Two features characterise Byzantine–Muslim relations between the seventh and ninth century: a finely tuned link between domestic strife and the external fortunes of war and diplomacy; and the fitful involvement of both polities’ leaders with their armed forces, without exercise of personal command. The Arabs’ dramatic conquest of Byzantium’s eastern territories in the 630s was followed by four further periods of Muslim expansion; by gradual stabilisation; and then by Byzantine strengthening and eventual territorial recovery. The four periods ofMuslim expansion were all brought to an end by bouts of civil war (fitna) among the Muslims, the first lasting from 656 until 661. The second expansionary period under the Sufyanid Umayyad caliphs was followed by almost ten years of civil war, from 683 until 692; the third, under the Marwanid caliphs – the final branch of the Umayyad dynasty – was broken by infighting for some two years between 718 and 720, only to be followed by a twenty further years or so of aggressive campaigning. The violent replacement of the Umayyads by the Abbasids in the mid-eighth century owed nothing to Byzantium, nor did it halt military and diplomatic interaction between the two polities; but it did transform Arab–Byzantine relations.