the besieging army suffered horrendous casualties from the cold and the lack of provisions. In spring, new reinforcements were sent by the new caliph, Umar ibn Abd al-Aziz (r. 717–720), by sea from Africa and Egypt and over land through Asia Minor. The crews of the new fleets were composed mostly of Christians, who began defecting in large numbers, while the land forces were ambushed and defeated in Bithynia. As famine and an epidemic continued to plague the Arab camp, the siege was abandoned on 15 August 718. On its return, the Arab fleet suffered further casualties to storms and an eruption of the volcano of Thera. Stabilization of the frontier, 718–863 The first wave of the Muslim conquests ended with the siege of Constantinople in 718, and the border between the two empires became stabilized along the mountains of eastern Anatolia. Raids and counter-raids continued on both sides and became almost ritualized, but the prospect of outright conquest of Byzantium by the Caliphate receded. This led to far more regular, and often friendly, diplomatic contacts, as well as a reciprocal recognition of the two empires. In response to the Muslim threat, which reached its peak in the first half of the 8th century, the Isaurian emperors adopted the policy of Iconoclasm, which was abandoned in 786 only to be readopted in the 820s and finally abandoned in 843. Under the Macedonian dynasty, exploiting the decline and fragmentation of the Abbasid Caliphate, the Byzantines gradually went on the offensive, and recovered much territory in the 10th century, which was lost however after 1071 to the Seljuk Turks. Raids under the last Umayyads and the rise of Iconoclasm Following the failure to capture Constantinople in 717–718, the Umayyads for a time diverted their attention elsewhere, allowing the Byzantines to take to the offensive, making some gains in Armenia. From 720/721 however the Arab armies resumed their expeditions against Byzantine Anatolia, although now they were no longer aimed at conquest, but rather large-scale raids, plundering and devastating the countryside and only occasionally attacking forts or major settlements. Under the late Umayyad and early Abbasid caliphs, the frontier between Byzantium and the Caliphate became stabilized along the line of the Taurus-Antitaurus mountain ranges. On the Arab side, Cilicia was permanently occupied and its deserted cities, such as Adana, Mopsuestia (al-Massisa) and, most importantly, Tarsus, were refortified and resettled under the early Abbasids. Likewise,