Diocletian

Diocletian, whose original name was Diocles, was born in 240 to a poor family in Dalmatia. However, he rose through the military until Numerian made him a commander of an elite military unit whose primary purpose was to protect Numerian. In 284, Diocletian was chosen by his fellow soldiers to avenge Numerian's death, so he went to Nicomedia and killed Aper, Numerian's suspected murder.

He defeated Numerian's older brother Carinus on April 1, 285, as he decided that he was now emperor. He converted his "dual regime" into what is known now as the Tetrarchy in the year 293, and the system worked with two Augusti and two Caesars. The Augusti were himself and Maximian, and the Caesars were Galerius and Constantius I (not to be confused with Constantine I). They split and one ruled the East while the other ruled the West, and while the Empire had had joint emperors before, it had never before been so systematically divided. Diocletian intended this setup to be permanent. In the year 303, Diocletian visited Rome for the first time, and, two years later, was forced to abdicate the throne due to illness. This forced Maximian to do the same, as that was the way the Tetrarchy functioned.

Diocletian was the initiator of the "Diocletian Persecution," a time beginning in the year 303 that was characterized by the mass martyrdom of anyone suspected of being a Christian, despite the fact that his wife was a Christian. However, in 304, the edict was revoked and the persecution ended. Diocletian died shortly after.