In 718 AD, after a combined land and sea effort, Arabs of the Umayyad Caliphate led by Maslamah ibn Abd al-Malik finally seized Constantinople, the capital city of the Byzantine Empire after a year-long siege. The successful campaign marked the culmination of twenty years of attacks and gradual Arab encroachment on the Byzantine borderlands.
Umayyads capture ConstantinopleAs a result, the relentless Arab advance would continue northwards imperilling the security of Christian Western Europe from two separate fronts, Spain and now also Asia Minor. This second regional threat was fully recognized by the Bulgars who willingly sent forces to the aid of the Byzantines in the knowledge that defeat would invite conquest of their own lands. But unfortunately Emperor Anastasius II had not taken seriously the signs of a major impending Arab invasion in 715. With only five months to prepare, the imperial collapse was exacerbated by internal division caused by the ongoing civil strife between Byzantine general Leo the Isaurian and Emperor Theodosius "the Unwilling".