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), with the accession of Diocletian in AD 284, his reign marking the beginning of the Tetrarchy. The constitution of the Dominate outrightly recognized monarchy as the true source of power, and thus ended the facade of dyarchy, in which emperor and Senate governed the empire together.
Diocletian's reforms to the Imperial government finally put an end to the period when the old Republican magistracies (e.g. consuls and praetors) held real powers. From then, the consuls had almost no real duties beyond that of presiding at Senate meetings and the duties of the lesser magistrates were effectively just hosting various games, e.g. chariot racing. Most other lesser magistracies simply disappeared.
Diocletian attempted to reform the Imperial system itself into a structure in which four emperors, consisting of two Augusti and two Caesares, each governed one fourth of the Empire. Known as the Tetrarchy, this constitutional structure, however, failed to 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 enacted major administrative reforms to the Empire. His division of the Empire into east and west, with each half under the command of a separate emperor, remained with brief interruptions of political unity. Although it remained the sole capital until Constantinople was elevated to that status in 359, the city of Rome ceased to be the seat of the Imperial government which in the West was usually in Mediolanum (now Milan), or sometimes in Augusta Treverorum (now Trier) if an emperor was resident there, or wherever the emperor happened to be since 4th century emperors moved within their realms, though Rome still had had its own Praefectus urbi above all other municipal governors and mayors and also its own Senate with Imperial level above all other municipal councils (except that of Constantinople from 359), maintaining the de jure capital status.
A vicar, later two vicars under Praetorian Prefect of Italy, headed the imperial administration of Italy, one in suburbicarian Italy (south of the Apennines and the Islands) and the other in Annonarian Italy (north of the Apennines and Raetia). The Senate and executive magistrates continued to function as Diocletian's constitution had originally specified. Diocletian's civil and military divisions of the empire remained in effect with little change though Upper Egypt from the mid-fifth was governed by a general, the dux, who also exercised civilian authority over the population. Later emperor Constantine would modify Diocletian's constitution by changing the roles of officials somewhat but not the administrative framework. It was not until Justinian I 527-565 that major changes that saw the near abolition of the regional tier of officials, and severe weakening of the Treasury () and Crown Estates.
Augusti and Caesares
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.
The Augusti were legally distinct from the old Princeps (Roman emperors under the Principate), because under the Principate, the Princeps took the place of the old republican magistrates. When a Princeps issued a decree, that decree was only valid so long as that Princeps was Emperor, whereas in contrast, under the Republic, any decree issued by a magistrate was only good so long as that magistrate was in office. Under the Republic and the Principate, only the Senate and legislative assemblies were continuous institutions, and thus only they could pass laws that remained in effect indefinitely. Under Diocletian's new Tetrarchy, the Augusti took the place of the Senate and the assemblies, and thus any decree of an Augustus remained in force even after that particular emperor left office. Such an act could only be invalidated by a future emperor. The logical extension of this concept meant that neither a magistrate, the assemblies, nor the senate, could legally restrain the emperor.
The old republican magistrates, as well as the Princeps, both had legal status. Under the Republic, the state gave the magistrates the authorization to hold their office, while under the Principate, the state gave the Princeps the legal authorization to be emperor. Any Augusti, in contrast, did not need authorization from the state to be emperor, because the Augusti became the state itself. The higher authority of the Augusti was illustrated by their robes (which were trimmed with precious stones) and the imperial diadem, as well as the elaborate ceremony required of anyone who approached them. Unlike the old Princeps, the Augusti were viewed as being more than mortal, which was illustrated by the honors that they received. These honors had, in the past, been reserved only for the Gods. While emperors had received such honors in the past, they only received these honors after their death, and yet, the Augusti could receive such honors while they were still alive.
In 293, Diocletian and Maximian appointed two Caesares, which resulted in an arrangement known as the "Tetrarchy" ("rule by four"). The Caesares were subordinate to their Augusti, and the only authority that they had was that which had been given to them by their Augusti. Their status was so inferior to the Augusti that they received a fixed salary. The powers that were delegated to them usually included the right to hear appeals, and a set of provinces were often assigned to them so that they could supervise the governors of those provinces. The reason why Diocletian created the office of Caesar was to create a method by which orderly successions could occur, so that when one Augustus died, one of the two Caesares replaced that Augustus. When a new Caesar was appointed, his Augustus adopted him. Diocletian had hoped that the Augusti would jointly resign at a given point in time, and allow their Caesares to replace them.
Administration of the Empire Diocletian separated the civil administrative apparatus and the military one in order to mitigate the risk that future generals might attempt to seize the throne through force, and then he reorganized both of them. While changes were made by subsequent Emperors, the basic structure established by Diocletian would govern the Empire, until its eventual fall in the West during the 5th century, and its overall reform in the East during the reign of Heraclius in 7th century. The government of the Empire was divided (in both civil and military structures) between central and provincial levels. The central government general refers to those civil officials directly associated with the Emperor's court and the highest-ranking military officers. The provincial government includes all levels of provincial governors and local military commanders.
The Imperial Court The central government of the Roman Empire consisted of the Imperial Court. At the top of the central government was the Emperor himself. From his presence all imperial authority flowed, both civil and military. The edicts of the Emperor were binding upon all persons throughout the Empire. To support the Emperor in the administration of the Empire, the Emperor was attended by numerous Court officials ("comes" or "counts").
Chief among these court officials was the Imperial Chancellor ("magister officiorum"). He was a kind of Interior Minister for State Security. His post had started out rather lowly as a tribune of the Palace Guard who was elevated by Constantine the Great in 320 AD to oversee the imperial secretariats. He functioned as a watchdog head of administration, although not able to order other branches of the administration such as the prefectures, Treasury and Crown Estates what to do without direct orders from the emperor. Ranked as a Count of the Empire, the Chancellor oversaw the entire civil service, the officia (however, the prefects and his subordinates had jurisdiction over staffs in civil and criminal suits). They were "the emperors' eyes" according to the orator, Libanius. The Chancellor's portfolio include a significant number of functions handled by modern government officials. The Chancellor was responsible for conducting court ceremonies and regulated audiences with the Emperor. All correspondences with foreign powers were sent by and embassies of foreign powers to the Emperor were received by the Chancellor. The Chancellor commanded the Imperial Intelligence Service corps of ("Agentes in rebus"), 'men of state affairs,' who handled communications between the Emperor and provincial governments as well as gathering intelligence as the Emperor's administrative policing force. They were courier/bureaucrats often deputed to other departments on special assignments. From the early 340s senior agentes in rebus were appointed as heads of the offices, principes, of prefects, vicars and two of three proconsulates (not of Asia). All business coming in and out of these offices were vetted by this officials; the wrote confidential reports the chancellor. Nothing could be issued without their counter-signature. Their small personal staffs were not part of the office staffs they whose activity they monitored. It has debated to what degree vicars had control over the principes. Their presence in the prefectures and proconsular offices connected them directly to the palatine administration headed by the masters of the offices, but also to the prefects and indirectly to the regional comptrollers of the Treasury and Managers of the Crown Estates whose offices were almost all located in diocesan see cities which were destination points for masses of information for processing for the upper administration echelon with the emperors.