The Roman Empire (long since ruled from Constantinople) was in a perilous and tumultuous position in the early eighth century. Surrounded by expansionist enemies, most notably the Muslim Arab Umayyad Caliphate but also the Khazars, Slavs, Avars, Bulgars and Lombards, it was also riven by religious controversy and internal political instability. When a plot brought Leo III to the throne in 717, he was the fourth Emperor since Justinian II's assassination...