Born to a family of low status in the Roman province of Dalmatia, Diocletian rose through the ranks of the military to become cavalry commander to the Emperor Carus. After the deaths of Carus and his son Numerian on campaign in Persia, Diocletian was proclaimed Emperor. The title was also claimed by Carus' other surviving son, Carinus, but Diocletian defeated him in the Battle of the Margus. With his accession to power, Diocletian ended the Crisis of the Third Century.
He appointed fellow officer Maximian Augustus his senior co-emperor in 285. He 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. 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. In 299 he sacked their capital, Ctesiphon. Diocletian led the subsequent negotiations and achieved a lasting and favorable peace.