Tetrarchy

The Tetrarchy (293-c.324 CE) describes the system of Government imposed by the Emperor Diocletian (284-305 CE).
He divided the Roman Empire into East and West, and placed an Emperor and a junior Emperor in each half.

During the Crisis of the Third Century, the precedent for splitting the Empire into East and West had already been laid down by Valerian. When he became Emperor on the 22 October 253 CE, he made his son Gallienus co-Emperor in the West, whilst Valerian went to retake control of the East.

When Diocletian became Emperor in 284 CE, he set about preventing civil war by making Reforms aimed at preventing the Provincial Governors from taking their Legions to Rome and declaring themselves Emperor.
He was also confronted by the Sassanid Empire (224-651 CE), who constantly threatened to invade from the East if the Empire was divided by civil war.

Diocletian realised from the beginning that attempting to rule the Empire single handed would lead to revolts. Therefore he created the Diarchy (The Rule of two) in 285 CE, appointing Maximian ‘Caesar’ of the Western Empire, whilst Diocletian was ‘Augustus’ of the Eastern Empire.
In 286 CE, Maximian was also promoted to ‘Augustus’, either by Diocletian, or by himself to position himself above Carausius, his deputy.
Later, in 293 CE, Diocletian created two more Caesars, giving each ‘Augustus’ a deputy. This then became known as the Tetrarchy.

Diocletian (284-305 CE) started removing the Military command of the Legions from Provincial Governors.
He created the Tetrarchy with four Co-Emperors governing the Roman Empire, which was then subdivided into 12 Dioceses (2-4 in each Tetrarchy)
- The Four Capitals:
  1. Augusta Treverorum: Trier, Germany
  2. Mediolanum: Milan, Italy
  3. Sirmium: Sremska Mitrovica, Serbia
  4. Nicomedia: Izmit, Turkey
Diocletian's Praetorian Prefectures

293-305 CE. Co-Emperor as ‘Caesar’ of the West with Maximian as ‘Caesar’ of the East under the Tetrarchy.
305-306 CE. Co-Emperor ‘Augustus’ of the West with Galerius as ‘Augustus’ of the East.

1 May 305 CE to Summer 306 CE as Caesar in the West, under Constantius I Chlorus
Summer 306 CE to March-April 307 CE as Augustus in the West
- competing against Maximian and Constantine I.

July 285 CE to 02 April 286 CE as Caesar in the West under Diocletian‘s Tetrarchy.
2 April 286 CE to 1 May 305 CE as Emperor of the West.
28 October 306 CE to 11 November 308 CE declared himself Emperor in opposition to Constantine I

308 to 311 CE as Augustus in the West with Galerius as Augustus in the East.
311 to 313 CE as Augustus in the West with Maximinus Daia as Augustus in the East.
313 to 324 CE as Augustus in the East with Constantine I as Augustus in the West.
315 and 324 CE he claimed both halves of the Roman Empire.

Constantine I completed the removal of military command of the legions from the Provincial Governors.
It was the same structure as the Tetrarchy but instead of the Praetorian Prefects assisting The co-Emperors, they themselves became the Chief Executives.
The 4 Praetorian prefectures were each administered by a Praetorian Prefect, reporting directly to the Emperor:
1. Prefecture of the Gauls:
  - Trier, Germany: Augusta Treverorum
2. Prefecture of Italy and Africa:
  - Milan, Italy: Mediolanum
3. Prefecture of Illyricum:
  - Sremska Mitrovica, Serbia: Sirmium
4. Prefecture of Oriens:
  - Izmit, Turkey: Nicomedia