”