Summerraids intoByzantine Anatolia diminished in intensity onceAbbasid power began to fragment both on the periphery and in the central cities during the internal conflicts of the middle decades of the ninth century. A slave revolt broke out in southern Iraq in 869 and the rebels, mostly of east African origin and known as the Zanj, took over Basra and even started striking their own coins before eventually being overwhelmed in 883. Then, from the 890s onwards for almost a century, the Carmathians (Qaramita) backed by bedouin tribes posed a constant challenge, sometimes amounting to a serious threat to the state. Even before inner turmoil diverted the caliphate’s resources, growing military and economic power enabled Byzantium to undertake offensive actions. One Byzantine expeditionary force penetrated to Amida, where it took many prisoners in 851, and a Byzantine fleet raided the Egyptian port of Damietta in 853 (see above, p. 298). The central government in Samarra or Baghdad could do little effectively to deter this. Ninth-century Byzantine–Muslim diplomatic practices and protocols are well documented in theGreek and Arabic sources.Muslim geographical texts such as that of Ibn Khurradadhbih, with its itineraries and other data concerning the empire, also testify to growing contacts and exchanges.70 Famous Byzantine embassies to Baghdad include those in the reigns of Leo Vand Theophilos.71 Two of the most important missionswere those of John the Grammarian of 829 and 831: returning from his first mission, John is said to have advised the emperor Theophilos to build the palace of Bryas ‘in imitation of those of the Saracens’.72 Not all the information passed through formal channels.Members of Syrian Christian communities, some of whose members also knew Greek, were important intermediaries between the two empires and cultures. They could carry out translations of ancient texts73 and they could also transmit intelligence of vital relevance to contemporary politics, war and commerce. The frontiers were not tightly sealed between Byzantium and the lands under Abbasid rule. In fact, in every century renegades fled from one side to the other: many of them were neither Greek nor Armenian by origin, and some moved repeatedly to and fro. The migrants included the Banu Taghlib in the seventh century, Tatzates in the late eighth century, Theophobos in the ninth century, Samonas in the early tenth century and Bardas Skleros in the late tenth century.74 Such defections reveal the potential for the movement of individuals and even, occasionally, of whole groups or communities across the frontiers. Precisely for this reason, attempts were made by the respective authorities to keep vigilant watch over the border zones. It is probable that the Byzantines had developed techniques for reporting and tracking, and for mobilising their own military forces to cut off and destroy enemy raiders in Anatolia, by the late seventh or earlier eighth centuries. Documentation of these practices exists from the tenth century, but the basic military defence measures were most probably in place much earlier.75 Emperors, caliphs and amirs took responsibility for major operations on the Byzantine–Arab frontiers intermittently between the seventh and the mid-ninth century, but none persisted in campaigning in person. Too many other priorities and pressures were in play, and important as the frontier was, it did not monopolise their attention. Constans II spent much of his time on or near scenes of military campaigning; the state of emergency required the emperor’s continuous personal involvement. Leo III and Constantine V managed to lead far more effectively than their seventh-century predecessors had done (see above, pp. 265–6, 273, 277), but only a few of the earlier ninth-century emperors came from a military background and those who scored significant successes did so in fighting the Bulgars, not the Arabs (see above, p. 257). So far as the Muslims were concerned, no leader after their campaigns of conquest in the seventh century, and occasional expeditions of the eighth and earlier ninth centuries, managed to assemble sufficient human and material resources to undertake further major offensives or to conquer fresh territories. The personal presence of a sovereign was necessary to make the respective military systems function effectively on both sides of the frontier, yet neither caliph nor basileus could long give the Byzantine–Muslim frontier his full attention in the light of pressures elsewhere. No single campaign, battle or other brilliant tactical or political feat could resolve the underlying military challenges and political tensions in northern Syria and upperMesopotamia. Processes of political and military deterioration undermined Muslim strength in the area, but limited resources, the cost and complexity of mobilisation and fear of military coups continued to frustrate the Byzantine emperors, too. Internal conflicts greatly complicated the conduct of warfare and diplomacy on their eastern frontier. No systematic institutional solution showed signs of emerging there for either Byzantines or Muslims, and indecisive if incessant warfare and diplomacy remained the norm. The approximate limits of Byzantine control to the south-east in the mid-ninth century were not radically different from those which had emerged some two centuries earlier, in the aftermath of the earliest Muslim conquests. Decisive change could wait.