The new emperor observed to a friend that "he had now killed the boar," punning on the word Aper, Which means a boar, and alluding to the prediction of a soothsayer in Gaul, who had told him that he would become emperor after having killed a boar (Vopiscus, in "Hist. Aug."). Diocletian, self-composed and strong-minded in other respects, was all his life an anxious believer in divination, which superstition led him probably to inflict summary punishment upon Aper with his own hands. He made his solemn entrance into Nicomedia in September, 284, which town he afterward chose for his favorite residence.

Carinus, the other son of Carus, who had remained in Italy, having collected a force to attack Diocletian, the two armies met at Margum, in Mœsia, where the soldiers of Carinus had the advantage at first, but Carinus himself being killed during the battle by his officers, who detested him for his cruelty and both armies joined in acknowledging Diocletian emperor in 285. Diocletian was generous after his victory, and, contrary to the common practice, there were no executions, proscriptions, or confiscations of property; he even retained most of the officers of Carinus in their places.

Diocletian, on assuming the imperial power, found the Empire assailed by enemies in various quarters—on the Persian frontiers, on the side of Germany and of Illyricum, and in Britain; besides which a serious revolt had broken out in Gaul among the rural population, under two leaders who had assumed the title of emperor. To quell the disturbance in Gaul, Diocletian sent his old friend Maximianus, a native of Pannonia, and a brave but rude uncultivated soldier. Maximianus defeated the Bagaudi, for such was the name the rustic insurgents had assumed. In the year 286, Diocletian chose Maximianus as his colleague in the Empire, under the name of Marcus Valerius Maximianus Augustus, and it is to the credit of both that the latter continued ever after faithful to Diocletian and willing to follow his advice. Maximianus was stationed in Gaul and on the German frontier to repel invasion; Diocletian resided chiefly in the East to watch the Persians, though he appears to have visited Rome in the early part of his reign. About 287 the revolt of Carausius took place. In the following year Maximianus defeated the Germans near Treviri, and Diocletian himself marched against other tribes on the Rhetian frontier; the year after he defeated the Sarmatians on the lower Danube. In the same year, 289, peace was made between Carausius and the two emperors, Carausius being allowed to retain possession of Britain. In 290 Maximianus and Diocletian met at Milan to confer together on the state of the Empire, after which Diocletian returned to Nicomedia. The Persians soon after again invaded Mesopotamia and threatened Syria; the Quinquegentiani, a federation of tribes in the Mauritania Cæsariensis, revolted; another revolt under one Achillæus broke out in Egypt; another in Italy under a certain Julianus.

Diocletian thought it necessary to increase the number of his colleagues in order to face the attacks in the various quarters. On the 1st of March, 292, or 291, according to some chronologists, he appointed Galerius as Cæsar, and presented him to the troops at Nicomedia. At the same time Maximianus adopted on his part Constantius called Chlorus. The two Cæsars repudiated their respective wives; Galerius married Valeria, Diocletian's daughter, adding to his name that of Valerianus; and Constantius married Theodora, daughter of Maximianus. Galerius was a native of Dacia, and a good soldier, but violent and cruel; he had been a herdsman in his youth, for which he has been styled, in derision, Armentarius. The two Cæsars remained subordinate to the two Augusti, though each of the four was entrusted with the administration of a part of the Empire. Diocletian kept to himself Asia and Egypt; Maximianus had Italy and Africa; Galerius, Thrace and Illyricum; and Constantius had Gaul and Spain. But it was rather an administrative than a political division. At the head of the edicts of each prince were put the names of all the four, beginning with that of Diocletian.

Diocletian resorted to this arrangement probably as much for reasons of internal as of external policy. For nearly a hundred years before, ever since the death of Commodus, the soldiers had been in the habit of giving or selling the imperial crown, to which any general might aspire. Between thirty and forty emperors had been thus successively made and unmade, many of whom only reigned a few months. By fixing upon four colleagues, one in each of the great divisions of the Empire, each having his army, and all mutually checking one another, Diocletian put a stop to military insolence and anarchy. The Empire was no longer put up to sale, the immediate and intolerable evil was effectually cured, though another danger remained, that of disputes and wars between the various sharers of the imperial power; still it was a smaller danger and one which did not manifest itself so long as Diocletian remained at the helm. Writers have been very free of their censure upon this emperor for parcelling, as they call it, the Empire; but this was the only chance there was of preventing its crumbling to pieces. Italy, and Rome in particular, lost by the change: they no longer monopolized the wealth and power of the world, but the other provinces gained. The Empire was much too large for one single man or a single central administration, under the dwindled influence of the Roman name, and amidst the numerous causes of local dissension and discontent, private ambition, social corruption, and foreign hostility, that had accumulated for three centuries, since the time of Augustus.

The new Cæsars justified Diocletian's expectations. Constantius defeated the Franks and the Alemanni, and soon after reconquered Britain. Galerius subjugated the Carpi, and transported the whole tribe into Pannonia. In the year 296, the Persians, under their king Narses, again invaded Mesopotamia and part of Syria. Galerius marched against them, but being too confident was defeated by superior numbers, and obliged to retire. On his meeting Diocletian, the emperor showed his dissatisfaction by letting Galerius walk for a mile, clad in purple as he was, by the side of his car. The following year Galerius again attacked the Persians, and completely defeated them, taking an immense booty. The wives and children of Narses, who were among the prisoners, were treated by Galerius with humanity and respect. Narses sued for peace, which was granted by Diocletian on condition of the Persians giving up all the territory on the right or western bank of the Tigris. This peace was concluded in 297, and lasted forty years.

At the same time Diocletian marched into Egypt against Achillæus, whom he besieged in Alexandria, which he took after a siege of eight months, when the usurper and his chief adherents were put to death. Diocletian is said to have behaved on this occasion with unusual sternness. Several towns of Egypt, among others Busiris and Coptos, were destroyed. Constantine, the son of Constantius, who was educated at Nicomedia, accompanied the emperor in this expedition. Diocletian fixed the limits of the Empire on that side at the island of Elephantina, where he built a castle, and made peace with the neighboring tribes, called by some Nubæ and by others Nabatæ, to whom he gave up the strip of territory which the Romans had conquered, of seven days' march above the first cataract, on condition that they should prevent the Blemmyes and Ethiopians from attacking Egypt. Maximianus in the meantime was engaged in putting down the revolt in Mauritania, which he effected with full success.

For several years after this the empire enjoyed peace, and Diocletian and his colleagues were chiefly employed in framing laws and administrative regulations, and in constructing forts on the frontiers. Diocletian kept a splendid court at Nicomedia, which town he embellished with numerous structures. He, or rather Maximianus by his order, caused the magnificent Thermæ at Rome to be built, the remains of which still bear Diocletian's name, and which contained, besides the baths, a library, a museum, public walks, and other establishments.

In February, 303, Diocletian issued an edict against the Christians, ordering their churches to be pulled down, their sacred books to be burnt, and all Christians to be dismissed from offices civil or military, with other penalties, exclusive however of death. Various causes have been assigned for this measure. It is known that Galerius had always been hostile to the Christians, while Diocletian had openly favored them, had employed them in his armies and about his person; and Eusebius speaks of the prosperity, security, and protection which the Christians enjoyed under his reign. They had churches in most towns, and one at Nicomedia in particular under the eye of the emperor. Just before the edict was issued, Galerius had repaired to Nicomedia to induce Diocletian to proscribe the Christians. He filled the emperor's mind with reports of conspiracies and seditions. The imperial palace took fire, Constantine ("Oratio ad Cœtum Sanctorum ") says, from lightning, and Galerius suggested to the emperor that it was a Christian plot.

The heathen priests on their part exerted themselves for the same purpose. It happened that on the occasion of a solemn sacrifice in presence of the emperor, while priests were consulting the entrails of the victims, the Christian officers in the imperial retinue crossed themselves; upon which the priests declared that the presence of profane men prevented them from discovering the auspices. Diocletian, who was very anxious to pry into futurity, became irritated, and ordered all his Christian officers to sacrifice to the gods under pain of flagellation and dismissal, which many of them underwent. Several oracles which he consulted gave answers unfavorable to the Christians. The church of Nicomedia was the first pulled down by order of the emperor. The rashness of a Christian who publicly tore down the imperial edict exasperated Diocletian still more: the culprit was put to a cruel death. Then came a second edict, ordering all magistrates to arrest the Christian bishops and presbyters, and compel them to sacrifice to the gods. This was giving to their enemies power over their lives, and it proved, in fact, the beginning of a cruel persecution, whose ravages were the more extensive in proportion to the great diffusion of Christianity during a long period of toleration. This was the last persecution under the Roman Empire, and it has been called by the name of Diocletian. But that emperor issued the two edicts reluctantly and after long hesitation, according to Lactantius's acknowledgment: he fell ill a few months after, and on recovering from his long illness he abdicated. Galerius, who had instigated the persecution, was the most zealous minister of it; the persecution raged with most fury in the provinces subject to his rule, and he continued it for several years after Diocletian's abdication, so that it might with more propriety be called the Galerian persecution. Legend says that he died of a horrible disease, filled with remorse and imagining himself haunted by the martyred spirits. The countries under the government of Constantius suffered the least from it.

In November of that year (303) Diocletian repaired to Rome, where he and Maximianus enjoyed the honor of a triumph, followed by festive games. This was the last triumph that Rome saw. The populace of that city complained of the economy of Diocletian on the occasion, who replied that moderation and temperance were most required when the censor was present. They vented their displeasure in jibes and sarcasms, which so hurt Diocletian that he left Rome abruptly in the month of December for Ravenna, in very cold weather. In this journey he was seized by an illness which affected him the whole of the following year, which he spent at Nicomedia. At one time he was reported to be dead. He rallied, however, in the spring of 305, and showed himself in public, but greatly altered in appearance. Galerius soon after came to Nicomedia, and it is said that he persuaded Diocletian to abdicate. Others say that Diocletian did it spontaneously.

On the 1st of May he repaired with his guards to a spot three miles out of Nicomedia, where he had thirteen years before proclaimed Galerius as Cæsar, and there addressing his officers and court, he said that the infirmities of age warned him to retire from power, and to deliver the administration of the state into stronger hands. He then proclaimed Galerius as Augustus, and Maximinus Daza as the new Cæsar. Constantine, who has given an account of the ceremony, which is quoted by Eusebius in his life of that prince, was present, and the troops fully expected that he would be the new Cæsar; when they heard another mentioned, they asked each other whether Constantine had changed his name. But Galerius did not leave them long in suspense; he pushed forward Maximinus and showed him to the assembly, and Diocletian clothed him with the purple vest, after which the old emperor returned privately in his carriage to Nicomedia, and immediately after set off for Salona in Dalmatia, near which he built himself an extensive palace by the sea-shore, in which he lived for the rest of his life, respected by the other emperors, without cares and without regret.

Part of the external walls which inclosed the area belonging to his palace and other buildings still remain, with three of the gates, as well as a temple, which is now a church at Spalatro, or Spalato, in Dalmatia, a comparatively modern town, grown out of the decay of the ancient Salona, and built in great part within the walls of Diocletian's residence, from the name of which, " Palatium," it is believed that "Spalato " is derived.

At the same time that Diocletian abdicated at Nicomedia, Maximianus, according to an agreement between them, performed a similar ceremony at Milan, proclaiming Constantius as Augustus, and Severus as Cæsar. Both Severus and Maximinus Daza were inferior persons, and creatures of Galerius, who insisted upon their nomination in preference to that of Maxentius and Constantine, whom Diocletian had at first proposed. Maximianus retired to his seat in Lucania, but not being endowed with the firmness of Diocletian he tried some time after to recover his former power, and wrote to his old colleague to induce him to do the same. "Were you but to come to Salona," answered Diocletian, "and see the vegetables which I grow in my garden with my own hands, you would no longer talk to me of empire." In his retirement he used to observe to his associates how difficult it is, even for the best-intentioned man, to govern well, as he cannot see everything with his own eyes, but must trust to others, who often deceive him.

Once only he left his retirement to meet Galerius in Pannonia for the purpose of appointing a new Cæsar, Licinius, in the place of Severus, who had died. Licinius, however, did not prove grateful, for after the death of Galerius, in 311, he ill-treated his widow, Valeria, Diocletian's daughter, who then, with her mother, Prisca, took refuge in the territories of Maximinus Daza. The latter offered to marry Valeria, but on her refusal exiled both her and her mother into the deserts of Syria, and put to death several of their attendants. Diocletian remonstrated in favor of his wife and daughter, but to no purpose, and his grief on this occasion probably hastened his death, which took place at his residence near Salona in July, 313. In the following year his wife and daughter were put to death by order of Licinius.

Diocletian ranks among the most distinguished emperors of Rome; his reign of twenty-one years was upon the whole prosperous for the empire, and creditable to the Roman name. He was severe, but not wantonly cruel, and we ought to remember that mercy was not a Roman virtue. His conduct after his abdication shows that his was no common mind. The chief charge against him is his haughtiness in introducing the Oriental ceremonial of prostration into the Roman court. The Christian writers, and especially Lactantius, have spoken unfavorably of him; but Lactantius cannot be implicitly trusted. Of the regular historians of his reign we have only the meagre narratives of Eutropius and Aurelius Victor, the others being now lost; but notices of Diocletian's life are scattered about in various authors, Libanius, Vopiscus, Eusebius, Julian in his "Cæsars," and the contemporary panegyrists, Eumenes and Mamertinus. His laws or edicts are in the "Code." Among other useful reforms, he abolished the frumentarii, or licensed informers, who were stationed in every province to report any attempt at mutiny or rebellion, and who basely enriched themselves by working on the fears of the inhabitants. He also reformed and reduced the number of the insolent Prætorians, who were afterward totally disbanded by Constantine.