Al-ʿAwāṣim (, "the defences, fortifications"; sing. al-ʿāṣimah, , "protectress") was the Arabic term used to refer to the Muslim side of the frontier zone between the Byzantine Empire and the Umayyad and Abbasid Caliphates in Cilicia, northern Syria and Upper Mesopotamia. It was established in the early 8th century, once the first wave of the Muslim conquests ebbed, and lasted until the mid-10th century, when it was overrun by the Byzantine advance. It comprised the forward marches, comprising a chain of fortified strongholds, known as al-thughūr (; sing. al-thagr, , "cleft, opening"), and the rear or inner regions of the frontier zone, which was known as al-ʿawāṣim proper. On the Byzantine side, the Muslim marches were mirrored by the institution of the kleisourai districts and the akritai border guards. The term thughūr was also used in the marches of al-Andalus and Mawara al-Nahr, and survived in historical parlance, to be revived by the Egyptian Mamluks in the 14th century, when the areas traditionally comprising the 'ʿawāṣim and thughūr in northern Syria and the northern Euphrates region came under their control. Arab–Byzantine frontier zone Creation of the frontier zone Already from late 630s, after the rapid Muslim conquest of Syria, a wide zone, unclaimed by either Byzantines or Arabs and virtually deserted (known in Arabic as al-Ḍawāḥī (, "of the Outer Lands") and in Greek as ta akra (, "the extremities") emerged between the two powers in Cilicia, along the southern approaches of the Taurus and Anti-Taurus mountain ranges, leaving the Anatolian plateau in Byzantine hands. Both Emperor Heraclius () and the Caliph ʿUmar () pursued a strategy of destruction within this zone, trying to transform it into an effective barrier between their realms. Nevertheless, the ultimate aim of the caliphs remained the outright conquest of Byzantium, as they had done with its provinces in Syria, Egypt and North Africa, and it was only the failure of the Second Arab Siege of Constantinople in 717–718 that forced a revision of this strategic objective: although raids into Anatolia continued, the goal of conquest was abandoned, and the border between the two powers began to acquire more permanent features. For the next two centuries, border fortresses might change hands between Byzantines and Arabs, but the basic outline of the Arab–Byzantine border remained essentially unaltered. Thus the term al-thughūr, which initially meant "fissures, clefts" (cf. their Greek name ta Stomia, , "the mouths/openings") and