After his rise through the ranks of military until he became the Roman Cavalry Commander to Emperor Carus, when Carus and his son Numerian died on campaign in Persia, Diocletian became emperor on November 20, 284. Of course, Carus surviving son, Carinus also claimed the title, but was defeated in the battle of Margus. Diocletian´s rule put an end to the Crisis of the Third Century.
Diocletian named his fellow officer Maximian co-emperor, Augustus, in 286. Afterwards, in 293, he named caesars Galerius and Constantius, or heirs of the augustus title. This new regime was called the Tetrarchy, or “government of four”, and it meant the geographical division of the empire into four parts.
Diocletian lead military campaigns against Sarmatians, Carpi, Alemanii, and the usurpers in Egypt, securing frontiers and eliminating threats against his empire. By 299 he established negotiations with the Sassanian Empire, securing a stable peace with one of Rome´s oldest enemies. He reorganized the provinces and created the biggest and most bureaucratized government in the history of Rome.
This increase, both bureaucratic and military, constant military campaigns, and architectural projects, raised the state expenses, and required a fiscal reform. By year 297, the tax system had been standardized, with a higher but fairer tax rate.
Nevertheless, not all of his reforms were successful. The tetrarchy collapsed as soon as Diocletian abdicated, resulting in a fierce fight for power between Maxentius and Constantine. His Edict on Maximum Prices (301), his attempt to curb inflation via price controls, was counterproductive and quickly ignored. The Diocletianic Persecution (303–11), the empire’s last, largest, and bloodiest official persecution of Christianity, failed to eliminate Christianity in the empire; indeed, after 324, Christianity became the empire’s preferred religion under its first Christian emperor, Constantine.
With all and this, Diocletian´s reforms and innovations changed the structure of Imperial Government and stabilized it both economically and militarily, allowing the Empire to survive 100 more years, although it had been near collapse a few years before.
In May 1, 305, Diocletian abdicated, becoming the first Roman Emperor to leave voluntarily his office.