The Tetrarchy was the system of rule by four emperors that Diocletian set up in AD 293. Two senior emperors, called Augusti, were each helped by a junior emperor, a Caesar. It ended in civil war after Diocletian retired in AD 305.
Key takeaways
- The Tetrarchy began in AD 293 when Diocletian added two junior emperors to the two senior ones.
- Diocletian and Maximian retired on 1 May AD 305, and the system broke down into civil war soon after.
- The four rulers used four capitals, including Nicomedia, Milan, Trier and Sirmium, so Rome was no longer the working centre.
- Our map shows the empire at AD 211 and AD 395, either side of the Tetrarchy, but does not draw its four regions.
How the Tetrarchy works
The Tetrarchy is the name for the rule of four that Diocletian built in the 290s, after the chaos described in the crisis of the third century. The empire was too big and the frontiers too long for one man to defend. In AD 293 Diocletian expanded the ruling group by appointing two junior emperors, called caesares, Galerius and Constantius Chlorus. Diocletian and Maximian were the two senior emperors, the augusti [1].
Each of the four had a region and a capital. Diocletian ruled from Nicomedia, Galerius from Sirmium, Maximian from Milan (Mediolanum) and Constantius from Trier [1]. The point was that an emperor was always close to a threatened frontier. It was also meant to settle who would rule next: a Caesar would move up to Augustus when a senior emperor retired.
On 1 May AD 305 Diocletian and Maximian retired, and Galerius and Constantius became the senior emperors [1]. The plan began to fail soon after. Constantius died in July AD 306, and his troops proclaimed his son Constantine. Civil wars followed, and by AD 313 only Constantine in the West and Licinius in the East were left [1]. Historians date the end differently. One date is 313, the other about 324, when Constantine beat Licinius and became sole ruler [1].
The Tetrarchy matters for two reasons. First, it was a working model of splitting the empire between several rulers, an idea that returned later with the division of the empire into East and West [2]. Second, it fits with Diocletian's wider reforms of the provinces and the army, described elsewhere under the dominate. For the later outcome, see why it fell twice.
An example from our data
The Tetrarchy happened between two dates on our slider. At AD 211, the empire covers about 4.81 million km² with 43 provinces. At AD 395 it covers about 4.72 million km² with 42, now coloured in Eastern and Western halves. The headline area barely moved, which shows that Diocletian's work was about government and not about conquest.
The province outlines on the map are those of AD 200. Diocletian split provinces into smaller ones, grouped them into dioceses and later prefectures, and none of that is drawn. The map has no layer for the Tetrarchy's four regions or for imperial capitals either. See the provinces page for the AD 200 units, and the Byzantine Empire for the Eastern half that outlasted the West.
The thing people get wrong
People sometimes say Diocletian split the empire into East and West. He did not. His Tetrarchy divided the work between four rulers, and the empire stayed legally one. The division between the sons of Theodosius I came in AD 395, which our map marks, and that was a different arrangement that came about under different circumstances. Diocletian also did not plan for civil war: the system was designed to stop it, and failed within a few years of his retirement [1].
Frequently asked questions
What does tetrarchy mean?
Rule by four. It comes from a Greek word, and here it means four emperors sharing the empire.
Who were the four rulers?
Diocletian and Maximian were the senior emperors, and Galerius and Constantius Chlorus the junior ones, from AD 293.
Does the map show the four regions?
No. The map's province outlines are those of AD 200, before Diocletian's reforms, and it has no layer for the Tetrarchy's divisions.
Sources
- Tetrarchy, Wikipedia.
- Crisis of the Third Century, Wikipedia.
- Every figure from our own map is worked out from the datasets on the data sources page (checked against Ancient World Mapping Center outlines and Pleiades).