Born to a family of low status in Dalmatia, Diocletian rose through the ranks of the military to become Roman cavalry commander to the Emperor Carus.
After the deaths of Carus and his son Numerian on campaign in Persia, Diocletian was proclaimed emperor.
In spite of these failures and challenges, Diocletian's reforms fundamentally changed the structure of Roman imperial government and helped stabilize the empire economically and militarily, enabling the empire to remain essentially intact for another 150 years despite being near the brink of collapse in Diocletian's youth.
Weakened by illness, Diocletian left the imperial office on 1 May 305, and became the first Roman emperor to abdicate the position voluntarily.
He lived out his retirement in his palace on the Dalmatian coast, tending to his vegetable gardens.
His palace eventually became the core of the modern-day city of Split in Croatia.
Not all of Diocletian's plans were successful: the Edict on Maximum Prices (301), his attempt to curb inflation via price controls, was counterproductive and quickly ignored.
He appointed fellow officer Maximian as Augustus, co-emperor, in 286.
Diocletian delegated further on 1 March 293, appointing Galerius and Constantius as Caesars, junior co-emperors.
Under this 'tetrarchy', or "rule of four", each emperor would rule over a quarter-division of the empire.
Diocletian secured the empire's borders and purged it of all threats to his power.
The title was also claimed by Carus' other surviving son, Carinus, but Diocletian defeated him in the Battle of the Margus.