Diocletian may be considered the real founder of the late empire, though the form of government he established—the tetrarchy, or four persons sharing power simultaneously—was transitory. His reforms, however, lasted longer. Military exigencies, not the desire to apply a preconceived system, explain the successive nomination of Maximian as Caesar and later as Augustus in 286 and of Constantius and Galerius as Caesars in 293. The tetrarchy was a collegium of emperors comprising two groups: at its head, two Augusti, older men who made the decisions; and, in a secondary position, two Caesars, younger, with a more executive role. All four were related either by adoption or by marriage, and all were Illyrians who had attained high commands after a long military career. Of the four, only Diocletian was a statesman. The unity of the empire was safeguarded, despite appearances, for there was no territorial partitioning. Each emperor received troops and a sector of operation: Maximian, Italy and Africa; Constantius, Gaul and Britain; Galerius, the Danubian countries; and Diocletian, the East. Practically all governmental decisions were made by Diocletian, from whom the others had received their power. He legislated, designated consuls, and retained precedence. After 287 he declared his kinship with the god Jupiter (Jove), who Diocletian claimed was his special protector. Diocletian, together with his Caesar Galerius, formed the “Jovii” dynasty, whereas Maximian and Constantius, claiming descent from the mythical hero Hercules, formed the “Herculii.” This “Epiphany of the Tetrarchs” served as the divine foundation of the regime. The ideological recourse to two traditional Roman divinities represented a break with the Orientalizing attempts of Elagabalus and Aurelian. Even though he honoured Mithra equally, Diocletian wanted to be seen as continuing the work of Augustus. In dividing power, Diocletian’s aim was to avoid usurpations, or at least to stifle them quickly—as in the attempt of Carausius, chief of the army of Britain, who was killed (293), as was his successor, Allectus (296), after a landing by Constantius.

The deification of the imperial function, marked by elaborate rituals, tended to set the emperors above the rest of mankind. But it was still necessary to avoid future rivalries and to assure the tetrarchy a legitimate and regular succession. Some time between 300 and 303 Diocletian found an original solution. After the anniversary of their 20-year reign the two Augusti abdicated (Maximian quite unwillingly), and on the same day (May 1, 305) the two Caesars became Augusti. Two new Caesars were chosen, Severus and Maximinus Daia, both friends of Galerius, whose strong personality dominated Constantius. In repudiating the principle of natural heredity (Maximian and Constantius each had an adult son), Diocletian took a great risk: absolute divine monarchy, which Diocletian largely established, implies the hereditary transmission of power, and the future was soon to demonstrate the attachment of the troops and even of the population to the hereditary principle.

In order to create a more efficient unity between subjects and administrators, Diocletian multiplied the number of provinces; even Italy was divided into a dozen small units of the provincial type. Rome, moreover, was no longer the effective capital of the empire, each emperor having his own residence in the part of the empire over which he ruled (Trier, Milan, Sirmium, Nicomedia). Although a few provinces were still governed by senators (proconsuls or consuls), the majority were given to equestrian praesides, usually without any military power but with responsibility for the entirety of civil administration (justice, police, finances, and taxes). The cities lost their autonomy, and the curiales administered and collected the taxes under the governor’s direct control. The breaking up of the provinces was compensated for by their regrouping into a dozen dioceses, under equestrian vicars who were responsible to the emperor alone. The two praetorian prefects had less military power but played an important role in legislative, judicial, and above all, financial matters: the administration of the annona, which had become the basis of the fiscal system, in fact gave them management of the entire economy. Within the central administration the number of offices increased, their managers being civilians who carried out their functions as a regular career. All officials were enrolled in the militia, whose hierarchy was to be outlined during the 4th century.

Great efforts were devoted to strengthening the borders, and the limes were outfitted with fortresses (castella) and small forts (burgi), notably in Syria. The army’s strength was increased to 60 legions (but with reduced personnel); and, in principle, each border province received a garrison of two legions, complemented by subsidiary troops. Adopting one of Gallienus’ ideas, Diocletian created an embryonic tactical army under the direct orders of the emperor whose escort (comitatus) it formed. The troops were most often commanded by duces and praepositi rather than by provincial governors and were mainly recruited from among the sons of soldiers and from barbarians who enlisted individually or by whole tribes. In addition, the landowners had to provide either recruits or a corresponding sum of money. All of these reforms were instituted gradually, during defensive wars whose success demonstrated the regime’s efficiency. Constantius put down Carausius’ attempted usurpation and fought the Alemanni fiercely near Basel; Maximian first hunted down the Bagaudae (gangs of fugitive peasant brigands) in Gaul, then fought the Moorish tribes in Africa, in 296–298, triumphing at Carthage; and on the Danube, Diocletian, and later Galerius, conquered the Bastarnae, the Iazyges, and the Carpi, deporting them in large numbers to the provinces. In the East, however, the opposition of the Persians, led by the enterprising Narses, extended from Egypt to Armenia. The Persians incited uprisings by both the Blemmyes nomads in southern Egypt and the Saracens of the Syrian desert and made use of anti-Roman propaganda by the Manichaeans and Jews. Diocletian succeeded in putting down the revolt in Egypt and fortified the south against the Blemmyes. But in 297, Narses, the heir to Shāpūr’s ambitions, precipitated a war by taking Armenia, Osroëne, and part of Syria. After an initial defeat, Galerius won a great victory over Narses, and in 298 the peace of Nisibis reinstated a Roman protégé in Armenia and gave the empire a part of Upper Mesopotamia that extended even beyond the Tigris. Peace was thus assured for some decades.

The wars, the reforms, and the increase in the number of officials were costly, and inflation reduced the resources of the state. The annona, set up by Septimius Severus, had proved imperfect, and Diocletian now reformed it through the jugatio-capitatio system: henceforth, the land tax, paid in kind by all landowners, would be calculated by the assessment of fiscal units based on extent and quality of land, type of crops grown, number of settlers and cattle, and amount of equipment. The fiscal valuation of each piece of property, estimated in juga and capita (interchangeable terms whose use varied by region and period of time), required a number of declarations and censuses similar to those practiced long before in Egypt. Each year, the government established the rate of tax per fiscal unit; and every 15 years, beginning in 312, taxes were reassessed. This complicated system was not carried out uniformly in every region. Nevertheless, it resulted in an improved accounting of the empire’s resources and a certain progress in fiscal equity, thus making the administration’s heavy demands less unbearable. In addition, Diocletian wished to reorganize the coinage and stabilize inflation. He thus minted improved sterling coins and fixed their value in relation to a gold standard. Nevertheless, inflation again became disturbing by the end of the century, and Diocletian proclaimed his well-known Edictum de Maximis Pretiis, fixing price ceilings for foodstuffs and for goods and services, which could not be exceeded under pain of death. The edict had indifferent results and was scarcely applied, but the inscriptions revealing it have great economic interest.

Diocletian’s reforms adumbrated the principal features of late Roman society: a society defined in all parts that could be useful to the state by laws fixing status and, through status, responsibility. The persons owning grain mills in Rome were (to anticipate developments that continued to unfold throughout the next two or three generations) responsible for the delivery of flour for the dole and could not bequeath or withdraw any part of their capital from their enterprise. Several other labour groups were similarly restricted, such as owners of seagoing vessels that served the supply of Rome, bargees in the Tiber, Ostian grain handlers, distributors of olive oil and pork for the dole, bath managers, and limeburners. A ban on moving to some other home or job along with production quotas were placed on people in trades serving state factories that made imperial court and army garments, cavalry equipment, and arms. Diocletian built a number of such factories, some in his capital Nicomedia, others in cities close to the groups whose needs they served. The laws imposing these obligations affected only labour groups serving the army and the capital (or capitals, plural, after the promotion of Constantinople); and, to identify them, induce them to serve, and hold them in their useful work, emperors as early as Claudius had offered privileges and imposed controls. Diocletian, however, greatly increased the weight and complexity of all these obligations.

Diocletian also changed the administrative districts in Egypt, in keeping with the model found elsewhere, by designating in each a central city to take responsibility for the whole. The last anomalous province was thus brought into line with the others. Everywhere, the imperial government continued to count on the members of the municipal senate to serve it, above all in tax collection but also in the supply of recruits, in rural police work, billeting for troops, or road building. As had been the case for centuries, they had to have a minimum of landed property to serve as surety for the performance of their administrative duties as well as to submit to nomination as senator, if it was so determined by the Senate. There had never been any one law to that effect, but by Diocletian’s time the emperor had at his command a body of long-established custom and numerous imperial decisions that served just as well. Local elites were thus hereditary, compulsory agents of his purpose, exactly like the Tiber bargees.

Two other groups were frozen into their roles in the same fashion: soldiers and farmers. The sons of soldiers were required to take up their fathers’ occupation (a law to that effect was in operation at least by 313); and the natural tendency of tenant farmers (coloni) to renew their lease on land that they, and perhaps their fathers and grandfathers, had worked was confirmed by imperial decisions—to such effect that, in 332, Constantine could speak of tenants on his Sardinian estates as bound to the acres they cultivated. This is the earliest explicit pronouncement on what is called the “colonate.” Soon the institution was extended beyond imperial estates to tie certain categories of tenants to private estates as well. The emperors wanted to ensure tax revenue and, for that, a stable rural labour supply.

The empire, as it is seen in abundant legislation for the period of Diocletian and beyond into the 5th century, has been called a “military dictatorship” or even a sort of totalitarian prison, in which every inhabitant had his own cell and his own shackles. This may well have been the rulers’ intent. By their lights, such a system was needed to repair the weaknesses revealed in the 3rd-century crisis. The principle of hereditary obligations was not, after all, so very strange, set against the natural tendencies of the economy and the practices that had developed in earlier, easier times. Yet Diocletian’s intentions could not be fully realized, given the limits on governmental effectiveness.

After a period of initial indifference toward the Christians, Diocletian ended his reign by unleashing against them, in 303, the last and most violent of their persecutions. It was urged on him by his Caesar Galerius and prolonged in the East for a decade (until 311) by Galerius as Augustus and by other emperors. As in earlier persecutions, the initiative arose at the heart of government; some emperors, as outraged by the Christians as many private citizens, considered it their duty to maintain harmony with the gods, the pax deorum, by which alone the empire flourished. Accordingly, Decius and Valerian in the 250s had dealt severely with the Christians, requiring them to demonstrate their apostasy by offering sacrifice at the local temples, and for the first time had directly struck the church’s clergy and property. There were scores of Christians who preferred death, though the great majority complied or hid themselves. Within a matter of months after he had begun his attacks, however, Decius had died (251), and the bloody phase of Valerian’s attacks also lasted only months (259/260). His son Gallienus had issued an edict of tolerance, and Aurelian was even appealed to by the church of Antioch to settle an internal dispute. Christianity had now become open and established, thanks to the power of its God so often, it seemed, manifested in miraculous acts and to the firmness with which converts were secured in a new life and community. The older slanders—cannibalism and incest—that had troubled the Apologists in the 2nd century no longer commanded credence. A measure of respectability had been won, along with recruits from the upper classes and gifts of land and money. By the end of the 3rd century Christians actually predominated in some of the smaller Eastern towns or districts, and they were well represented in Italy, Gaul, and Africa around Carthage; all told, they numbered perhaps as many as 5 million out of the empire’s total population of 60 million. Occasional meetings on disputed matters might bring together dozens of bishops, and it was this institution or phenomenon that the Great Persecutions sought to defeat. The progress of a religion that could not accept the religious basis of the tetrarchy and certain of whose members were imprudent and provocative, as in the incidents at Nicomedia (where a church was built across from Diocletian’s palace), finally aroused Galerius’ fanaticism. In 303–304 several edicts, each increasingly stringent, ordered the destruction of the churches, the seizure of sacred books, the imprisonment of the clergy, and a sentence of death for all those who refused to sacrifice to the Roman gods. In the East, where Galerius was imposing his ideas more and more on the aging Diocletian, the persecution was extremely violent, especially in Egypt, Palestine, and the Danubian regions. In Italy, Maximian, zealous at the beginning, quickly tired; and in Gaul, Constantius merely destroyed a few churches without carrying reprisals any further. Nevertheless, Christianity could no longer be eradicated, for the people of the empire and even some officials no longer felt the blind hatred for Christians that had typified previous centuries.

Struggle for power

The first tetrarchy had ended on May 1, 305; the second did not last long. After Constantius died at Eboracum in 306, the armies of Britain and Gaul, without observing the rules of the tetrarchic system, had hastened to proclaim Constantine, the young son of Constantius, as Augustus. Young Maxentius, the son of Maximian (who had never wanted to retire), thereupon had himself proclaimed in Rome, recalled his father into service, and got rid of Severus. Thus, in 307–308 there was great confusion. Seven emperors had, or pretended to have, the title of Augustus: Maximian, Galerius, Constantine, Maxentius, Maximinus Daia, Licinius (who had been promoted Augustus in 308 by Galerius against Constantine), and, in Africa, the usurper Domitius Alexander.

This situation was clarified by successive eliminations. In 310, after numerous intrigues, old Maximian was killed by his son-in-law Constantine, and in the following year Alexander was slain by one of Maxentius’ praetorian prefects. In 311 Galerius died of illness a few days after having admitted the failure of his persecutions by proclaiming an edict of tolerance. There remained, in the West, Constantine and Maxentius and in the East, Licinius and Maximinus Daia. Constantine, the best general, invaded Italy with a strong army of faithful Gauls and defeated Maxentius near the Milvian Bridge, not far from Rome. While attempting to escape, Maxentius drowned. Constantine then made an agreement with Licinius, and the two rallied the Eastern Christians to their side by guaranteeing them religious tolerance in the Edict of Milan (313). This left Maximinus Daia, now isolated and regarded as a persecutor, in a weak position; attacked by Licinius near Adrianople, he fell ill and died soon afterward, in 313. This left the empire with two leaders, Constantine and Licinius, allied in outward appearances and now brothers-in-law as a result of Licinius’ marriage to Constantine’s sister.