Constitution of the Late Roman Empire

The constitution of the late Roman Empire was an unwritten set of guidelines and principles passed down mainly through precedent which defined the manner in which the late Roman Empire was governed. As a matter of historical convention, the late Roman Empire emerged from the Roman Principate (the early Roman Empire), in AD 284, with the accession of Diocletian in 284, with his reign marking the beginning of the Dominate. The constitution of the Dominate ultimately recognized monarchy as the true source of power, and thus ended the fiction of dyarchy in which emperor and Senate governed the empire.

Diocletian's reforms to the imperial government finally ended the fiction that the old republican magistracies (e.g. consuls and praetors) were anything more than municipal officials with powers beyond Rome itself. By the late Empire, the consuls had no real duties beyond that of presiding at Senate meetings and the duties of the lesser magistrates were effectively just the organisation of various games. Most other magistracies simply disappeared.

Diocletian attempted to reform the imperial system itself into a structure in which there were four emperors, consisting of two Augusti and two Caesares, each governing one fourth of the Empire. Known as the Tetrarchy, this constitutional structure, however, did not even outlast Diocletian, who lived to see the collapse of his system and the civil wars that followed in his retirement after abdication in AD 305.

He also effected major administrative reforms to the Empire. His division of the Empire into west and east, with each half under the command of a separate emperor, remained with brief interruptions of political unity. The capital of the Western Roman Empire was never returned to Rome, the Senate and executive magistrates continued to function as Diocletian's constitution had originally specified, and Diocletian's civil and military divisions of the empire remained in effect. Later emperors like Constantine the Great and Justinian I modified Diocletian's constitution.

Under Diocletian's new constitution, power was shared between two emperors called Augusti. The establishment of two co-equal Augusti marked a rebirth of the old republican principle of collegiality, as all laws, decrees, and appointments that came from one of the Augusti, were to be recognized as coming from both conjointly. One Augustus was to rule the western half of the Empire, and the other Augustus was to rule the eastern half of the Empire. Diocletian made Maximian his co-Augustus, and gave him the Western Empire, while Diocletian took the Eastern Empire. Diocletian made Nicomedia his capital, and Maximian made Milan his capital. To make the two halves symbolically appear to be one, Diocletian called his territory patres Orientis, while Maximian called his territory patres Occidentis.

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