Diocletian was Roman emperor from 284 -
In 285, Diocletian divided and
reorganized the Roman empire.
He abdicated on May 1, 305.
After his abdication a fresh series of civil wars and
ensued. Constantine, the first Christian emperor, reunited the
empire in 324.
And here is the map:
BOUNDARIES OF THE ROMAN PROVINCES BEFORE DIOCLETIAN
Click to enlarge