In June the leader of Islamic State declared the creation of a caliphate stretching across parts of Syria and Iraq - Abu Bakr al-Baghdadi named himself the caliph or leader. Edward Stourton examines the historical parallels and asks what is a caliphate, and what is its appeal? When Islamic State (IS) declared itself a caliphate in June this year, and its leader Abu Bakr al-Baghdadi claimed the tit