 Following the Muslim conquests of the 7th century , the Arab – Byzantine conflict had featured constant raids and counter @-@ raids along a relatively static border roughly defined by the line of the Taurus and Anti @-@ Taurus Mountains . Until the 860s , superior Muslim armies had placed the Byzantines on the defensive . Only after 863 , with the victory in the Battle of Lalakaon , did the Byzantines gradually regain some lost ground against the Muslims , launching ever @-@ deeper raids into Syria and Upper Mesopotamia and annexing the Paulician state around Tephrike ( now <unk> ) . Furthermore , according to historian Mark Whittow , " by 912 the Arabs had been pinned back behind the Taurus and Anti @-@ Taurus " , encouraging the Armenians to switch their allegiance from the Abbasid Caliphate to the Empire , in whose service they entered in increasing numbers . The revival of Byzantine power was further facilitated by the progressive decline of the Abbasid Caliphate itself , particularly under al @-@ Muqtadir ( r . 908 – 932 ) , when the central government faced several revolts . In the periphery of the Caliphate , the weakening of central control allowed the emergence of semi @-@ autonomous local dynasties . In addition , after the death of the Bulgarian Tsar Simeon in 927 , a peace treaty with the Bulgarians allowed the Empire to shift attention and resources to the East . 
