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 succession, he compelled his rather unwilling co-Augustus, Maximian, to do so as well, to make way for the two Caesars, Galerius and Constantius, to rise to the senior offices.  For their replacements, according to the principles that Diocletian had devised, two Caesars were chosen as junior emperors:   Maximin Daia (not to be confused with the out-going Augustus Maximian) to serve with Galerius in the East, and Severus to serve with Constantius in the West.   There was now a “Second Tetrarchy.”

At the time it may have seemed like a smooth and unproblematic transition, and in a sense it was — until one of the new Augusti died.  Then the plan of succession based on qualifications ran afoul of both the dynastic principle and the army.

The background to the story involves the new Augustus of the West, Constantius, and his son Constantine.  Constantine had …

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