The history of Assyrian Baghdad, as a metropolis, coincides with the history of the rise and fall of the Abbasid Caliphs, for in the East it would appear to be almost a necessity of the case that every new dynasty should found a new capital. In the earlier annals of Islam the Era of the Flight commemorates the date when the Prophet Muhammad, being forced to leave Mecca, went to take up his abode in the little hamlet of Yathrib. The manifold advantages of the position of Baghdad are a theme on which Moslem geographers and historians fondly expatiate. Mukaddasi, for instance, states that the Caliph took the advice of those who had had experience from living here both in summer and in winter, and allKasreed in its praise, that geographer summing up in the following terms said to have been addressed to Mansur.