The most vulnerable period for Byzantium came immediately after the disastrous battle of the river Yarmuk in 636, during the imperial succession crisis triggered by Heraclius’ death in 641 and in the earliest years of his successor Constans II (641–68) (see above, pp. 230–1). After the withdrawal of their armies from Syria and northern Mesopotamia, the Byzantines had managed to regroup by the late 630s and early 640s and create new Anatolian defences, taking advantage of the Taurus mountains and key fortified points in the interior. Although limited truces had previously been struck with the Arabs, no formal, linear frontier was ever established and hostilities persisted. Fortunately for the Byzantines, the Muslims had priorities elsewhere. They needed to consolidate their vast territorial gains in Syria and northernMesopotamia and to complete their conquest of Egypt, both more attractive and easier goals than the continued seizure of territory in Byzantine Anatolia. By the time thatMu‘awiya (661–80) emerged as caliph, the Muslims had missed their chance of outright conquest of Byzantium: the empire was stabilising, as Constans II rapidly gained military experience and judgement, and developed his defences against the Arab threat from Syria. Caliph ‘Umar (634–44) reportedly believed that the Muslims needed to consolidate their territorial expansion of the 630s before pursuing further conquests at the expense of Byzantium. Tradition has it that during his caliphate ‘Umar restrained Mu‘awiya from attempting an invasion of the island of Cyprus.2 This is plausible, and consistent with the well-known story that ‘Umar also tried to restrain the very able military commander ‘Amr ibn al-‘As from invading Egypt.3 Mu‘awiya only succeeded in implementing his far more aggressive policy towards Byzantium after ‘Umar’s death, prompted no doubt in part by a calculation of his own interests, as well as of the advantages he believed such a policy would gain for Islam. Political, topographical and logistical impediments combined with Byzantine military resilience to halt major Arab advances into Anatolia in the seventh century, even though the Arabs made significant territorial conquests in the central andwesternMediterranean at Byzantium’s expense. They initially used a combination of force and diplomacy to overcome the Byzantine defences, being prepared to engage in fierce combat, while also negotiating separate terms with both local civilians and military commanders. However, these tactics ceased to be effective once the Muslim armies tried to penetrate and establish permanent control north of the Taurus and Anti-Taurus mountains in Asia Minor. It is difficult to define the style of Arab–Byzantine warfare in the seventh century. Muslim methods involved a broad conformity to Islamic principles, including the spreading of the faith by force, together with use of combined military and political initiatives. The Muslims would divide their opponents both on and off the battlefield, identifying those willing to conclude separate peace terms and then allowing them to negotiate their submission, thus reducing the likelihood of costly, bloody resistance.However, using political pressure to control the Byzantine civilian population was not inconsistent with fighting Byzantium’s forces and demolishing their strong points, destroying their opponents’ equilibrium. There tended to be close consultation, as far as was practicable, between field commanders and the highestMuslim leadership. The Arabs generally tried to avoid positional warfare, such as slow-moving sieges, except when they were forced onto the defensive; their strategy was to drive their opponents into decisive battle, with an eye to exploiting military victories to the full. The Byzantines, for their part, tended to avoid the risk of major land battles after their defeat at Yarmuk, preferring to seek refuge in fortified positions. They made cautious efforts to identify, cut off, attack and destroy smaller detachments of Muslim raiders, using relatively modest-sized mobile units. Concepts of holy war and crusade did not dominate Byzantine warfare between the seventh and ninth century.4 It is similarly hazardous to superimpose later concepts of jihad onto seventh- and eighth-century Arab– Byzantine warfare, as both jihadi practices and concepts in this period are poorly documented. They become better documented with the expansion and stabilisation of the Muslim territories, when communal obligation to perform jihad was increasingly focused on the frontier regions, and it was in areas such as northern Syria and upperMesopotamia that the most zealous Muslim soldiers tended to be concentrated. Areas further back from the frontiers, although theoretically supportive of military expansion, were in practice less involved on a daily basis. It became increasingly difficult to engage the whole Muslim community actively in the process of jihad. Between the late 630s and 650s both empire and caliphate periodically created zones of devastation between their territories. Local inhabitants would occasionally be allowed to stay, but only if they agreed to act as informers and refused to help the enemy. However, these more charitable arrangements do not appear to have been successful: both powers expelled inhabitants they regarded as hostile, leaving either a total wasteland, or settling their own armed troops and loyal populations. ‘Umar reportedly wanted to create at least a temporary zone of destruction between Byzantium and the caliphate, just asHeraclius (610–41) had done in the remaining imperial territories after the Arabs overran Syria. According to al-Ya‘qubi, whenever ‘Umar spoke of the Byzantines, he voiced the hope that God would ‘turn the passes between us and them into burning coals; this side [of the passes] for us and what is behind [the passes] for them’.5 The ninthcentury historian al-Baladhuri reports that ‘Umar ordered that Arabissos be destroyed and its inhabitants forcibly removed, after learning of their refusal to give information on Byzantine troop movements to theMuslims, while continuing to act as informants for the empire.6 The inhabitants of nearby Duluk and Raban, in northern Syria, apparently honoured a similar arrangement with the Muslims.7 And while governor of Syria in 649, Mu‘awiya forced the Cypriots to stop giving aid to the Byzantines and to inform on them, an arrangement which they failed to respect.8 Byzantine andMuslim governing circles thus had an equal interest in creating zones of devastation. The resulting attempts to tighten governmental control on either side of the de facto border, to counteract the emergence of independent borderland powers, helped to strengthen state-building for both caliphate and empire. From Heraclius’ reign onwards, the Byzantines started to appoint military commanders in place of those civil governors who proved too willing to come to terms with theMuslims. Through such appointments, the Byzantines hoped to concentrate power in the hands of military leaders who were dependent on the emperor: they would therefore make no local settlements with the Muslims without having received explicit imperial authorisation and approval.9 Not all early Arab–Byzantine contacts were violent, and despite extensive military engagement, limited maritime trade, exchange and travel – especially pilgrimages – persisted. Some Christian churchmen and ascetics managed to cross the frontiers at transit points such as Cyprus, and smugglers and renegades played their part in creating a porous frontier. Diplomacy coexisted with warfare. Prisoner- and hostage-exchange was a complex challenge for both Byzantine andMuslim authorities in the seventh and eighth centuries.Diplomatic negotiations generally took place either atDamascus or Constantinople at the highest level and were conducted by the caliph and emperor – or their envoys – but ad hoc exchanges could also occur occasionally between local commanders.10 Accommodating such political realities committed neither side to any fundamental theoretical or religious concessions. Diplomatic protocol was highly formalised by the tenth century, as witness Constantine VII Porphyrogenitus’ Book of ceremonies, but it is likely that this protocol owed its origins to seventh- and eighth-century practices.