This regime change in the Islamic world had very important consequences for Byzantium. The dynamics of the relationship between the two central governments changed, as the Abbasids initially sought to consolidate their own leadership through championship of the jihad against Byzantium. Late in the reign of their first caliph, al-Saffah (749–54), the large army that had been mustered for use against Byzantium was diverted by its commander, ‘Abdallah bin ‘Ali, in an attempt to seize power for himself. His efforts were thwarted,57 but this abortive coup d’´etat diverted Muslim resources at a time when logistical considerations were making it increasingly difficult for the Abbasids to wage war on Byzantium from their new capital under construction at Baghdad. Under Caliph al-Mansur (754–75) the Byzantine frontier was regarded as an area for Muslim settlement and fortification rather than a theatre for major campaigning. Warfare became positional, while the borders were now relatively static; some would argue that the raiding became virtually ritualised.58 The Abbasid leaders had to reckon with possible Byzantine invasions, and they also had to keep a close eye on the Muslim armies from northern Mesopotamia, who still maintained their loyalties to the Marwanids. Various border warlords also gave them cause for concern, particularly those from the region of Samosata. To counter all these threats, al-Mansur tried to coopt supporters of the last Umayyad caliph, Marwan II (744–50), and he also imported troops from Khorasan. The result, however, was chaos and anarchy on the borders. The caliph was forced to call on Abu Muslim – who had led the Abbasids’ revolt against the Umayyads in 747, establishing al-Mansur’s predecessor al-Saffah on the throne, and who was now governor of Khorasan – for support to crush the rebellion of ‘Abdallah bin ‘Ali. Al-Mansur proceeded to develop his own network of border commanders, of disparate origins, to serve as a counterbalance to the warlords. No single Muslim commander was to lead an expedition against Byzantium more than twice in succession. This rotation system was designed to prevent any border commander from gaining control of really substantial human and material resources. Yet it was also a precarious system, provoking jealousy and competition among the local commanders and it did not make for maximum military efficiency against the Byzantines. The rotation system had been dropped by 769, towards the end of al-Mansur’s caliphate, and thereafter al-Mansur sought to control the frontier regions from a distance. Expeditions and leaders of expeditions had proliferated because anyone with sufficient resources could try to mount an expedition against Byzantium. The Abbasids now attempted to make permission from the imam a necessary precondition for embarking on an expedition against Byzantium. During the caliphates of al-Mahdi (775–85), al-Hadi (785–86), and Harun al-Rashid (786–809) warfare against the Byzantine empire became a priority for the leadership. Al-Mahdi ordered the stationing of 2,000 new troops at Mopsuestia, to be maintained by stipend, and his first campaign against Byzantium was launched with aplomb in 776, with the caliph lending his symbolic presence to the proceedings. In 778 the Byzantines, under the Isaurian dynasty’s favoured commanderMichael Lachanodrakon, penetrated the Hadath pass and attacked Germanikeia. Eventually the siege was lifted (through the garrison commander’s bribery of Lachanodrakon, according to Theophanes),59 but not before the Byzantines had deported many Jacobite Christians to reside on the other side of the empire, in Thrace. Emperor Leo IV (775–80) celebrated a triumph for his generals at the palace of Sophianai on the Bosporus, distributing rewards. In the following year, the Byzantines again penetrated to theHadath pass, provoking a Muslim counter-expedition under Hasan bin Qahtaba, which reached – but failed to capture – Dorylaion. Henceforth Caliph al-Mahdi appointed frontier regional commanders from his own household and family, aiming to raise them above the level of the local border warlords. He accompanied an expedition as far as the frontier region in 780 and from there he sent on his son, Harun al-Rashid, who penetrated Byzantine territory and managed to besiege and capture Semalous. In 781/2 al-Mahdi sent Harun to engage the Byzantine forces at the head of a huge force, allegedly some 100,000 strong. After penetrating to Chrysopolis, on the Asiatic side of the Bosporus opposite Constantinople, and after seizing many captives, Harun imposed expensive and embarrassing terms on Empress Irene in 782: the truce was to last for three years, and involved an annual tribute payment by the Byzantines of 100,000 dinars; in addition, Harun’s army kept its prisoners and considerable booty.60 Warfare against the Byzantines now counted for more than it had done at any time since the Umayyad caliphate of Hisham. Harun al-Rashid resumed military operations in 785–6, in response to an alleged violation of the truce in 785 and the Byzantines’ seizure and destruction of the fortress of Hadath. From the Abbasids’ point of view, the frontier system was not working very well and the Barmakid family gained effective control of the caliphate’s north-west frontier provinces. Nonetheless, Byzantium faced a formidable opponent when Harun al- Rashid became caliph in 786; he had won great renown for his personal participation in the jihad against Byzantium, and he had taken shrewd advantage of Empress Irene’s weakness and her serious problems with her military units (see above, p. 270). Around the time that Constantine VI (780–97) was reigning in uneasy equilibrium with his mother,Harun chose to make Raqqa his residence in order to control access to the frontier. In 797/8 Harun led the expedition that captured the Byzantine fortress at Safsaf, not far from the Cilician Gates. The Muslims are even said to have reached the Bosporus again, and Harun appointed commanders for further raids. He agreed to negotiate with Byzantium only because of the pressure he was coming under from the Khazars to his north. The caliph led expeditions in person against Irene’s successorNikephoros I (802–11) in 803 and 806; during the latter expedition, Harun’s forces captured Heraclea- Cybistra and Tyana, and the emperor was compelled to accept peace and to pay a humiliating poll tax on himself and his son (see above, p. 256). Harun took institutional measures to strengthen the caliphate’s position against Byzantium in the long term. In the very first year of his caliphate, he created a new frontier district called al-‘Awasim with Membij as its centre. This marked the beginnings of a new system of frontier organisation. Opposing the Byzantines along a straggling line from Tarsus northeastwards as far as Theodosioupolis were the strongholds of the thughur, subdivided into the thughurs of Syria and of the al-Jazira.61 These fortifications ran through mountainous country from the Taurus in Cilicia towards Germanikeia, and then on to Melitene. In 799 Harun built the town of Haruniyya, named after himself, between Germanikeia and Anazarbos, as part of his programme to organise and improve the defences of the northern Syrian frontier. These strongholds on the front line were the culmination of a long process that had already been underway before Harun’s caliphate. But behind them Harun instituted the ‘awasim; these formed a second, more compact buffer zone between northern Syria and the Cilician thughur, extending from Antioch to Membij. Harun sought to break up the conglomeration of north-west frontier provinces and to impose his own personality directly on the frontier area and on the waging of jihad, thus cutting down to size the figures of the local commanders and warlords. The frontier became a centre of unprecedented attention in part because of the accession to the throne of Nikephoros I in 802. Harun al-Rashid was provoked by an insulting letter from Nikephoros, demanding the return of the tribute that Irene had paid, but more important may have been a Byzantine raid against Anazarbos and Kanisa al-Sawda. There was an exchange of prisoners in 805 but, as mentioned above, in the following year Harun imposed tribute of 30,000 dinars on Byzantium, in addition to the poll tax that was payable by Nikephoros and his son. In 808 an exchange of prisoners and a summer expedition into Byzantine territory took place, but no further major Muslim expeditions into Byzantine territory would occur until 830. Harun’s commitment of so much energy and so many resources to his wars against the Byzantines reflected his sense of duty, although some scholars see his warfaring primarily as propaganda intended for internal consumption. 62 His wars brought few concrete territorial gains, but demonstrated his personal involvement with the jihad.Harun may be characterised as the first ghazi-caliph.63 For Hisham and theMarwanids, military service had been a personal obligation, but Harun adopted the role of ghazi. He owed his accession to the power base that he and his supporters controlled in the north-western frontier regions, facing the Byzantines. He held himself out as a ruler whose power was coterminous with Islam and also as an imam-volunteer. Irene andNikephoros I could not match him as commanders themselves and they lacked generals who were capable of resisting such an effective leader.64 In this period some Muslim scholars and saints also settled on the frontier, a trend that had not been typical of the Umayyad era. Tarsus andMelitene emerged as the principal centres of the thughur by the mid-ninth century, and part of their population was made up of ghazivolunteers. Some commercial goods passed through these strongholds to and from Byzantine territory, and professional military men tried to wield power there.