The orderly system of two senior and two junior rulers endured until Constantius died in July 306, and his son Constantine was unilaterally acclaimed augustus and caesar by his father's army.

Maximian's son Maxentius contested Severus' title, styled himself princeps invictus, and was appointed caesar by his retired father in 306.

Ultimately the tetrarchic system lasted until c. 324, when mutually destructive civil wars eliminated most of the claimants to power: Licinius resigned as augustus after losing the Battle of Chrysopolis, leaving Constantine in control of the entire empire.

The Constantinian dynasty's emperors retained some aspects of collegiate rule; Constantine appointed his son Constantius II as another caesar in 324, followed by Constans in 333 and his nephew Dalmatius in 335, and the three surviving sons of Constantine in 337 were declared joint augusti together, and the concept of the division of the empire under multiple joint emperors endured until the Fall of the Western Roman Empire.

The term tetrarchy (from the Greek: τετραρχία, tetrarchia, "leadership of four [people]")[a] describes any form of government where power is divided among four individuals.

The term was understood in the Latin world as well, where Pliny the Elder glossed it as follows: "each is the equivalent of a kingdom, and also part of one" (regnorum instar singulae et in regna contribuuntur).

Although Edward Gibbon pioneered the description of the Diocletianic government as a "New Empire", he never used the term "tetrarchy"; neither did Theodor Mommsen.

It did not appear in the literature until used in 1887 by schoolmaster Hermann Schiller in a two-volume handbook on the Roman Empire (Geschichte der Römischen Kaiserzeit), to wit: "die diokletianische Tetrarchie".

[4] The first phase, sometimes referred to as the diarchy ("rule of two"), involved the designation of the general Maximian as co-emperor—firstly as caesar (heir apparent) in 285, followed by his promotion to augustus in 286.

In 293, Diocletian thought that more focus was needed on both civic and military problems, so with Maximian's consent, he expanded the imperial college by appointing two caesares (one responsible to each augustus)—Galerius and Constantius I.

The four tetrarchs based themselves not at Rome but in other cities closer to the frontiers, mainly intended as headquarters for the defence of the empire against bordering rivals (notably Sassanian Persia) and barbarians (mainly Germanic, and an unending sequence of nomadic or displaced tribes from the eastern steppes) at the Rhine and Danube.

In terms of regional jurisdiction there was no precise division among the four tetrarchs, and this period did not see the Roman state actually split up into four distinct sub-empires.

Although power was shared in the tetrarchic system, the public image of the four members of the imperial college was carefully managed to give the appearance of a united empire (patrimonium indivisum).

One of the greatest problems facing emperors in the Third Century Crisis was that they were only ever able to personally command troops on one front at any one time.

Both the dyarchic and the tetrarchic system ensured that an emperor was near to every crisis area to personally direct and remain in control of campaigns simultaneously on more than just one front.

Similarly, Constantius defeated the British usurper Allectus, Maximian pacified the Gauls, and Diocletian crushed the revolt of Domitianus in Egypt.

This agreement proved disastrous: by 308 Maxentius had become de facto ruler of Italy and Africa even without any imperial status, and neither Constantine nor Maximinus—who had both been caesares since 306 and 305 respectively—were prepared to tolerate the promotion of the augustus Licinius as their superior.

The tetrarchic system was at an end, although it took until 324 for Constantine to finally defeat Licinius, reunite the two halves of the Roman Empire and declare himself sole augustus.

Constantine was denied the promotion to augustus even after Severus' death in September, as Galerius had decided to exclude him from the system altogether.

[8] At the council of Carnutum, Diocletian decides that Licinius will be the new augustus of the west (although his western domains only consist of the Diocese of Pannonia).

[8] After the death of Galerius' (who died of natural causes), Licinius acquires parts of his domains, thus ruling over territories both in the East and West.