How Constantine Became Emperor

As background to the conversion of the emperor Constantine I have been explaining how Diocletian had set up the Tetrarchy with a sensible order of succession, so that the Roman emperors would be chosen on a rational basis rather than simply because of accidents of birth or the whims of the army. His plan ended up not working.

Because of health issues, after a long and successful reign of over two decades, Diocletian decided to retire from office on May 1, 305. For the sake of a smooth success...