The history of the conquest of the Islamic east, like that of other phases of the Muslim wars of expansion, is difficult to reconstruct and to interpret. The Arab conquests in what would become the Islamic east entailed a number of demographic, social, economic, political and cultural changes that would help determine the parameters for the development of this area. The administration of the fiscal apparatus depended heavily on the same class that had played that role in Sasanian times. Political economy, rather than fiscal administration, provides a better guide to distinguishing the various regions of the Islamic east and following their development. Following al-Mamun's accession to the caliphate and return to Baghdad, the history of the Islamic east becomes primarily that of largely autonomous, hereditary, regional dynasties, namely the Tahirids, Saffarids, Samanids and Ghaznavids. The Saffarids represented in almost every conceivable way the antithesis of the Tahirid version of regionalism.