In 275, as Aurelian was leading an army across Thrace to settle matters with Persia, a group of officers, misled into thinking that he planned to execute them, assassinated him. Shocked by its own accumulated crimes, the army asked the Senate to appoint a successor. None wanted an honor that so regularly heralded death; finally Tacitus, being seventy-five years old, consented to serve. He claimed descent from the historian, and illustrated all the virtues preached by that laconic pessimist; but he died of exhaustion six months after taking the crown. The soldiers, repenting their repentance, resumed the prerogative of force, and saluted Probus as emperor (276).

It was an excellent choice and a merited name, for Probus stood out in courage and integrity. He expelled the Germans from Gaul, cleared the Vandals from Illyricum, built a wall between the Rhine and the Danube, frightened the Persians with a word, and gave peace to the whole Roman realm. Soon, he pledged his people, there would be no arms, no armies, and no wars, and the reign of law would cover the earth. As a prelude to this utopia he compelled his troops to clear wastelands, drain marshes, plant vines, and perform other public works. The army resented this sublimation, murdered him (282), mourned him, and built a monument to his memory.

It now hailed as imperator one Diocles, the son of a Dalmatian freedman. Diocletian, as he henceforth called himself, had risen by brilliant talents and flexible scruples to the consulate, a proconsulate, and command of the palace guards. He was a man of genius, less skilled in war than in statesmanship. He came to the throne after a period of anarchy worse than that which had prevailed from the Gracchi to Antony; like Augustus, he pacified all parties, protected all frontiers, extended the role of government, and based his rule on the aid and sanction of religion. Augustus had created the Empire, Aurelian had saved it; Diocletian reorganized it. His first vital decision revealed the state of the realm and the waning of Rome. He abandoned the city as a capital, and made his emperial headquarters at Nicomedia in Asia Minor, a few miles south of Byzantium. The Senate still met in Rome, the consuls went through their ritual, the games roared on, the streets still bore the noisome pullulation of humanity; but power and leadership had gone from this center of economic and moral decay. Diocletian based his move on military necessity: Europe and Asia must be defended, and could not be defended from a city so far south of the Alps. Hence he appointed a capable general, Maximian, as his coruler (286), charging him with defense of the West; and Maximian made not Rome but Milan his capital. Six years later, to further facilitate administration and defense, each of the two Augusti chose a “Caesar” as his aide and successor: Diocletian selected Galerius, who made his capital at Sirinium (Mitrovica on the Save), and was responsible for the Danube provinces; and Maximian appointed Constantius Chlorus (the Pale), who made his capital at Augusta Trevirorum (Treves). Each Augustus pledged himself to retire after twenty years in favor of his Caesar, who would then appoint a “Caesar” to aid and succeed him in turn. Each Augustus gave his daughter in marriage to his “Caesar,” adding the ties of blood to those of law. In this way, Diocletian hoped, wars of succession would be avoided, government would recapture continuity and authority, and the Empire would stand on guard at four strategic points against internal rebellion and external attack. It was a brilliant arrangement, which had every virtue but unity and freedom.

The monarchy was divided, but it was absolute. Each law of each ruler was issued in the name of all four, and was valid for the realm. The edict of the rulers became law at once, without the sanction of the Senate at Rome. All governmental officials were appointed by the rulers, and a gigantic bureaucracy spread its coils around the state. To further fortify the system, Diocletian developed the cult of the Emperor’s genius into a personal worship of himself as the earthly embodiment of Jupiter, while Maximian modestly consented to be Hercules; wisdom and force had come down from heaven to restore order and peace on earth. Diocletian assumed a diadem- a broad white fillet set with pearls- and robes of silk and gold; his shoes were studded with precious gems; he kept himself aloof in his palace, and required visitors to pass the gantlet of ceremonious eunuchs and titled chamberlains, and to kneel and kiss the hem of his robe. He was a man of the world, and doubtless smiled in private at these myths and forms; but his throne lacked the legitimacy of time, and he hoped to buttress it, to check the turbulence of the populace and the revolts of the army, by enduing himself with divinity and awe. “He had himself called dominus,” says Aurelius Victor, “but he behaved like a father.” This adoption of Oriental despotism by the son of a slave, this identification of god and king, meant the final failure of republican institutions in antiquity, the surrender of the fruits of Marathon; it was a reversion, like Alexander’s, to the forms and theories of Achaemenid and Egyptian courts, of Ptolemaic, Parthian, and Sassanid kings. From this Orientalized monarchy came the structure of Byzantine and European kingdoms till the French Revolution. All that was needed now was to ally the Oriental monarch in an Oriental capital with an Oriental faith. Byzantinism began with Diocletian.