The second Abbasid caliph, Abu Ja'far Abdullah al-Mansur (754–775), was the genuine architect of the dynasty's institutional form. Where his elder brother al-Saffah had been the figurehead of the revolution, al-Mansur consolidated the system: a new capital built on a previously empty site at the confluence of the Tigris and the Euphrates; an administrative apparatus drawn largely from the Sasanian bureaucratic tradition; a standing army recruited principally from Khurasan; a centralised treasury that the Umayyad system had lacked; and a deliberate distance from both the Arab tribal politics of the earlier dynasty and the Shia partisans of the original revolution. The reign was twenty-one years; the institutional creation lasted five centuries.
The site of Baghdad
The principal capital of the early Umayyads had been Damascus; the early Abbasids governed for a decade from Kufa and the temporary residence at Hashimiyya. A permanent capital was politically necessary — to consolidate the new dynasty in a location separate from its predecessor's cultural and tribal base, to anchor the administrative apparatus, and to demonstrate the dynasty's permanence. Al-Mansur's selection of the site at the confluence of the Tigris and the Tigris-Euphrates canal system, twenty miles upstream of the ancient city of Ctesiphon (the Sasanian capital, destroyed in the Arab conquest of 637), was made in 762 after a personal survey of candidate sites and consultation with astrologers and engineers.
The chosen site combined four advantages: it sat at the head of the Mesopotamian alluvial plain, the most productive agricultural region of the empire; it controlled the river routes both to the Persian Gulf (via the Tigris and the Shatt al-Arab) and to the upper Tigris and the Khurasan land route; the climate was substantially more tolerable than the southern Iraqi cities; and the land was empty, which permitted the deliberate planned construction of a new capital. The site was unproductive of legend or factional claim — a deliberate political quality.
The Round City
Construction of the new capital, formally named Madinat al-Salam ("the City of Peace") but universally called by its colloquial name Baghdad (a Persian word meaning roughly "the gift of God"), began on the 30th of July 762. The plan was the Round City: a circular fortified citadel with four gates oriented on the cardinal points, with the caliphal palace and the great mosque at the centre, the administrative buildings and military quarters in radial wedges around the centre, and the residential and commercial quarters extending in an arc outside the walls.
The circular plan was without precedent in the Arab Islamic tradition and was probably drawn from the Sasanian circular fortifications of which Firuzabad in Fars was the principal exemplar. The construction was managed by four engineering teams under four supervisors drawn from the principal ethno-administrative groupings of the empire (an Arab from Syria, a Persian from Khurasan, an Iraqi from Kufa, and a Khurasani from Merv); the labour force was approximately a hundred thousand men. The principal phase of construction took four years; the city was substantially complete by 766.
The Round City itself — the inner walled enclosure of approximately two miles in diameter — was the administrative and ceremonial centre. Around it, on both banks of the Tigris, the residential and commercial quarters extended over the following decades into what would become, by Harun al-Rashid's reign of 786–809, the largest city west of the Tang Chinese capital at Chang'an, with a population estimated at between three hundred thousand and five hundred thousand by the early ninth century.
The administrative apparatus
The Abbasid administrative system as al-Mansur consolidated it drew its principal forms from the Sasanian Persian inheritance rather than from the Arab-tribal Umayyad pattern. The central institution was the diwan system — separate offices for the principal functions of state. The Diwan al-Kharaj managed the land tax; the Diwan al-Jund managed the army payroll; the Diwan al-Rasa'il managed the official correspondence; the Diwan al-Khatam managed the chancery seals. A wazir (vizier, from the Persian) coordinated the diwans on behalf of the caliph; the first formal vizier was Abu Salama al-Khallal under al-Saffah, but the office was institutionalised under al-Mansur with the appointment of Khalid ibn Barmak (founder of the Barmakid family that would dominate the administration under Harun al-Rashid).
The provincial administration replaced the Umayyad reliance on Arab tribal governors with a more bureaucratic system: salaried governors appointed and removed at caliphal pleasure, separated from local military command (the military and civil functions were typically held by different officers, with both reporting to the caliphal centre), with detailed administrative correspondence flowing weekly from each provincial capital to Baghdad. The provincial revenues — chiefly the land tax (kharaj) on agricultural production and the poll tax (jizya) on non-Muslims — were collected by provincial officials and forwarded net of provincial expenditure to the central treasury.
The army
The Abbasid army as al-Mansur reorganised it was recruited principally from Khurasan, on the basis of the Khurasan provincial army that had carried out the revolution. The Khurasani regiments — the abna' al-dawla, "the sons of the dynasty" — were professionally salaried, organised into permanent regiments, and stationed both at Baghdad and at the frontier garrisons. The principal field force was about thirty thousand men, with the substantial provincial garrisons bringing the total imperial military establishment to approximately a hundred and twenty thousand. The army was the largest and most professionally organised in the world of the period.
The Khurasani character of the army was a deliberate political choice. The Arab tribal levies of the Umayyad period had been factionally divided and politically unreliable; the Khurasani regiments were drawn from a more uniform background (Arab tribesmen settled in Khurasan as garrison troops during the conquest of the 670s and 680s, plus their descendants and the Persian client converts) and had a direct dynastic loyalty to the Abbasid house that the Arab tribes did not. The political consequence was that the Khurasani military elite became, over the following century, one of the principal forces in caliphal politics — frequently more powerful than the caliph himself.
The succession and Mahdi
Al-Mansur died on the 7th of October 775 while on the road to the Mecca pilgrimage, near the wells of Bi'r Maymun. The succession to his son al-Mahdi (775–785) was uncontested. Al-Mahdi continued his father's administrative system, expanded the frontier campaigns against Byzantium under the command of his son Harun (the future Harun al-Rashid), conducted a moderate accommodation with the Shia community after a generation of conflict, and presided over the substantial economic and cultural growth that the new capital had been intended to produce. The ten-year reign was, in the dynastic memory, the period of consolidation before the dynastic high-water mark under Harun al-Rashid that is the subject of chapter III.
End of Chapter II