This page is an attempt to simplify and summarize a complicated period in Roman history. For a thorough and exhaustive discussion see T.D. Barnes, Constantine and Eusebius. See also:

Diocletian, who single-handedly pulled the Empire back from the brink of destruction, decided that the Empire needed a different organizational structure. He decided that the Empire should be broken up into more manageable chunks. Each half of the empire would be ruled by a different ruler, known as an Augustus. Each Augustus, in turn, was to be assisted by a junior “ruler-in-training” known as a Caesar.

Diocletian instituted this policy by naming himself Augustus for the Eastern half of the Empire and Maximian Augustus in the West. Diocletian chose Galerius to be his Caesar while Maximian chose Constantius, the father of Constantine the Great.

The Caesars were subordinate to the Augusti, but maintained a large degree of autonomy. Each one of the four rulers was given territory to govern. Diocletian ruled the East, Galerius ruled Illyricum, Maximian ruled Italy, and Constantius ruled Gaul.

The original imperial college looked like this:

293 – 304 West East
Augustus Maximian (Italy) Diocletian (East)
Caesar Constantius (Gaul) Galerius (Illyricum)

In 305, Diocletian and Maximian retired. Constantius and Galerius then ascended to the post of Augustus, and Severus and Maximinus were named as Caesars.

305- 306 West East
Augustus Constantius (Gaul) Galerius (Illyricum)
Caesar Severus (Italy) Maximinus (East)

As it turned out, the imperial college rotated successfully only once. In 306, soon after becoming Augustus, Constantius died, apparently bequeathing his title and dominion to his son, Constantine. Constantine wasted no time in calling himself the new Augustus, even though this decision had not been agreed upon by the other members of the imperial college. Galerius, who was now the senior emperor in the East, did not wish to recognize Constantine as Augustus, and named Severus as Augustus. Constantine was then named as the new Caesar.

During this time Maxentius, Maximian’s son, grew upset that he had been passed over for yet another imperial appointment, and so he usurped Italy. He then appointed himself Augustus, but was denied formal recognition by Galerius. Maxentius knew he would need help in defending his newly gotten territory, and enlisted the help of his father, retired Augustus Maximian. Technically, the imperial college did not change, although Maxentius had more power in Italy than Severus.

306 – 307 West East
Augustus Severus (Italy, fighting for control)(Maxentius/Maximian take over Italy) Galerius (Illyricum)
Caesar Constantine (Gaul) Maximinus (East)

Maxentius and Maximian ruled jointly and repelled Severus, ultimately killing him in 307. After Severus’ defeat, Maximian attempted to overthrow his own son and lost. He then fled to Constantine for protection. Galerius became so desperate to impose some order on the West that he called Diocletian out of retirement. The intent was not for Diocletian to rule again, but for Diocletian to use his influence in order to choose a new Augustus for the West that would actually be able to exert power. This temporarily resulted in the famous “rule of six” that is mentioned in Eusebius.

307 – 308 West East
Augustus Diocletian (attempting to re-take Italy)(Maxentius/Maximian control Italy) Galerius (Illyricum)
Caesar Constantine (Gaul) Maximinus (East)

Through all of this turmoil, Constantine remained as uninvolved as possible. He did not want to do anything that would jeopardize the legitimacy of his rule in Galerius’ eyes, because Galerius was the senior member of the Imperial College. Diocletian and Galerius, along with Maximian, elevated Licinius to the post of Augustus of the West. The college now looked like this:

308 – 309 West East
Augustus Licinius (Italy, in theory)(Maxentius controls Italy) Galerius (Illyricum)
Caesar Constantine (Gaul) Maximinus (East)

Although he lacked any real power, Maximian joined forces with Constantine and named him Augustus of the West, an appointment which was not recognized by Maximinus and Galerius in the East. Maximian used his position to become a general in Constantine’s army. He soon used his position to try and overthrow Constantine, but the uprising was quickly quelled by Constantine, and Maximian subsequently hanged himself. During this time Maximinus was also promoted to the rank of Augustus. So much for the Caesars until 317:

309 – 311 West East
Augustus Licinius (Italy, in theory)Constantine (Gaul)(Maxentius controls Italy) Galerius (East/Illyricum)Maximinus (East)

Galerius died in 311. Maximinus immediately took the opportunity to seize Illyricum. Licinius also wanted Illyricum, and the stage seemed set for battle. The battle never happened, however, and Maximinus and Licinius came to terms. Maximinus remained ruler of Illyricum.

While Licinius was preoccupied in Illyricum, Maxentius took a gamble and attacked Constantine. Constantine went to war and won a decisive victory over Maxentius in 312. Finally, the usurper was out of the picture. A new problem arose, however. Constantine had now conquered the territory that was theoretically ruled by Licinius. Constantine was very astute, however, and made an alliance with Licinius by giving Licinius the hand of his sister in marriage. Constantine was securely in charge of the entire West, and Licinius was prepared to make war on Maximinus in the East.

311 – 313 West East
Augustus Licinius (Italy, in theory)Constantine (Italy/Gaul) Maximinus (Illyricum/East)

In 313 Licinius crossed into Asia Minor and declared war on Maximinus. Licinius won and took over Illyricum. He continued East and eventually conquered the Orient. Maximinus, realizing that the end was near, committed a slow and painful suicide.

313 – 317 West East
Augustus Constantine (Italy/Gaul) Licinius (Illyricum/East)

Soon after Licinius consolidated his power in the East, Constantine declared war on him under dubious circumstances. He invaded Asia Minor, and Licinius named Valens as Caesar to defend the frontier. Valens was defeated, however, and murdered under the subsequent treaty between Constantine and Licinius. Asia Minor was now Constantine’s. As senior emperor, Constantine chose the two new Caesars, his sons Constantius and Crispus.

317 – 324 West East
Augustus Constantine (Gaul) Licinius (East)
Caesar Constantius (Italy) Crispus (Illyricum)

This new arrangement lasted for approximately seven years. Licinius began persecuting his Christian subjects, however, which gave Constantine the perfect excuse to finally eradicate him. In 324 Constantine charged into Nicomedia and overthrew Licinius. Constantine was now at the helm of the entire Empire, with his sons as his deputies. This effectively ended the idea of the Tetrarchy.

324 – 337 West East
Augustus Constantine (Gaul) Constantine (East)
Caesar Constantius (Italy) Crispus (Illyricum)

Created by JRZ

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