The Roman Emperor Severus II was one of the first Emperors to be raised to the rank of Augustus from a subordinate position of Caesar. This system was based on the division of the Roman Empire into four parts that was put in place by the Roman Emperor Diocletian.Diocletian, who became emperor of Rome in 284, was the creator of the division of the Roman Empire into the four parts we refer to as the Tetrarchy. A year after becoming emperor, Diocletian appointed Maximianus his assistant or Caesar and then made him co-Augustus in 286. Diocletian took the eastern half of the empire and gave Maximianus the western half.
In 293 Diocletian and Maximianus appointed Galerius and and Constantius their heirs and Caesars. The Eastern contingent -- Diocletian and Galerius -- had no sons, while the Western pair -- Maximianus and Constantius -- did, respectively, Maxentius and Constantine.
Then, when the Augusti (Diocletian and Maximianus) abdicated in 305, their Caesars, who became Augusti, appointed new heirs. According to Lactantius, the appointments were made by Galerius, first of the Illyrian-born Severi (who took the name Flavius), characterized by Diocletian "as a drunken reprobate," to which Galerius responded that he was a loyal commander. Galerius also chose the other replacement, Maximinus Daia, probably a relation through marriage.
Severus ruled with Constantius I in the West, while Galerius ruled with Maximinus Daia in the East.
Constantius I died in the summer of 306 and was succeeded by Severus as junior Augustus. In Severus' place, Constantius' son Constantine I became Caesar in the West.
Meanwhile, the son of Maximinianus, Maxentius, claimed the throne and was declared emperor at Rome on October 28, 306. This meant the usurper had taken control of the area Severus controlled. Maxentius tried to get his father Maximianus to take back the title of Augustus and fight with him against Severus and Galerius. Maximianus agreed. He convinced Severus to surrender, brought him to Rome, and had him put to death in 307 on September 16.