Ancient History & Civilisation

Part IV

Heredity and Absolutism

The Tetrarchy


The Tetrarchs hang together.


Maximian (286–311).


Diocletian (285–305).

The British Secession


Carausius (286–293).


Allectus (293–296).


Constantius I Chlorus (305–306).

Chapter Twelve

The Tetrarchy

The new ruler who seized power in 284, Diocletian, benefited from the hard work of his recent predecessors, notably Probus and Aurelian but also Gallienus, and even the senatorial emperors of 238. One of his major achievements was to stay alive and in power for the next twenty years. (It was an even greater achievement to retire and then to die a natural death several years later; a unique pair of achievements for a Roman emperor, except for Tetricus.) It was the facility with which emperors had been murdered that had been one of the root causes of the imperial instability in the recent past, and one of the reasons for that was their practice of living in the midst of their forces and appearing in public audiences. Diocletian survived in part by removing himself from such soldierly temptation; he appeared relatively little in public and inhabited a palace rather than a camp. He was a soldier, of course, though not a great one, although he was able to find and appoint capable generals to command in his name, and he slowly devised a method of organizing the imperial succession.

His own accession was particularly brutal and intimidating. He was a senior officer, called at the time Diocles, in the army of Carus and Numerian in the Persian War. Carus died and was succeeded by his son, but Numerian was dominated by his father-in-law Lucius Aper, his Praetorian Prefect. Numerian died, probably by Aper’s hand, and when the death was discovered, the soldiers at once arrested Aper; he had been the one with access, after all. However, it is probable that Diocles the general was also involved, at least in the arrest.1

The army consulted over a successor. Diocles had convened the council of senior officers and he was their choice as emperor. The army was paraded. He was loudly acclaimed by the troops, and the purple cloak was placed on his shoulders. He swore by his gods that he had had no hand in the death of Numerian (which implies that he had been accused of it). He brought Aper forward and accused him of the murder, then personally stabbed him to death. As an accession to power it was a display of vengeance and a warning to plotters quite magnificent in its brutality. It also conveniently silenced Aper.2

This gave Diocletian, as he now called himself, control of the Eastern half of the Empire through the army he had inherited from Numerian. Carinus, Numerian’s brother, still controlled the West. The two met in battle at Margus, near Sirmium. Carinus’ army was winning the fight when Carinus himself was murdered by one of his own officers, who had a personal grievance against him. Carinus’ army then stopped fighting and at once acclaimed Diocletian.3

This was one of the most difficult and dangerous imperial accessions since that of Septimius in 193–197. It was a long time since two Roman armies had fought a set battle. As a seizure of power it was messy and costly, and if anything was able to convince the new ruler that he was lucky to have survived it was this. The complexity and unpleasantness of this were the background to the reforms introduced by Diocletian and the new and extraordinary system of succession that he designed.

This system did not emerge fully grown, but was developed piece by piece in response to events. As such, it was reminiscent of the way Augustus’ system had developed. It was a sensible response to the problems that had brought the Empire close to collapse, but it was so complicated that it soon became a hindrance. The pressures on the frontiers as a result of the continuing internal problems of the Empire had become incessant, from Hadrian’s Wall to the Upper Nile, from the Crimea to Morocco, and it was physically impossible for the emperor to cope with them all. Expedients devised in the past had included joint emperors and super-governors, and Diocletian used them, as well as dividing the larger provinces into smaller sections that could more easily be directly controlled and would reduce the immediate armed support available to a potential usurper. At the same time the lack of resources of a local governor made his province the more vulnerable and demanded the creation of super-governors with wide regional authority; this could well produce even more powerful usurpers.

The real problem, though, was not the administrative system but that the barbarian invasions were actually in most cases a response to trouble within the Empire. In 284–285 four emperors and a pretender died, and Roman armies twice fought each other; not surprisingly, there was now trouble on the Danube and the Rhine, in the East, in Africa and in Britain. The secession of the Gallic Empire had been explicitly intended to defend Gaul against barbarian attack, but the Gallic go-it-alone solution would not work in the long run. The rise of the Palmyrene power in the East was a response to the Persian invasions that had briefly conquered large parts of Syria. In turn, the occurrence of rebellions was in almost every case a local response to local trouble – an invasion, usually – which the central government was unable to combat. Diocletian learned this lesson also, but the only way to solve this problem was to stop the internal dissent, a task that was by no means easy. At the time it was particularly acute in Gaul, where there was a widespread insurrection by people referred to as bacaudae. Diocletian’s attempted solution to the problem of imperial power and succession is a major element in tackling the problem.

He began by adopting an old solution, one that went back to Augustus and Agrippa or even in a sense to the consuls of the Republic: he appointed a Caesar, M. Aurelius Maximianus, a friend and fellow soldier, as emperor of the West. He was explicitly to deal with the bacaudae. Two of their leaders, perhaps as a last resort, like Aureolus, claimed the title of Augustus but only briefly before their destruction.4

Maximian’s fleet commander in British waters, M. Aurelius Carausius, seized control of Britain and part of northern Gaul in 286, after Maximian’s successful campaign against the bacaudae. Carausius had gained a substantial reputation in his military post and no doubt he was well-liked in Britain for his campaigns against Frisian and Saxon raiders, who were always stigmatized as ‘pirates’. His seizure of power came as a result of his belief that Maximian intended to arrest him. Carausius’ regime was stable for several years, possibly because he portrayed himself as a colleague of Diocletian and Maximilian, though this was not an interpretation of the situation that they shared.5

It may have been Carausius’ seizure of power locally or the spectre of bacaudian Augusti fighting a ‘legitimate’ Caesar, but in April 286 Diocletian raised Maximian to the rank of Augustus,6 theoretically therefore Diocletian’s equal, though Maximian was clearly the junior of the two for it was Diocletian who raised him to Augustus. The title of Caesar implied in the old scheme that Maximian was Diocletian’s heir, a matter that also led to the assumption that he had been adopted as Diocletian’s son. This might be all the more cunning since Diocletian had no sons of his own. Maximian’s promotion to Augustus happened in the aftermath of a victory over the bacaudae, rather as, back in the defunct Republic, soldiers saluted a victorious consul as imperator. The two men trusted each other, and neither of them succumbed to plots or seems to have intended plots. These various moves were all within the repertory of the imperial government as conducted over the previous three centuries.

Maximian was defeated in 290 in one of the various attempts to suppress Carausius, and this provoked further thoughts about the system that Diocletian was organizing. The two Augusti met in conference in Milan in 291. A public display of amity and unity was put on, the two men riding cheerfully together through the city. Diocletian had come west to display his support for his colleague after his defeat, and Maximian certainly had the more difficult task. His defeat by Carausius was a dangerous moment that would surely encourage invasions and plots if precautionary measures were not instituted. This was a situation that had happened in the previous half-century, after all; more frequently than most would care to recall. It was the very situation that normally provoked an army to look round for a new commander. Diocletian’s visit west was thus a pre-emptive move, designed to show that he and his forces would support Maximian despite his defeat.

Diocletian’s was the political brain and it was presumably he who devised the next development. Again, he only needed to drop back into the Roman imperial past to find precedents. Augustus had employed Agrippa and Tiberius as his military arms, and Tiberius had used Drusus and Germanicus in the same way. The purpose had been to allow Augustus – never an outstanding commander in the field – to control matters in Rome and avoid the danger of defeat, while more competent commanders took on the military problems. It also, of course, allowed Roman power to be projected even when the emperor was elsewhere. It was, in fact, only since Marcus Aurelius that emperors had regularly commanded the armies in the field, the only real exception earlier being Trajan.

Diocletian and Maximian were both commanders and emperors; defeat in the field would therefore damage or destroy the prestige of both of them. The answer was to separate, as in the past, the two imperial roles. So the two men set about developing plans for distancing themselves from campaigning; instead they selected deputies who would do that for them. The defeat of a deputy would be much less politically damaging than the defeat of an Augustus, and to show that the deputies were junior they were given the title of Caesar.

The plan was put into effect in 293. Maximian chose M. Flavius Constantius Chlorus as his deputy. Constantius was an experienced soldier, a former governor of Dalmatia (who may well have deserted Carinus for Diocletian in 285), and had been Maximian’s Praetorian Prefect; he also became Maximian’s son-in-law. Diocletian chose C. Galerius, who divorced his wife and married Diocletian’s only daughter. The two deputies were then given the title of Caesar and proclaimed principes iuventutes, even though both were hard-bitten soldiers not all that much younger than the two Augusti.7 The whole scheme is perhaps most closely parallelled by the brief period in 238 when there were two Augusti – Pupienus and Balbinus – with Gordian III as Caesar; if this was in Diocletian’s mind one wonders if he noted the irony, for he was wholly dismissive of any senatorial pretensions to power.

There are here echoes of past succession methods, with more or less subtle changes, but how else was the public to understand the situation unless it had been wrapped up in familiar terms? The essential difference from the past was that this was all done, so far as can be seen, with no reference at all to the Senate. The crucial meeting between the emperors took place in Milan, a convenient city in which the rulers of the East and West could meet and not be too far from their separate responsibilities and their frontier problems. It was actually more convenient for Maximian but then it was his military and political problem that was at the heart of the crisis. The designation of the two middle-aged Caesars as ‘princes of youth’ may have been rather entertaining, but it was another title that had been detached from its literal and original meaning. Such titles’ connections were, however, clear: the successors of emperors had often had such titles and Diocletian was organizing a new system of succession under the old titles.

The absence of consultation with the Senate as a body does not mean that senators were not involved. Diocletian had deliberately chosen a senatorial colleague as his partner-consul when he first seized power, and at Milan there were enough senators present as individuals to have held a Senate meeting if it had been felt necessary. These were further traditional gestures to the past but with a Diocletianic twist: this time the senators attended on the emperors, whereas before the emperors had headed for Rome to be invested by the Senate. Diocletian probably did not visit Rome until nearly the end of his reign (though he did rebuild the Senate House when it burned down in 285, and he just might have been in the city in that year).

Carausius had therefore twice provoked major crises and changes in the Diocletianic regime, first in 286 by provoking the promotion of Maximian to Augustus, and then again in 293 and the appointment of new Caesars. The Caesar Constantius justified his promotion at once by clearing Carausius’ supporters and forces from northern Gaul, and then besieging and capturing his main continental base at Gesoriacum (Boulogne). This opened the way for the invasion of the island. Carausius himself died at about this time, though how, where and why is not known. His successor was Allectus, otherwise wholly unknown, said to have been one of Carausius’ financial officials. He is assumed, without much evidence other than later assumptions, to have been responsible for Carausius’ death.8 In fact, he held on to the island empire for three more years and finally succumbed to an invasion commanded by Constantius in 296.

The length of time this ‘British Empire’ lasted – ten years – was due entirely to the command with which Carausius began: the local Roman fleet. He will also have been able to persuade the army in Britain either to support him or to acquiesce in his pretensions. Similarly, Allectus clearly had the same sort of local support. However, without acceptance by Diocletian, the senior emperor, it would be impossible for the regime to last long, and Carausius’ method of promoting himself, by usurpation and self-proclamation, was exactly what Diocletian, having achieved power by those very means, was determined to prevent. Carausius’ regime lasted ten years or so in part only because the emperors on the continent were busy with other affairs.

The ‘Tetrarchy’, as the Rule of Four Emperors is known in modern historical studies, was devised in 293, and so it lasted for twelve years, until Diocletian retired in 305. Its success was due in part to the military victories of its members, but above all, to the fact that the former rulers all survived long enough for their work to become embedded and generally accepted. The Caesars were given territorial responsibilities, so that all the several frontier areas were covered. (See also Genealogical Table XVI.)

This government system, as it had developed by 293, was the result of a series of expedients devised to cope with crises that threatened the regime. The most serious, in the sense that it was a direct challenge to the legitimacy of Diocletian’s rule, was the rival imperial regime of Carausius. Carausius tried to muscle in onto the wider scheme, and had to be excluded for that very reason. The British challenge was a most threatening one for the ruling emperors, for this rival regime, first under Carausius himself and then under his successor Allectus, laid claim to an equal legitimacy with that of Diocletian and Maximian, and it is best, as with the similar Gallic regime initiated by Postumus, to see it as a normal Roman government; it had originated, after all, in a military putsch, just as had Diocletian’s regime. In terms of origin, the two were the same. Yet it could only be regarded by Diocletian and Maximian as illegitimate, because it was of an origin independent of them; hence its importance and the necessity of its destruction.

This is not necessarily how we should regard them. To take Caurausius as a normal Roman emperor of the type of Postumus is not a plea for the belated installation of these men as ‘legitimate’ emperors, but a suggestion that to classify one group as legitimate emperors and another as usurpers is, as ever, pointless, especially at this distance in time. It is more useful to look at what they did, what their authority was, and at the way they made themselves into emperors. For if a man could maintain himself as emperor over several years, have soldiers and civilians obey him over a relatively geographically extensive area and pass on his power and position to his successor, he was actually as much a ‘legitimate’ ruler as any other.


The question of legitimacy is, of course, at the heart of the problem. In the Empire, any man who claimed the throne in defiance of a ruler was automatically a rebel, until he won. So Diocletian, to Carinus, was a rebel, until the battle of Margus and Carinus’ death; and Carausius was a rebel or a rival to Diocletian, though he was clearly not so to his British subjects. The only indication between 285 and 305 that a ruler had ‘legitimacy’ was by his acceptance by Diocletian as a colleague. However, in the decade before Diocletian, and for Diocletian himself at the start of his reign, it was acclamation by the army that counted, together with installation by the Senate. Both before then and after, it was inheritance and designation by a blood relative that were crucial. The criteria for legitimacy therefore altered with the changes in actual power – this was one of Diocletian’s achievements – and these changes means it is wasted effort to distinguish rulers into legitimate emperors and usurpers. The case of Carausius and Allectus is to the point, for between them they maintained a successful Roman imperial regime for a full decade, rather longer than most third-century ‘legitimate’ emperors. To their subjects, from whom they collected taxes and to whom they dispensed justice and provided defence, they were clearly legitimate rulers.

Diocletian himself in the East was faced with a great revolt in Egypt that began in 297. In the course of it the Alexandrians promoted one of their leaders, L. Domitius Domitianus, to the position of emperor. He and the city were then subjected to a grim siege and assault by Diocletian, who was angry at what he may well have seen as treachery in his rear as he was about to launch an attack on Persia. (He may also have been annoyed that the Egyptian rebellion blew up just as the British problem was solved.) Domitianus vanished from the scene fairly early on, after a reign of only four or five months, and the Alexandrian resistance was then led by a man called Aurelius Achilleus, who had been corrector of Egypt under Domitianus. He apparently did not take the imperial title, though he and the city held out until the spring of 298. His regime seems to have had deliberate reminiscences of the old Ptolemaic kingdom.9

The reasons for the revolt were, suitably enough given the curious Ptolemaic ideology, almost entirely backward-looking. Diocletian was engaged in a widespread programme of reforms, a process that always upsets people whose positions and/or comforts are threatened. Part of those reforms involved a new census for the purpose of bringing the taxation system up to date.

Table III: Unsuccessful Emperors, 284–330.


That is, the reforms were going to be more efficient at extracting tax revenues, and the population as a whole quite reasonably did not like the idea. This seems to have been the basis of the revolt in Egypt, though which came first, the city revolt or the claim of Domitianus to the throne, is uncertain. The revolt and the regime it set up fell to pieces during Diocletian’s vengeful siege and capture of Alexandria, a fate that also affected a considerable area of the city. The appearance of a local Roman emperor in the city had therefore little to do with the revolt, and he posed no real threat to the wider imperial regime. Yet again, this was merely a local matter, even though it was apparently triggered by a tax dispute. Yet it was exactly these local disputes that had so destabilized the Empire during the previous half-century, and Diocletian’s rage at Alexandria was perhaps a response to this localism and his way of making clear that such disputes were not to be the source of revolts. Its failure permitted the Diocletianic reforms, including the census, to go ahead. (See Table III for the rebels of this period.)

Two other events that have been classified as ‘usurpations’ took place in Syria and nearby in 303. Neither is well-attested. One took place at Melitene (modern Malatya) in the province of Armenia Minor, but that is all that is known about it; the Church historian Eusebius claimed that it was a danger to the Empire, but what it amounted to is quite unknown. The other problem was an attempt by an army unit stationed at Seleukeia in Syria, where the commander, Eugenius, was proclaimed emperor by his men and then led them on a march against Antioch. This is better attested, and was, given the army involvement, rather more serious than that at Melitene. It was quickly defeated. Both of these revolts took place in 303, and it is perhaps best to associate them, as Eusebius does, with the imperial orders designed to suppress Christianity that were issued that year.10 Diocletian was more concerned over a fire in the imperial palace at Nikomedeia, and did not allow himself to be diverted by these events from a projected visit to Rome. His estimate of the importance of the events in Syria seems to have been quite accurate.

Of the rebellions during Diocletian’s reign, therefore, only that of Carausius (and Allectus) was of any real political importance. Like the Egyptian rebellion, it was in all likelihood essentially a resistance movement devoted to preventing Diocletian’s reforms from having local effect; the Syrian and Melitenean risings, if at base connected with Diocletian’s anti-Christian policy, were thus also essentially resistance to unwelcome imperial measures. Diocletian nevertheless succeeded in imposing those reforms, and in recovering control in Britain and Egypt by main force; this was, of course, the most effective way of discouraging future attempts, and the damage to Alexandria was a graphic warning to others. So far as can be seen, the reforms were also imposed by ignoring the Senate, though the succession scheme might have been a worthwhile subject for senatorial legislation, particularly one as formal and prescriptive as that Diocletian apparently intended to leave behind.

So, amid wars and campaigns, the Empire remained in the grip of Diocletian and his system of Augusti and Caesars until Diocletian fell ill in 304. The Rule of Four Emperors, two Augusti and two Caesars, had worked well for more than a decade, and had been instrumental in recovering rebellious provinces and defeating invaders. The system, however, had been developed in response to particular political crises, notably the problem of how to deal with Carausius. Exactly when Diocletian realized that he also had devised a system by which he could hand over power and ensure a peaceful succession system is not clear. It seems highly unlikely that he thought of the Rule of Four as a scheme of succession at the beginning, but he certainly saw it in that light later.

He had floated the idea of retirement sometime before his long and serious illness, which began in late 304. Once it was in the air, of course, the question of the succession automatically became urgent, but in this case it was not the question of his immediate successor that was at issue but who should be his next successor but one. He persuaded a reluctant Maximian to retire with him, so that their Caesars should inherit full power as the next Augusti, but first it was necessary to pick two new Caesars. Like Augustus and Hadrian, Diocletian was clearly intent on organizing the succession for the foreseeable future; that is, for at least two more generations. If his immediate successors could reign for twenty years as he and Maximian had, and if their successors could then do so as well, he was envisaging the organization of the succession for at least the next forty years.

So on 1 May 305, Constantius and Galerius each moved up to the rank of Augustus, the former as the senior of the two, and each of them was given a new Caesar. The whole process was yet again reminiscent of several earlier occasions, going back to Augustus and Tiberius, but also to the second-century emperors, whereby it was supposed the emperor would select a suitable successor and adopt and train him. Only after a careful investigation does it turn out that in most of these cases the suitable successor proved to be a very close relative of the emperor who selected him.

The new Caesars were not the sons of any of the various emperors. Constantius had an adult son, as did Maximian: Constantine and Maxentius respectively. They were the obvious candidates to be Caesars, but neither was chosen. They had none of the experience that the promoted Caesars had had ten years before when they took the office. In particular, no one except Maximian and Maxentius himself was impressed by Maxentius, and if the son of one emperor was to be excluded, the other would have to be as well. It might also be that Diocletian, who had no son of his own, was intent on avoiding the dynastic principle, though this seems less likely when the various marriages between the four emperors are taken into account.

This idea of the careful selection and judicious weighing of merits in potential successors is, of course, untenable, for it was necessarily a personal decision made, it appears, by Diocletian, partly under the influence of Galerius. They chose men they knew, and tried to impose their choices on their colleagues. The two new Caesars of 305, Severus and Maximin Daia, were both in fact associates of Galerius: Maximin was his nephew and Severus was a military colleague, possibly his Praetorian Prefect. This would seem to be a coup for Galerius, and Constantius clearly felt that he could not rely on his new Caesar, Maximin Daia, in the way that Maximian had been able to trust Constantius. Also, of course, the scheme took no heed of either human mortality or of the deeply-ingrained Roman (and human) affection for heredity.

The whole scheme has all the hallmarks of one that was bodged together out of emergency measures developed to deal with problems as they arose, and based on a set of half-remembered and only half-understood historical memories, just like other aspects of Diocletian’s imperial ‘scheme’, developed in response to events rather than being organized from particular principles. The old Roman preference for double magistracies, the supposed Antonine preference for adoptive succession, the idea of selecting experienced men from a pool of eligible candidates, all are involved, but none of these really existed during the imperial period at any time. No pairs of emperors were ever equal, adoption was only resorted to in emergencies (as with Nerva and Trajan) or when direct family succession failed.

Beyond this, the scheme was unworkable without the authority of a senior emperor with the personal force and power of a man like Diocletian to enforce it, and it actually failed within a year of his retirement. Neither of the passed-over sons of the emperors was prepared to submit to rejection.

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