In the third century the vast Roman Empire had at least 50 million inhabitants. Since Caracalla’s Constitutio Antoniniana of 212 almost all of these possessed Roman citizenship, provided that they were freeborn. The large majority of the people lived in the country or in one of the many hundreds of small towns throughout the Empire, most of which had populations never exceeding 10,000 inhabitants. There were just a couple of truly large cities, with populations of over 100,000 citizens. The most important of these were Carthage in North Africa, Alexandria in Egypt, Antioch in Syria and of course Rome, a city that in the third century still must have had over 500,000 inhabitants, in spite of the many crises that it was faced with. Although trade was of major importance in the Roman Empire, Roman society was still largely agrarian. The large estates (latifundiae) worked by slaves had in the third century mostly disappeared. They had been replaced, especially in the Latin west, by a kind of proto-feudalism, which involved wealthy landowners leasing land to coloni (‘serfs’). It was a fact of life that the rich and powerful dominated the much more numerous poor. The big question was how their rule could be made as effective as possible.
The Roman elite was composed of several hundred senators, a few thousand knights (equites) and tens of thousands of local magistrates. Together these formed at most one percent of the total Roman population. Roman provincial governors usually had just a small staff and delegated many tasks to local authorities. Judged by modern standards, the system for levying taxes was primitive, but at the same time a large amount of money was need for upkeep of the Roman army, which not only had to protect the Empire against foreign attacks, but also constituted the power base of each and every emperor.
As soon as he had ascended the throne at the end of the year 284, Diocletianus must have realised that governing the immense Roman Empire was a gargantuan task. It seems rather unlikely that at the time he already had a ‘blueprint’ in mind for modernising and improving the administration of the Empire. The emperor probably came up with new ideas along the way and then implemented these on an ad hoc basis. The appointment of Maximianus as caesar (junior emperor) in 285 and then as augustus (co-emperor) in 286 can be seen in that light. But in 293 Diocletianus went one step further by creating a system of government that involved no fewer than four emperors. The system is known as the Tetrarchy. In 293 Diocletianus must have been around 50 years old. The emperor had no son and therefore no heir in the immediate family. It seems highly plausible that these factors played a role in the way the Tetrarchy was designed.
The Tetrarchy led to the Roman Empire being split informally into four parts. Each part was governed by either a senior emperor (an augustus) or a junior emperor (a caesar). When an augustus died or abdicated, he was normally succeeded by a caesar. The latter then nominated a new caesar. In a solemn ceremony on 1 March, probably in Mediolanum (Milan), Flavius Valerius Constantius was appointed caesar under Maximianus. As he was married to Maximianus’ stepdaughter Theodora, he was already his son-in-law. Now he also became Maximianus’ son by adoption. Around the same time Diocletianus, residing in Nicomedia, made one Galerius his caesar. The new caesar was probably from Dacia Aureliana, a relatively new province that had been created by the emperor Aurelianus in northern Moesia. Galerius subsequently married Valeria, the daughter of his augustus, and was then also adopted by his father-in-law. Constantius and Galerius from now on began styling themselves Herculius and Jovius too, after Hercules and Jupiter, the most important gods of both the imperial family and the Roman Empire as a whole.
When the Empire was split, Gaul and Britannia were granted to Constantius. Maximianus was charged with governing Italy (including Sicily and Sardinia), North Africa and Spain, while Galerius ruled over the Balkan area with the Danube provinces. The most important provinces in the east – in Asia Minor, Syria, Egypt and Libya – fell to Diocletianus. It should be noted that there were no rigid boundaries between the four respective ‘realms’. Especially Diocletianus, who remained the most senior emperor within the Tetrarchy, often campaigned in the territories of his ‘son’ Galerius and vice versa. It cannot be denied that the creation of the Tetrarchy was also motivated by military factors. Emperor living at the end of the third century had to be everywhere at the same time. The new system allowed Constantius to defeat the rebel Carausius and retake Britannia, while Maximianus could focus on both the Rhine borders and the troublesome Berbers in North Africa. Galerius kept an eye on the Danube, so that Diocletianus in his turn could watch the Persians. In 293 king Bahram II, always eager to avoid conflict with the Romans, had died. He had been succeeded by his uncle Narses, a much more bellicose ruler. A few years later a new war with the Romans would break out.
As long as the four emperors cooperated harmoniously and considered themselves scions of one and the same family with the same interests, the risk that the Tetrarchy would degenerate into four separate empires was manageable. It is nevertheless possible that Diocletianus deliberately added a few extra safeguards to the system. Constantius had a son from a previous relationship with Helena, a lowborn woman from Bithynia. This Constantine, who was about twenty years old, could one day succeed his father as caesar and had to be prepared for this future task. The young man was therefore sent to the east and alternately served under Galerius and Diocletianus. Although this service was formally part of Constantine’s administrative and military education, it has been assumed that the young man was effectively a hostage, to ensure his father’s loyalty to the regime.
Organisation of the provinces and other reforms
Possibly in 293-294, but perhaps earlier, Diocletianus also reorganised the Roman provinces. When he became emperor, there were about 50 provinces. Diocletianus now split almost all of these provinces into smaller units, raising the total number of provinces to over 100. Of course this also meant that the number of provincial governors was doubled. These governors were recruited from among the senators and knights, and were henceforth called correctores. A new layer of government was then created between the emperors on the one hand and the correctores on the other. This reorganisation involved the recently split provinces being joined again into groups that were known as dioceses and that were administered by a vicarius. In this way a number of ‘super provinces’ were created in Britannia, Gaul, Spain, Italy, North Africa, the Balkans and Asia Minor. Perhaps the most important super province was Oriens, the new name for the large diocese that comprised Libya, Egypt, Syria, Cilicia and Mesopotamia.
Both the introduction of the Tetrarchy and the provincial reforms must have led to a greater number of government officials and to a larger bureaucracy composed of, among others, secretaries, clerks and lawyers. Each emperor, whether he was an augustus or a caesar, had his own court and praetorian prefect, while each vicarius or corrector had his own staff. Diocletianus had furthermore taken several measures to increase the size of the Roman army by several tens of thousands of men. Military service had been made hereditary and members of Germanic and other tribes living within the borders of the Empire had been recruited. Since these tribal warriors did not possess Roman citizenship, they were enrolled among the auxiliary forces (auxilia), which made them a lot cheaper, but certainly not less effective. The idea that a process of ‘barbarisation’ of the Roman army made it a less formidable fighting force, inclined to disloyalty, has few supporters left these days.
It was also during Diocletianus’ long reign that the foundations were laid for the later distinction between border troops, that were associated with a certain region and were called limitanei, and the mobile field army, the comitatenses. The border troops were led by commanders that were called duces. These ‘dukes’ – the modern English word ‘duke’ derives from Latin dux – were responsible for the security of their own border district. The comitatenses were commanded by a comes or count. Diocletianus’ army reforms very likely continued a process that had already been initiated under Valerianus and Gallienus. The reforms were presumably not completed until the reign of Constantine the Great (306-337).
The reforms created a much sharper distinction between civil offices on the one hand and military offices on the other. Young Roman noblemen now opted either for a career in government or in the army. Another development was that, while the Roman army itself grew in size, its constituent units became progressively smaller. In the fourth century most of the large legions of ca. 5,000 men only existed on paper. In practice they had been split into smaller units of no more than 1,000 soldiers. Nevertheless, there must have been exceptions, as the fourth-century writer Vegetius mentions two large ‘legions’ under Diocletianus and Maximianus that were supposedly composed of 6,000 men. Vegetius calls them the Joviani and Herculiani and claims they were stationed in lllyricum. The legions were probably the elite units of the two emperors. Vegetius’ claim that they excelled at fighting with the martiobarbuli (leaded javelins) is evidence that the equipment of the Roman soldiers had also changed. The heavy javelin or pilum had for instance disappeared.
Diocletianus’ system was costly, but it was intended to be self-supporting and sought to streamline and modernise tax collection within the Empire. Its ultimate aim was for the benefits to exceed the costs. Under previous emperors the amount of taxes to be paid by the provinces was usually fixed in advance for a certain period of time. Diocletianus gradually adjusted the system to te needs of the Empire and the costs of the army and bureaucracy, making it much more flexible. Part of the process was Italia Annonaria, the part of Italy above Rome, losing its traditional tax exemption (possibly in 297). The other part of Italy, Italia Suburbicaria, kept its privileges for the moment.
Campaigns in 293-295
Although the newly appointed caesar Constantius was charged with governing Gaul and Britannia, the harsh reality was that part of the former and all of the latter were controlled by the rebel emperor Carausius. An attempt by Maximianus in 289 to expel the rebel from his stronghold of Gesoriacum (Boulogne) had failed spectacularly. In 293 Constantius tried to do better than his father-in-law and was in fact successful. The caesar commanded a strong fleet and succeeded in closing off the harbour of Gesoriacum with a mole just long enough to force the city to surrender. The surrender came just in time, as not much later Constantius’ mole was swept away by the tide. Soon after, Carausius was murdered by one of his own subordinates, one Allectus. The fact that the rebel emperor had been unable to defend his territories in Gaul may have contributed to the murder. Although Constantius had won an important victory, he still had to retake Britannia, where Allectus had now proclaimed himself the new emperor.
In – presumably – 294-295 Diocletianus and Galerius jointly campaigned along the Danube against the Bastarnae and Carpi. After several confrontations they managed to make a treaty with the latter. Groups of Carpi settled on Roman territory and many of them joined the Roman army. After this campaign Diocletianus went east, where he probably took up residence in the important city of Antioch. It is generally assumed that the young Constantine was serving on his staff at the time.
- Aurelius Victor, De Caesaribus 39 (translated and annotated by H.W. Bird);
- Eutropius, Breviarium Historiae Romanae 9.22;
- Vegetius, De Re Militari 1.17.
- Adrian Goldsworthy, The Fall of the West, p. 160-165 and p. 206-214;
- Henk Singor, Constantijn, p. 40-43, p. 143-144 and p. 160-161;
- Timothy Venning, A Chronology of the Roman Empire, p. 627-630.
 Henk Singor, Constantijn, p. 40-41.
 Timothy Venning, A Chronology of the Roman Empire, p. 630.
 Adrian Goldsworthy, The Fall of the West, p. 164; Henk Singor, Constantijn, p.160-161; Timothy Venning, A Chronology of the Roman Empire, p. 627.
 Vegetius, De Re Militari 1.17.