Born to a poor family, Maximian made a career in the army until 285, when the new emperor Diocletian, a friend of his, made him caesar (sub-emperor), making him the ruler of the western part of the empire. The next year Maximian became augustus next to Diocletian, and in 293, when Diocletian introduced the Tetrarchy, Constantius Chlorus became his caesar.
During his reign, Maximianus had several military successes, against the Alemanni and Burgundians in northern Germany, against the Carpi on the Danube frontier and against Carausius, who had rebelled in Britain and declared himself emperor there. He also strengthened the frontier defenses in Africa.
On May 1, 305, Diocletian and Maximian retired together; it is clear that this was not a voluntary act of Maximian's, but that he was forced to do so by Diocletian. Galerius and Constantius Chlorus became the new emperors; Flavius Valerius Severus and Maximinus Daia became their caesars. When Constantius died the next year, Maximian's son Maxentius took the western emperorship, and named Maximian to be his augustus. Maximian resolved the conflicts around this emperorship by defeating Severus and Galerius in battle and bringing Constantinus' son Constantine on his side by having Constantine marry his daughter Fausta.
However, in 308 Maximian rebelled against his own son, and marched upon Rome, but was beaten and forced to find refuge with Constantine in Gaul. In 310 he declared himself emperor for the third time, but was unable to defend himself against Constantine, who forced him to commit suicide.
Diocletian (284 - 305)
Constantius Chlorus (305 - 306),
Galerius (305 - 311)