Arab–Byzantine wars
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, in Upper Mesopotamia, places like Germanikeia (Mar'ash), Hadath and Melitene (Malatya) became major military centers. These two regions came to form the two-halves of a new fortified frontier zone, the thughur.

Both the Umayyads and later the Abbasids continued to regard the annual expeditions against the Caliphate's "traditional enemy" as an integral part of the continuing jihad, and they quickly became organized in a regular fashion: one to two summer expeditions (pl. ṣawā'if, sing. ṣā'ifa) sometimes accompanied by a naval attack and/or followed by winter expeditions (shawātī). The summer expeditions were usually two separate attacks, the "expedition of the left" (al-ṣā'ifa al-yusrā/al-ṣughrā) launched from the Cilician thughur and consisting mostly of Syrian troops, and the usually larger "expedition of the right" (al-ṣā'ifa al-yumnā/al-kubrā) launched from Malatya and composed of Mesopotamian troops. The raids were also largely confined to the borderlands and the central Anatolian plateau, and only rarely reached the peripheral coastlands, which the Byzantines fortified heavily.

Under the more aggressive Caliph Hisham ibn Abd al-Malik (r. 723–743), the Arab expeditions intensified for a time, and were led by some of the Caliphate's most capable generals, including princes of the Umayyad dynasty like Maslama ibn Abd al-Malik and al-Abbas ibn al-Walid or Hisham's own sons Mu'awiyah, Maslama and Sulayman. This was still a time when Byzantium was fighting for survival, and "the frontier provinces, devastated by war, were a land of ruined cities and deserted villages where a scattered population looked to rocky castles or impenetrable mountains rather than the armies of the empire to provide a minimum of security" (Kennedy).

In response to the renewal of Arab invasions, and to a sequence of natural disasters such as the eruptions of the volcanic island of Thera, the Emperor Leo III the Isaurian concluded that the Empire had lost divine favour. Already in 722 he had tried to force the conversion of the Empire's Jews, but soon he began to turn his attention to the veneration of icons, which some bishops had come to regard as idolatrous. In 726, Leo published an edict condemning their use and showed himself increasingly critical of the iconophiles. He formally banned depictions of religious figures in a court council in 730.

This decision provoked major opposition both from the people and the church, especially the Bishop of Rome, which Leo did not take into account. In the words of Warren Treadgold: "He saw no need to consult the church, and he appears to have been surprised by the depth of the popular opposition he encountered". The controversy weakened the Byzantine Empire, and was a key factor in the schism between the Patriarch of Constantinople and the Bishop of Rome.

The Umayyad Caliphate however was increasingly distracted by conflicts elsewhere, especially its confrontation with the Khazars, with whom Leo III had concluded an alliance, marrying his son and heir, Constantine V (r. 741–775) to the Khazar princess Tzitzak. Only in the late 730s did the Muslim raids again become a threat, but the great Byzantine victory at Akroinon and the turmoil of the Abbasid Revolution led to a pause in Arab attacks against the Empire. It also opened up the way for a more aggressive stance by Constantine V (r. 741–775), who in 741 attacked the major Arab base of Melitene, and continued scoring further victories. These successes were also interpreted by Leo III and his son Constantine as evidence of God's renewed favour, and strengthened the position of Iconoclasm within the Empire.
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