Complex sentence: It is indicative that the expeditions against Byzantium were the only external campaigns where the Caliph or his sons participated in person, and that in official propaganda these campaigns closely paralleled the leadership by Abbasid family members of the annual pilgrimage ("hajj") to Mecca.
Split into simpler sentences: It is indicative that the expeditions against Byzantium were the only external campaigns where the Caliph or his sons participated in person. These campaigns were closely paralleled in official propaganda by the leadership by Abbasid family members of the annual pilgrimage ("hajj") to Mecca.