The Chaldeans now entered upon a second period of prosperity, which lasted 200 years, and during which under the early Khalifas they attained premier positions in all matters of philosophy, learning, and even statesmanship, causing more than once complaints from the less gifted and, consequently, the less favoured Musulman Arabs. The 2nd century of this period, from the time of the battle of the Zab (see p. 107), when a Persian dynasty reigned, was as well the golden age of the Khalifate as of the later Chaldean Church. Under the beneficent rule of the earlier Abbasid Khalifas (among whom were the renowned Harun al Rashid and Ma’mun) the patriarchate was transferred to Bagdad, and a new bishopric was founded at Kufa, the very heart and centre of Islam. Under the Khalifas Ma’mun and Harun al Rashid particularly, the Chaldeans found themselves in the greatest favour. Their colleges were protected, and as they were versed in many languages and sciences, their priests and philosophers were given the translation into Arabic of books from the Greek, Persian, and Chaldean languages. It is to the Chaldeans of this time that Islam is indebted for many of the Greek authors’ works, particularly Aristotle, whose philosophical treatises have been ever popular among the Arabs. Undoubtedly, both Arabian letters and the Chaldean Church reached their highest point in the period A.D. 809–813, the Khalifate of Ma’mun, and we may here remark upon the extent of the Church at that date. There were then, or soon after, as many as twenty-five bishoprics all over Asia,[40] for the missionaries sent out in the 5th century had not been idle, as we shall see later. However, this, the brightest period of Islam and Christianity, was sadly darkened by the accession of one of the monsters of history to the Khalifal throne. After the Khalifa Ma’mun came Al Mu’tasim, a famous ruler also, who transferred the capital to Samerra, higher up the Tigris than Bagdad, and he was followed by Al Wathiq, and then by Al Mutawakkil in A.D. 847, who degraded the high office he held as much as his greater predecessors had exalted it. [Sidenote: PERSECUTION OF CHRISTIANS]