Complex sentence: Relations were very strained in the mid- 8th century between the papacy and the Byzantine emperors of the Isaurian dynasty, and the Byzantine Empire itself was beset by the Abbasid Caliphate and Bulgars; no help came from Constantinople.
Split into simpler sentences: Relations were very strained in the mid- 8th century between the papacy and the Byzantine emperors of the Isaurian dynasty. Due to the fact that the Byzantine Empire itself was beset by both the Abbasid Caliphate and Bulgars, no help came from Constantinople.