Building on third-century trends towards absolutism, he styled himself an autocrat, elevating himself above the empire's masses with imposing forms of court ceremonies and architecture.
Bureaucratic and military growth, constant campaigning, and construction projects increased the state's expenditures and necessitated a comprehensive tax reform.
Born to a family of low status in Dalmatia (Roman province), 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.
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.
He raised his sword to the light of the sun and swore an oath disclaiming responsibility for Numerian's death.