Tetrarchy - Regions and Capitals

Regions and Capitals

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 endless procession from the eastern steppe; many nomadic or elsewhere chased tribes) 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 urbis, later copied in Constantinople).

The four Tetrarchic capitals were:

• Nicomedia in northwestern Asia Minor (modern Izmit 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 pretorian 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 Chlorus, 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 between 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 theatre'. Each Tetrarch was himself often in the field, while delegating most of the administration to the hierarchic bureaucracy headed by his respective Pretorian Prefect, each supervising several Vicarii, the governors-general in charge of another, lasting new administrative level, the civil 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 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.

However, it appears that some contemporary and later writers, such as the Christian author Lactantius, and Sextus Aurelius Victor (who wrote about fifty years later and from uncertain sources), misunderstood the Tetrarchic system in this respect, believing it to have involved a stricter division of territories between the four emperors.