Diocles was born in Dalmatia around the year 244. Diocles came from a family of low status. Little is known about the first forty years of Diocletian's life, though many suspect he held a military rank of some sort. In January of 284, the brothers Numerian and Carinus took control over the Roman Empire from their father. Unfortunately, Numerian died later in that year. Numerian, at the time of his death, was coming back from combat in Persia. When he died, his army voted on who should be succeed him as Emperor, and Diocles was chosen. After being chosen as Emperor, Diocles changed his name to Diocletianus, making his full name Gaius Aurelius Valerius Diocletianus. Toady, we know him as Diocletian.
After Numerian's army had named Diocletian as emperor, Carinus was not to happy. To Carinus, Diocletian was a usurper opposing Carinus's rule, so Carinus decided to meet Diocletian in battle. Although Carinus had a stronger army and he had control over the Roman Senate, he was unpopular. Carinus and Numerian were the sons of the unpopular Emperor Carus, and neither the sons nor the father were too popular with the people. Diocletian and Carinus met in battle at the Battle of Margus, during which most of Carinus's generals defected to Diocletian. When Diocletian won the battle, both the western and eastern armies accepted Diocletian as the Augustus, or ruler.
When Diocletian came to power, Rome was filled with problems. In the East, Rome was on the brink of war with Persia and the west faced constant threat from barbarian tribes. On top of that, the entire empire seemed to be on the brink of revolt. Diocletian decided that the job of Emperor was too big for one person, so Diocletian named Maximian as co-ruler of the Roman Empire in 285. Diocletian decided to rule over the East, because that part of the Empire was the more stable of the two. Maximian was assigned the Western Roman Empire. It was this division that divided the empire in two and allowed the Byzantine Empire to survive when the Western Roman Empire fell. Unfortunately, even though the empire divided, Rome's situation worsened. Persia had declared war on the Roman Empire and the ex-general Carausius had proclaimed himself Augustus and spurred Britain and Northern Gaul into open revolt. Though both Diocletian and Maximian had begun to retake Gaul and were winning the war in Persia, they needed more hands to deal with conflicts, so Maximian took Flavius Constantius as his second in command in 293, and Diocletian did the same with Galerius. Though Constantius and Galerius were below Diocletian and Maximian, all four had their own independent army and were pretty much independent from one another.
The Tetrarchy with Diocletian, Maximian, Constantius, and Galerius added great benefit to the Empire. While Galerius was fighting battles in Persia, Diocletian was quelling revolts in the Balkans. Though this organization made the job of Diocletian easier, it also gave him time to persecute Christians. Diocletian and Galerius created a lot of persecutionary edicts towards Christians along with killing several of the Christian clergy and razing several Christian churches. Diocletian also issued an arrest for all Christian clergy. The persecutions, luckily, were for the most part unsuccessful. Most Christians escaped punishment, and people of other religions were mainly unsympathetic to the persecution. Also, Maximian and COnstantius did not apply most of the persecutionary edicts, leaving Western Christians unharmed. In 311, Galerius repealed the edicts, stating that they had failed to bring Christians back to traditional religion.
In 304, Diocletian had just finished dedicating the opening of a circus when he collapsed. Diocletian's health was deteriorating, so on May 1, 305, Diocletion brought his generals, officers, and representatives to the hill, five kilometers out of Nicomedia, where he was appointed as Emperor after the death of Numerian. On that hill, he announced that he would be abdicating from his role as Augustus. Diocletian retired to Dalmatia where he created a palace for himself. After Maximian retired, Diocletian watched the political situation turn to chaos. The Tetrarchy collapsed and Maximian was forced to commit suicide after his third attempt to reclaim the throne. Diocletian died on December 3, 311, soon after Maximian. Diocletian made our list because he made the division in the Roman Empire that probably saved it for several hundred years and kept the Byzantine Empire alive after the fall of Rome in the West.