Diocletian (244-312) was born as Diocles to a family of low status from Dalmatia. He rose through the ranks of the military to become a cavalry commander under Emperor Carus; and after the deaths of Carus and his son Numerian in 284; Diocletian was proclaimed emperor; defeating Carus' other surviving son Carinus to sanctify his claim.

Diocletian's rule would stabilise the Roman Empire after the Crisis of the Third Century; and he made fellow officer Maximian co-emperor in 286 to help rule. He further appointed Galerius and Constantius as junior co-emperors in 293; establishing a tetrarchy (rule of four) which saw the quarter-division of the empire. He defeated many threats to Rome and secured the empire's borders. He sacked Ctesiphon; capital of the Sassanid Empire; before negotiating a lasting peace arrangement.

Diocletian's rule saw the establishment of the largest and most bureaucratic government in the empire's history; as well as overseeing the Docletianic Persecution (303-311); the largest and bloodiest official persecution of Christianity in the empire's history. He eventually abdicated in 305 after falling sick; becoming the first emperor to voluntarily abdicate his throne; living the rest of his years in his private palace on the Dalmatian coast.