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 raised his sword to the light of the sun and swore an oath disclaiming responsibility for Numerian's death.
He asserted that Aper had killed Numerian and concealed it.
Augustus, the first Emperor, had nominally shared power with his colleagues, and more formal offices of Co-Emperor had existed from Marcus Aurelius on.
Most recently, Emperor Carus and his sons had ruled together, albeit unsuccessfully.
The first time Diocletian's whereabouts are accurately established, in 282, he was made by the newly Emperor Carus commander of the Protectores domestici, the élite cavalry force directly attached to the Imperial household – a post that earned him the honor of a consulship in 283.
– left his sons Numerian and Carinus as the new Augusti.
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.
The title was also claimed by Carus' other surviving son, Carinus, but Diocletian defeated him in the Battle of the Margus.
Diocletian's reign stabilized the empire and marks the end of the Crisis of the Third Century.
He defeated the Sarmatians and Carpi during several campaigns between 285 and 299, the Alamanni in 288, and usurpers in Egypt between 297 and 298.
Galerius, aided by Diocletian, campaigned successfully against Sassanid Persia, the empire's traditional enemy. Diocletian led the subsequent negotiations and achieved a lasting and favorable peace.