The Later Roman Empire




Constantine’s successors

Further study

The Emperor Diocletian

After the crisis of the third century, when the Roman empire was overrun by invasions from beyond its borders and looked as if it would break up, a succession of soldier-emperors restored a measure of unity and pushed the invaders back over the frontiers. The emperor Diocletian (reigned 284 to 305 CE), like his immediate predecessors, came from the Danubian provinces, and like them he had had a career as a professional soldier. Unlike them, he succeeded in remaining in power for twenty years, and so gave the Roman empire a much-needed period of peace and stability. How did he do this?

The Tetrarchy

Successful though the efforts of the soldier-emperors were in reuniting the empire, one of the basic problems besetting the Roman empire in the third century persisted. This was that armies operating in their own theatre of war were tempted to acclaim their commander emperor, who would then march, not against Rome’s enemies, but on their rivals.

Diocletian set about dealing with this problem by dividing the empire into two parts, east and west, and, with himself based in the east, appointed a fellow emperor in the west. These two emperors each had a junior emperor to support them. The senior emperors had the title of Augustus, while the junior emperors had that of Caesar. The two Caesars were the designated heirs of the two Augusti. This emphasised the fact that anyone aiming for the throne from outside this charmed circle started from a position of blatant illegality.

This solution was the result of trial and error and emerged over a number of years.  It dew on many precedents from the past, in which emperors had appointed others, usually sons or close relatives, as co-emperors. Indeed, under Diocletian family relationships were created to underpin the arrangement by marriage alliances and adoptions between the co-emperors.

Modern scholars call this system of co-emperors the tetrarchy, or “rule of four”. There was never any doubt amongst contemporaries that Diocletian remained supreme amongst them. The arrangement gave almost twenty years of much needed stability to the empire.

None of these emperors now lived in Rome. Diocletian based himself largely in Nicomedia, in northwest Asia Minor; his western colleague Galerius, traveled around near the frontiers, with Milan or Trier, in Gaul, as his preferred headquarters. The Caesars went as and where they were needed.

The system of administration

Diocletian and has colleagues were able to bring some system to the chaotic administrative condition the empire had fallen into. With the collapse of the silver coinage, the army was now paid and supplied partly by requisitions. In the crisis conditions of the third century, this was bound to be haphazard and a source of great distress to local civilian populations. Diocletian made tax payments – in both money and kind – regular and predictable. This alleviated the suffering for civilians and made the administrative more efficient.

The better to deal with the immense burden of tax collection and other administrative duties which now fell to the provincial governors and their staffs, which in previous times had been the preserve of city councils, Diocletian progressively reduced the size of provinces. This of course increased the number of provinces (Britain, for example, now had four provinces instead of two), so co-ordinating provincial administration became an issue. He therefore grouped the provinces into administrative regions called diocese. These were supervised by senior officials called vicarii, who reported to the Praetorian Prefects (of which there were two, one for each of the Augusti).

The military responsibilities of governors was now handed to army commanders called duces (pl. of dux). From now on, for the first time in Roman history, military and administrative careers were separated (although civil servants were still officially viewed as military personnel, due to the militarization of the administration in the third century crisis; they wore military-style uniforms and had the rank and pay of soldiers,). This process of separation was not quite complete: it would have to await the reign of Constantine before officials of the topmost rung, the Praetorian Prefects, lost their military powers.

This new system of provincial government required a much greater number of officials to run it than in the early empire.

The army

In the past Diocletian was credited with vastly increasing the size of the army, but scholars now think that he oversaw no more than a modest expansion. He did create many new legions, but these were not the huge units of six thousand men of the early empire; they were about one thousand men strong, probably in line with the example of late third century emperors, under whom the old legions had been dispersed and considerably reduced in size.

Also in line with developments in the third century, field armies now formed a major element in imperial defences. However, Diocletian seems to have strengthened frontier defences, building military roads and forts, and strengthened natural barriers. The eastern frontier in particular received a great deal of attention, and would remain very heavily defended throughout the fourth century.

Social and religious legislation

The overriding need to pay for the defences of the empire, and the expanded imperial administration which went with it, determined the nature of Diocletian’s social legislation. It was aimed at ordering society in such a way that it was as easy as possible to raise taxes from it.

Much of the legislation from Diocletian’s time onwards was aimed at preventing various social groups from moving from place to place, or from one line of work to another, thus keeping them as regular taxpayers. Peasants were prohibited from leaving the estates on which they lived and farmed; they were (in law, at least; reality was a different thing) tied to their estates, making them similar to the serfs of later medieval times. Some professions regarded as crucial to the smooth operation of the state, such as shippers, bakers and soldiers, were made hereditary. Town councillors, who still had a vital role to play in the collection of taxes from their communities, were not allowed to leave their positions. From being a highly privileged elite in the early empire this had become a harassed and, in some parts, even impoverished class now, being required to personally guarantee their towns’ taxes from their own pockets. To keep them in place they were forbidden to enter the civil service, the army or the church.

That similar laws were repeated again and again later in the fourth century, in a shriller and shriller manner, points to this legislation not being very effective. Notably, town councillors left their positions in a steady trickle to take up a career the civil service – highly desirable as it freed them from their tax obligations and potentially opened the way to high office, much improved status and substantial wealth.

Another piece of Diocletian’s legislation that even he realised was ineffective was his attempted to control prices of a range of goods. He had the relevant laws withdrawn after a few years.

Diocletian’s attempts to dragoon society into supporting the military effort attests to the greater tax take in the later Roman empire than in the early empire. With a larger army and administration, this was inevitable. In the past, scholars have concluded that from Diocletian’s time onwards there was a harsh regime which created a caste-like society consisting of an oppressed majority, its vitality sapped by an unproductive minority of officials, courtiers, landowners, soldiers and clergy. However, the archaeological evidence gives a more nuanced picture; in most places it suggests an economy not much less vigorous than that of the early empire. In the east, indeed, the fourth and fifth centuries seem to have been ones of economic expansion. The west’s economy was a great deal more fragile, and seems never to have recovered all the ground lost in the third century; but even here, for example in Africa, southern Spain and much of Italy, above all Rome itself, this period shows signs of a great deal of prosperity.

Diocletian initiated the last great persecuted of Christians. This fits in with the ordered mind which seems to be apparent in his other policies: he wanted uniformity of belief as well as behaviour.

The end of the Tetrarchy

In the final analysis, Diocletian’s system of co-emperors rested on consent. After Diocletian retired in 305, to allow the Caesars to become Augusti in an orderly fashion, the system almost immediately began to break down. The ambitions of one of the Caesars, in particular, ensured that this was the case by rebelling against the Augustus of the west, Maxentius. In 312 Constantine fought and won the battle of the Milvian bridge, just outside Rome, and so established himself as the Augustus in the west. By 324 he had defeated the Augustus of the east and made himself the sole emperor of the Roman world. 


Constantine’s reign (from 312 in the west, from 324 in the whole empire, until his death in 337) is one of the pivotal periods in European, even world, history. This is for two reasons. First, he became a Christian, and from his time until the modern age almost all European rulers have followed him in this, enabling the Christian Church to dominate religion and culture to such an extent that Europe has for most of its history been synonymous with the term “Christendom”.

Secondly, he founded a new capital, Constantinople, which would remain the centre of the Roman (or, as scholars call this phase, the Byzantine) empire for another thousand years. With its superb location and massive walls it would act as a strong defence against the advance of Muslim for all of that time.

The first Christian emperor

In the course of his struggles for power, Constantine was converted to Christianity, He would not actually be baptized into the Christian church until right at the end of his life, but this was a fairly common practice at the time (for fear of committing a mortal sin).

Under Constantine, all persecution of the Christian church ceased, and Christianity became a legal religion. All subsequent Roman emperors except one, Julian “the Apostate” (reigned 361-3), would also be Christians.

Under Constantine and his successors, the Christian church began to receive official patronage. The Clergy were exempted from the duties and responsibilities of town councillors, and Constantine himself embarked on a major church building programme. The bishops of the Christian church started their rise to become prominent figures in their own towns and cities.

However, Constantine soon found himself drawn into the internal disputes between different groups of Christians. The main controversies revolved around attempts to define the precise nature of Christ in relation to God on the one hand, and mankind on the other. Constantine tried to resolve these issues by calling councils of Bishops, which would be regular occurrences in the later Roman empire. The Council of Nicaea of  325 CE, over which he presided, was the first of these, and probably the most influential: it hammered out a position (that Christ is both man and God) to which most of the the bishops, drawn from all over the Roman world, could sign up to, and which has remained broadly the orthodox view of mainstream churches ever since.

Pagans were not persecuted by Constantine nor any of his immediate successors, and certainly in Constantine’s time and for some time afterwards Christianity remained a minority religion. However, Constantine initiated a church building programme which would soon start to make the Christian religion a key part of the physical townscapes of the empire.

The founding of Constantinople

As we have seen, the later third century emperors were too busy campaigning on the frontiers to live in Rome for any long periods; and none of the emperors of the Tetrarchy made Rome their residence. Constantine’s foundation of Constantinople was therefore building on long-established practice going back at least a generation.

Yet there was something different about this act. It was no ad-hoc choice of a city as a temporary headquarters, but the deliberate selection of a permanent, second, imperial capital for the mighty empire. This is seen in the way it deliberately mimicked the features of Rome itself, above all the installation of second senate, and the appointment of the Prefect of the City to govern it, like the historic Prefect of Rome, the most senior (if not the most powerful) official since Augustus’ day.

Constantinople was in fact the ancient city of Byzantium, renamed, refurbished and much upgraded (hence the name of the empire it ruled, the Byzantine empire). From the start this was to be a Christian city; but it was also to be a well-defended city. It was located on a piece of land jutting out into the sea, with only one landward side. Stout walls (later rebuilt on a massive scale) surrounded the entire city; these were to prove impervious to attack for more than a thousand years (unless treachery was involved).

Constantine’s administrative and military policies

In his secular policies, Constantine mostly followed in the footsteps of Diocletian, though with some departures of his own. He consolidated Diocletian’s provincial arrangements by dividing the empire into three sectors, each one under a Praetorian Prefect. One was responsible for the west, Spain, Gaul and Britain; another for the central trunk, Africa, Italy and the Balkans, and the other for the Eastern provinces in Asia Minor, the Levant and Egypt. Their military responsibilities were taken away from them altogether and handed over to new officers, a magister militum, also one for each of the sectors. The Praetorian Prefects were left as the top civilian officials of the empire, viceroys of their respective sectors. This measure completed the separation of military and civilian offices in the empire.

One departure from Diocletian’s practice was to expand the senatorial order. He removed the obligation for senators to attend meetings of the senate, or even to live in Italy; and from now on most members of the order were high officials and generals who held senatorial rank as a social cache rather than a substantive position. But Constantine also appointed senators who were actual members of the Roman senate, and who came from Roman senatorial families based in Rome (though probably of provincial ancestry), to governorships much more frequently than Diocletian had done. Later in the fourth century such senators would reached very high office indeed, as members of the emperors’ inner council and as Praetorian Prefects.

Perhaps this expansion of the senatorial order and appointment of senators was linked to his introduction of taxation for this group (as a sort of quid pro quo). Since this was by far the wealthiest class within Roman society, this would have greatly strengthened the imperial treasury. Indeed, perhaps, in turn, this new taxation (which also brought certain merchants into the taxable fold) was linked to his issuing a new gold coinage, the solidus, which began to put an end to the chronic and disabling inflation the empire had experienced for so long.

In the military sphere, he seems to have placed more emphasis on field armies stationed behind the lines than Diocletian had done, who is noted for having strengthened the frontiers.

Constantine’s successors

The fourth century CE was certainly more stable for the Roman empire than the third century had been; even so, it fell a long way short of the stability and peace of the first and second centuries CE.

On Constantine’s death in 337, fighting between his sons left the empire divided between two of them, Constans in the west and Constantius II in the east. In 350 a usurper, Magnentius, rose against Constans and killed him; but, after winning control of much of the west, he himself was defeated and killed by Constantius (353), who thus became sole emperor.

In 355 the commander on the Rhine frontier, Silvanus, rebelled, but was killed soon after; this event probably prompted Constantus to realise that ruling alone was not a practical proposition by this time. He appointed his cousin Julian as Caesar. In 361 Julian succeeded Constantius as Augustus, but lasted only two years, dying on his return from a disastrous invasion of Persia.

With Julian’s death, the family of Constantine came to an end. The high command of the eastern army chose an officer called Jovian to succeed him (361-2), and then Valentinian (364-75), who reigned with his brother Valens as co-emperor. Valens remained in the east while Valentinian took ruled the west.

In 375 Valentian died, and his 16 year old son Gratian, took over control of the west. At same time his younger brother, the infant Valentinian II, was proclaimed emperor by the troops on the Danube. In 378, when Valens was defeated and killed at the battle of Adrianople, Gratian hurriedly appointed his general Theodosius to be his co-emperor in the east.

In 383 a usurper Magnus Maximus proclaimed himself emperor in Britain, and invaded Gaul. Gratian, a strict disciplinarian, was promptly murdered by his own troops, who joined Maximus.

Theodosius was forced to recognize Maximus as Augustus until the later started expanding his power. Theodosius then moved decisively against him, and in 383-5 overpowered him. After Maximus’ defeat, Theodosius ruled as sole emperor until his own death in 395.

Struggles within and without

It can be seen from the above that internal struggles were frequent; they had a serious impact on the empire’s efforts to repel invaders. In Constantine’s last years, hostilities broke out between Rome and Persia. His son Constantius II inherited the war here, which was prolonged by his struggles with his brothers. The rebellion of the commander of the Rhine frontier, Silvanus (355), prompted Germanic tribes to invade, and had to be pushed back by Julian, meanwhile Constantius was dealing with invasions in the Danubian sector. Constantius then had to return to the east, where the Persians had again invaded. He managed to patch things up there temporarily.

In 363 Julian, now sole emperor, embarked on a Persian campaign of his own, invading deep into Persian territory as far as the capital, Ctesiphon. This, however, he was unable to take it, and was forced to retreat. His successor, Jovian (361-2), concluded a peace with the Persians, which was generally considered humiliating for the Romans, but which in fact stabilised the eastern frontier for a long time to come (though fighting did break out here form time to time).

The co-emperors Valentinian and his brother Valens, and later his son Gratian, were constantly engaged in fighting on all frontiers, and on occasion, rebels. In 378, however, Valens was defeated and killed by the Goths at the battle of Adrianople, and his army destroyed,. This was an enormous shock to the Romans, and left a gapping hole in their defences along the eastern Danube. The Goths were able to rampage at will through the Balkans as far as the walls of Constantinople.

Valen’s successor in the east, Theodosius,  patched up the situation in the Danube region as best he could, settling the Goths on their own land within the empire so that they caused no more destruction.

A major civil war occurred in 383-5 in which Theodosius defeated the usurper Magnus Maximus, which further weakened frontier defences along the Rhine and the Danube.


These later fourth century emperors broadly continued the policies of Diocletian and Constantine. All, except Julian “the Apostate” (360-3) were Christians, and Julians’ attempts to turn back the clock to paganism came to nothing. In fact, this period saw the Christian church become firmly embedded into the government and society of the Roman empire. In 385 it was proclaimed the state religion of the Roman empire. At both a local and empire-wide level, Christian bishops rose to prominence, with famous bishops such as Ambrose of Milan becoming having a major influence on policy, and within cities across the empire, bishops becoming central figures within their communities. The growing wealth of local churches meant that bishops were now the prime source of patronage, with money now no longer devoted to the building and upkeep of temples and public baths, and the funding of games, but in the construction and maintenance of churches, which now came to dominate the townscapes of the empire, and in charity to the poor.

The overriding need to pay for the defences of the empire, and the expanded imperial administration which went with it, determined the nature of Diocletian’s social legislation. It was aimed at ordering society in such a way that it was as easy as possible to raise taxes from it.

As in Diocletian and Constantine’s time, much legislation continued to be aimed at fixing social groups in place on a hereditary basis – peasants on their land, soldiers and traders in their professions, town councillors in their cities – so that tax collection could be more efficient. It is quite clear that this legislation was only partially successful, at best. Such legislation was part of the great struggle to keep invaders out of the empire, which continued throughout the fourth century. By the end of the century this struggle was showing clear signs of being lost, at least in the west. The Rhine and Danube frontiers were increasingly being given over to Germans and other barbarians to defend. In the next century these defences would give way altogether.

Further study

Click here for the Rise and Fall of the Roman Empire in maps

Click here for a Timeline on Ancient Roman civilization

Articles on Ancient Rome:

The Rise of Rome

Government and Warfare under the Roman Republic

The History of the Early Roman Empire

Government and Warfare under the Roman Empire

The Society and Economy of Ancient Rome

The Crisis of the Roman Empire in the Third Century

See also:

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