Editing Tetrarchy (section)

==Demise==
{{Main|Civil wars of the Tetrarchy}}
When in 305 the 20-year term of Diocletian and Maximian ended, both abdicated. Their ''caesares'', Galerius and Constantius Chlorus, were both raised to the rank of ''augustus'', and two new ''caesares'' were appointed: [[Maximinus Daza]] (''caesar'' to Galerius) and [[Valerius Severus]] (''caesar'' to Constantius). These four formed the second tetrarchy.

However, the system broke down very quickly thereafter. When Constantius died in 306, [[Constantine the Great|Constantine]], Constantius' son, was proclaimed ''augustus'' by his father's troops; however, Galerius instead chose to promote Severus to ''augustus'' while granting Constantine the position of caesar to Severus. At the same time, [[Maxentius]], the son of Maximian, resented being left out of the new arrangements, so he rebelled against and defeated Severus before forcing him to abdicate and then arranging his murder in 307. Maxentius and Maximian both then declared themselves ''augusti''. By 308 there were therefore no fewer than four claimants to the rank of ''augustus'' (Galerius, Constantine, Maximian and Maxentius), and only one to that of ''caesar'' (Maximinus Daza).

In 308 Galerius, together with the retired emperor Diocletian and the supposedly retired Maximian, called an imperial "conference" at [[Conference of Carnuntum|Carnuntum]] on the River Danube. The council agreed that [[Licinius]] would become ''augustus'' in the West, with Constantine as his ''caesar''. In the East, Galerius remained ''augustus'' and Maximinus remained his ''caesar''. Maximian was to retire, and Maxentius was declared a usurper. 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.

After an abortive attempt to placate both Constantine and Maximinus with the meaningless title ''filius augusti'' ("son of the ''augustus''", essentially an alternative title for ''caesar''), they both had to be recognised as Augusti in 309. However, four full Augusti all at odds with each other did not bode well for the tetrarchic system.

Between 309 and 313 most of the claimants to the imperial office died or were killed in various civil wars. Constantine forced Maximian's suicide in 310. Galerius died naturally in 311. Maxentius was defeated by Constantine at the [[Battle of the Milvian Bridge]] in 312 and subsequently killed. Maximinus committed suicide at [[Tarsus (city)|Tarsus]] in 313 after being defeated in battle by Licinius.

By 313, therefore, there remained only two rulers: Constantine in the West and Licinius in the East. 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''.<ref>[[Edward Gibbon|Gibbon, Edward]] (1776). "Chapter XIV". ''The History of the Decline and Fall of the Roman Empire''. Vol. II.</ref>