In 717–18, Western civilization was hanging by a thread.Today, August 15, marks the anniversary of Constantinople’s victory over Muslim invaders in what historians commonly call the “Second Siege of Byzantium,” 717–18. Prior to this massive onslaught, the Muslims had been hacking away at the domains of the Byzantine empire for nearly a century. The Muslims’ ultimate goal was the conquest of Constantinople — for both political and religious reasons. Politically, Islam had no rival but the “hated Christians” of Byzantium, known by various appellations — including al-Rum (the Romans), al-Nassara (the Nazarenes), and, most notoriously, al-Kilab (the “dogs”). The eastern...