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Diocletian, commanding an army near the Bosphorus, is proclaimed emperor by his own troops when news comes in AD 284 that his predecessor has been murdered. He marches west, killing a rival claimant in battle in 285. The sequence of events reflects many previous occasions in this century, when usurpers, supported by their own armies, have laid claim to the throne. But Diocletian breaks this pattern of anarchy.

He is fortunate in that the frontiers of empire have recently been pushed back to their well established lines in the north and the east. He uses an unusual period of stablity to introduce a bold and but highly schematic reform in the administration of the empire.

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He not only divides the empire geographically into east and west (following the example of Valerian). He even divides the traditional imperial title, Augustus Caesar, into two ranks - a senior one, Augustus, and a junior Caesar.

He appoints his friend Maximian as co-emperor, giving him the western empire and the title Augustus. Officially Diocletian and Maximian are to be equal, but a subtle addition to the title makes plain who is in charge. Diocletian in the east is now Augustus Jovius, representing Jupiter on earth. Maximian in the west is Augustus Herculius, standing in for Hercules. As any Roman knows, Hercules is a muscular hero; but the supreme god is Jupiter.

Diocletian, commanding an army near the Bosphorus, is proclaimed emperor by his own troops when news comes in AD 284 that his predecessor has been murdered. He marches west, killing a rival claimant in battle in 285. The sequence of events reflects many previous occasions in this century, when usurpers, supported