The Umayyad dynasty fell rapidly in the face of the Hashimite-Khurasani revolution in 132/750, the Abbasid dynasty's hold on power took until 145/762 to become firmly established. Baghdad was meant to be the fortress of the new dynasty in times of crisis, as well as a strategically situated city in times of peace in economic and political terms. Iraq was the wealthiest province of the empire, and had been undergoing a process of agricultural development since the Umayyad period. Al-Mahdi's decade-long reign was by all accounts a prosperous time for the caliphate. When the Abbasid succession passed on to Harun al-Rashid, it was finally the anticipated moment which different factions wanted. After achieving reconciliation with the Abbasid family and granting amnesty to former opponents in Baghdad, al-Mamun dispatched Abd Allah ibn Tahir on the mission of reunification. Just as al-Mamun's political achievements radically transformed the Abbasid government, his religious policies were equally new and daring.