by volunteers, drawn by the religious motivation of jihad against the Byzantines but often paid a salary by the state as well. All this entailed a heavy financial burden on the Abbasid government. Under Harun al-Rashid, taxation from the Cilician sector brought in 100,000 gold dinars every year, which were all spent locally for public works, salaries, espionage etc. In addition, the costs of cross-border expeditions typically ranged between 200,000 and 300,000 dinars annually. The Mesopotamian sector's revenue amounted to some 70,000 dinars, to which the central government added 120,000–170,000 dinars each year for the upkeep of the fortifications and the salary of the frontier troops. Military operations By the 9th century, the Arab raiding expeditions launched against Byzantium from the frontier zone had gradually assumed an almost ritual character and were strictly organized. According to Qudama ibn Ja'far, the conventional pattern of Arab incursions included a first expedition in spring (10 May–10 June), when horses could find abundant fodder, followed after about a month's rest by a summer raid (10 July–8 September), usually the main campaign of the year, and sometimes by a winter raid in February–March. The importance of these raids is summarized by Islamic scholar Hugh N. Kennedy: "the ṣāʿifa (summer raid) was as much a part of the symbolic and ritual functions of the Caliph as was organising and providing leadership for the annual hajj to Mecca". The frontier zone was fiercely contested between the Arabs and the Byzantines. Raids and counter-raids were a permanent fixture of this type of warfare. Forts on either side of the notional frontier were captured and razed, or sometimes occupied, but never for long. As a result, the region was often depopulated, necessitating repeated resettlement. There is nevertheless evidence of some prosperity, based on agriculture and commerce, especially during the second half of the 9th century, when the borderlands became a node in a commercial route linking Basra with northern Syria and even Constantinople. After 842 and for most of the later 9th century, the decline of Abbasid power meant that control over the Thughūr gradually devolved to semi-independent border emirates, chiefly Tarsus, Malatya and Qālīqalā, which were left largely to fend on their own against a resurgent Byzantium. The Battle of Lalakaon in 863 broke the power of Malatya, altering the balance of power in the region, and signalled the beginning of a gradual Byzantine encroachment on the Arab