For once, his successor, the aged senator Tacitus, was chosen by the Senate—at the army’s request and on short notice; he reigned only for a few months. After him, Probus, another Illyrian general, inherited a fortified empire but had to fight hard in Gaul, where serious invasions occurred in 275–277. Thereafter, Probus devoted himself to economic restoration; he attempted to return abandoned farmland to cultivation and, with the aid of military labour, undertook works of improvement. To remedy the depopulation, he admitted to the empire, as had Aurelian, a great number of defeated Goths, Alemanni, and Franks and permitted them to settle on plots of land in Gaul and in the Danubian provinces. After the assassination of Probus in 282 by soldiers, Carus became emperor and immediately associated with himself his two sons, Carinus and Numerian. Carus and Numerian fought a victorious campaign against the Persians but died under unknown circumstances. Carinus, left behind in the West, was later defeated and killed by Diocletian, who was proclaimed emperor in November 284 by the army of the East. Diocletian
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.