On November 20, 284 CE, Diocles, an Illyrian officer who had risen to high command in the Roman army, was elevated to the purple by the soldiers at Nicomedia. The new emperor took the name of Gaius Aurelius Valerius Diocletianus (284–305) and is known as Diocletian. Having been acclaimed by the military, Diocletian was the first emperor to disregard the Senate in Rome by not seeking its customary confirmation. Going a step further, he also pronounced that his elevation to the throne of Rome enjoyed divine sanction, having come about through the will of Jupiter Optimus Maximus, Rome's highest deity.

After defeating Carinus, the son of the former Emperor Carus, and warding off several attempts by would-be usurpers to seize power, the emperor implemented a program to restore the tottering empire. Diocletian was a superb administrator, but he was also an autocratic ruler. He expanded the army in order to better defend Rome's...