October 28 marks the anniversary of the Battle of the Milvian Bridge (AD 312) at which Constantine the Great defeated the usurper Maxentius who had set himself up as emperor in Rome. Son of the emperor Maximian Herculius, Maxentius claimed the same right to the throne that Constantine had claimed from his own father, the emperor Constantius I Chlorus. The difference was that Constantius conferred the imperial power upon his son Constantine on his deathbed, whereas Maxentius took his father's authority by force and drove the old man from Rome. After several failed attempts of the eastern emperors to end...