Diocletian would have received little education beyond an elementary literacy.
Diocletian became commander of Emperor Numerian's bodyguard. When the Emperor was murdered by his praetorian prefect, the troops chose Diocletian in November 284 to succeed and avenge his master.
By early 285 Diocletian had circumvented all opposition and determined to take immediate steps to bring to an end the 50 years of military anarchy (235-284) that had seen 26 emperors gain the throne, and scores of unsuccessful pretenders. He therefore decided to appoint as his caesar (successor-designate) a man of his own age, his old fellow soldier Maximian. The wisdom of this policy was immediately demonstrated by Maximian's military successes in Gaul, Germany, and North Africa between 286 and 290. Diocletian, meanwhile, controlled the Danubian and eastern frontiers. His satisfaction with the arrangement led him in 286 to raise Maximian to the rank of augustus, or coemperor.
In 293 Diocletian and Maximian adopted as their caesars and aides Galerius and Constantius (I) Chlorus, respectively, and each young man was prevailed upon to divorce his wife and become the son-in-law of his augustus. Maximian assumed the general supervision of the West (prefecture of Italy) with headquarters in Milan; Constantius had special responsibility in Gaul and Britain and Galerius in the Balkans (Illyrium). Diocletian was in general control of the East with headquarters at Nicomedia (modern Izmir, Turkey), but the others also regarded him as their superior and guide.
Diocletian's innovation proved a military success: in 296 Constantius returned Britain, which had split away nearly a decade before, to the empire; Maximian triumphed over Moorish revolts in 297; and Diocletian suppressed insurrections in Egypt in 295 and 297. Galerius held the Danubian frontier successfully, and in 297 he so thoroughly defeated Narses I of Persia that more than 50 years of peace was achieved for that area.
Diocletian had lived and fought for many years in the East, and he had observed that the secluded Oriental potentates were victimized by their subjects far less frequently than the more democratic Romans.
In 303 Diocletian visited Rome for the first time to observe his twentieth anniversary as emperor. The following year he suffered from a very severe illness, probably a stroke, which seems to have convinced him that it was high time to turn over the reins of government to the caesars. On May 1, 305, therefore, he abdicated at Nicomedia, and by prearrangement Maximian performed the same act simultaneously at Milan. Galerius and Constantius Chlorus were elevated to the rank of augusti, while Flavius Valerius Severus became caesar in the West and Maximin Daia in the East.
Diocletian retired to the palace that he had prepared for himself in Spalato. There he busied himself with his vegetable garden, refusing to return to the political scene except for one brief peacemaking conference in 308 between his squabbling successors. He died at Spalato, probably in 313.
Diocletian was considered the special spokesman on earth for Jupiter, the king of the gods, and he assumed the epithet "Jovius"; his friend, Maximian, became "Herculius" as the representative of Hercules, the industrious son and helper of Jupiter, and who, as the benefactor of mankind, was running a close race with Christ for the allegiance of the Roman masses.
For most of his reign Diocletian was tolerant of dissident religious sects, including the Christians. But some Romans, especially Galerius, felt that the Christians were subverting Diocletian's attempt to emphasize the religious basis of his government to strengthen the state. In 303 Diocletian finally was prevailed upon to issue an edict banning Christian churches, assemblies, and sacred books. This ban was soon followed by two fires of mysterious origin in the Emperor's own palace in Nicomedia, which probably suggested the need for three further and progressively more severe edicts. These edicts were observed in a very uneven fashion, however, being strictly enforced only in Galerius's domain.
Diocletian extended and formalized the system of joint leadership by the establishment of the so-called tetrarchy (or "rule of four", each emperor would rule over a quarter-division of the empire), which proved a military success.
During his reign the Senate had progressively been deprived of its privileges. Diocletian designated the consuls; the senators no longer collaborated in the making of laws; the imperial counsellors (consilia sacra) were distributed among specialized offices, and their functions were strictly defined so that the power of the praetorian prefects (personal bodyguards to the emperor) was limited. This was the beginning of the bureaucracy and technocracy that was eventually to overrun modern societies.
During the third century governors of the larger provinces of the empire had repeatedly become rival claimants for the throne. Diocletian sought to correct this danger by splitting up the provinces into far smaller units - the number rose from less than 50 to well over 100 - and within these units civil and military administrations were carefully separated. The smaller units fostered more careful and personal administrative and judicial work by governors and promoted imperial stability, but resultant proliferation of bureaucratic machinery effected a severe strain on the economy.
Diocletian also began to systematize a new organization of the army, formalizing tendencies that constant third-century warfare had brought about. The old legions, now sedentary and in effect a militia of farmers, were stationed along the frontiers to absorb the first shock of external attack. New, mobile, and much smaller legions (1,000 to 1,500 men, as opposed to the old 6,000) were stationed in garrison cities to back up the frontier troops.
Diocletian also developed the use of mounted troops and began the organization of special crack troops, the comitatenses, or friends of the emperor, to serve as an imperial bodyguard. All this raised the size of the army from about 400,000 to about 500,000 men. It also increased the financial burdens of the state, though the frontier troops undoubtedly largely supported themselves from the land.
Diocletian undertook an ambitious building program, which included the enormous Baths of Diocletian at Rome and his palace for retirement at Spalato (modern Split) in Dalmatia, and he also encouraged his colleagues to sponsor public works. This program, with the demands of the bureaucracy and the army, severely strained the empire's finances, and Diocletian undertook a complete reform of the tax structure to meet these needs.
His new system was based on the establishment of units of approximately equal value of land or of living things: that is, the unit of land (a jugum) could equal 20 acres of first-class plowland, 5 acres of vineyard, or 225 olive trees; or the head unit (caput) could equal the labor of one man, two women, or the sale value of a given number of animals. The value of the nation's resources was to be reviewed periodically; and the emperor and his advisers, after determining the national budget, each year could then set the tax rate per jugum and caput.
A steady debasement of the coinage during the third century had undermined all public confidence in the monetary system. Diocletian instituted a complete currency reform, and a uniform currency for the whole empire was devised. It appears, though the details are obscure, that this reform sent prices skyrocketing, probably because much of the old coinage was still in circulation and was now suspect. In any case, the desperate plight of soldiers and bureaucrats, who were on a fixed salary, forced Diocletian in 301 to issue an edict setting maximum prices for almost every conceivable article and service throughout the empire. The penalty for nonobservance was death. The efficacy of the measure appears to have been disappointing and the need brief. The extant fragments of the edict are of immense value in calculating the standard of living in the Roman world.
Though himself a man of simple tastes, Diocletian determined to surround the throne with all the trappings of Oriental monarchy. He seldom appeared in public, but when he did it was with diadem, royal purple, and robes embroidered with gold.
Quotes from others about the person
A.H.M. Jones: "It is perhaps Diocletian's greatest achievement that he reigned twenty-one years and then abdicated voluntarily, and spent the remaining years of his life in peaceful retirement."
Diocletian had a wife Prisca and a daughter Valeria, both of whom reputedly were Christians.