Editing Tetrarchy (section)

== Regions and capitals ==
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[[File:Tetrarchy map3.jpg|thumb|left|Map of the Empire under the Tetrarchy, showing the dioceses and the four tetrarchs' zones of influence.]]

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 [[Sassanid dynasty|Sassanian Persia]]) and [[barbarian]]s (mainly Germanic, and an unending sequence of nomadic or displaced tribes from the eastern steppes) at the [[Rhine]] and [[Danube]]. These centres are known as the tetrarchic capitals. Although Rome ceased to be an operational capital, Rome continued to be nominal capital of the entire Roman Empire, not reduced to the status of a province but under its own, unique Prefect of the city (''[[praefectus urbi]]'', later copied in Constantinople).

The four tetrarchic capitals were:

*[[Nicomedia]] in northwestern Asia Minor (modern [[İzmit]] in Turkey), a base for defence against invasion from the Balkans and Persia's Sassanids was the capital of Diocletian, the eastern (and most senior) ''augustus''; in the final reorganisation by Constantine the Great, in 318, the equivalent of his domain, facing the most redoubtable foreign enemy, [[Sassanid Persia]], became the praetorian prefecture Oriens, 'the East', the core of later Byzantium.
*[[Sirmium]] (modern [[Sremska Mitrovica]] in the [[Vojvodina]] region of modern [[Serbia]], and near [[Belgrade]], on the [[Danube]] border) was the capital of Galerius, the eastern ''caesar''; this was to become the Balkans-Danube prefecture Illyricum.
*[[Mediolanum]] (modern [[Milan]], near the Alps) was the capital of Maximian, the western ''augustus''; his domain became "Italia et Africa", with only a short exterior border.
*[[Augusta Treverorum]] (modern [[Trier]], in Germany) was the capital of Constantius, the western ''caesar'', near the strategic Rhine border; it had been the capital of Gallic emperor [[Tetricus I]]. This quarter became the prefecture Galliae.

[[Aquileia]], a port on the Adriatic coast, and [[Eboracum]] (modern [[York]], in northern England near the Celtic tribes of modern Scotland and Ireland), were also significant centres for Maximian and Constantius respectively.

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. Each emperor had his zone of influence within the Roman Empire, but little more, mainly high command in a 'war theater'. Each tetrarch was himself often in the field, while delegating most of the administration to the hierarchic bureaucracy headed by his respective [[praetorian prefect]], each supervising several [[vicarii]], the governors-general in charge of another, lasting new administrative level, the civil [[Roman diocese|diocese]]. For a listing of the provinces, now known as [[eparchy]], within each quarter (known as a praetorian prefecture), see [[Roman province]].

In the West, the ''augustus'' Maximian controlled the provinces west of the [[Adriatic Sea]] and the [[Gulf of Sidra|Syrtis]], and within that region his ''caesar'', Constantius, controlled Gaul and Britain. In the East, the arrangements between the ''augustus'' Diocletian and his ''caesar'', Galerius, were much more flexible.