Why Did Diocletian Institute the Tetrarchate?

Diocletian's groundbreaking reform was to split the Empire into four subunits, with the two co-Augustii assisted by the junior Caesars. Why did he do this? The conventional explanation is that, after the crises of the third century, the army and bureaucracy were drained. But why would this necessitate a four-way split? Most of all, why did Diocletian (who had risen from basically nothing, and so, presumably, would be loath to give power away) even agree to this scheme?
 
Well, he understood that governing the empire alone required someone with the energy of an Aurelian, and even then it would make him more liable to get offed. So why not appoint his friend to rule the west and just chill out in the east? Well, it turned out Maximian was pretty shitty at his job, so he needed someone with an ounce of competency in military matters to handle the problem with the Franks and Carausius. I think it was about then that he stumbled into a system he thought would be a great way to solve the succession crises that plagued the empire.

Really, it was about making the empire more manageable and having a stable system of succession.
 
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