[SunkenCiv]

Diocletian stopped the disintegration of the Roman Empire in its Crisis of the Third Century, that had began with the reign of Septimius Severus, a highly effective war leader who was proud of his Carthaginian roots, and the rest of the Severan dynasty wasn't up to the job.

After reorganizing the provincial borders and administration, and dividing the Empire into four administrative districts, Diocletian set up a system of orderly succession, prepared a fortified retirement villa that still stands in Split, Croatia, then he retired to test the system he'd set up. He compelled his eastern colleague to do the same. D had to emerge from retirement for a time to straighten it out.

[Antoninus]

After reorganizing the provincial borders and administration, and dividing the Empire into four administrative districts, Diocletian set up a system of orderly succession,

The system of orderly succession failed almost immediately. If you read the source quoted in this article (Lactantius), that becomes extremely clear.

[Antoninus]

...then he retired to test the system he'd set up. He compelled his eastern colleague to do the same.

Not according to Lactantius, who maintains that he was forced into retirement by sickness and his overbearing colleague, Galerius.

[Antoninus]

D had to emerge from retirement for a time to straighten it out.

There is no source that says that Diocletian emerged from retirement. In fact, they all say the opposite -- that when the second Tetrarchy was in the process of failing, Maximian attempted to convince Diocletian to emerge from retirement to fix things. Diocletian refused, famously saying, "If you could see the cabbages that I have raised with my own hands, you would know that a return to power is no enticement."