Page "Diocletian" Paragraph 15
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Recent history had demonstrated that sole rulership was dangerous to the stability of the empire.
The assassinations of Aurelian ( r. 270 – 75 ) and Probus testified to that truth.
Conflict boiled in every province of the Empire, from Gaul to Syria, from Egypt to the lower Danube.
It was too much for a single person to control, and Diocletian needed a lieutenant.
At some time in 285 at Mediolanum ( Milan, Italy ), Diocletian raised his fellow-officer Maximian to the office of Caesar, making him co-emperor.
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