Abt 244 - 311 (~ 66 years)
Has no ancestors but 2 descendants in this family tree.
||Gaius Aurelius Valerius Diocletianus |
||Abt 22 Dec 244
||3 Dec 311
||9 Nov 2009 |
- Diocletian was probably born near Salona in Dalmatia (Solin in modern Croatia), some time around 244. His parents named him Diocles, or possibly Diocles Valerius. The modern historian Timothy Barnes takes his official birthday, 22 December, as his actual birthdate. Other historians are not so certain. Diocles' parents were of low status, and writers critical of him claimed that his father was a scribe or a freedman of the senator Anullinus, or even that Diocles was a freedman himself. The first forty years of his life are mostly obscure. The Byzantine chronicler Joannes Zonaras states that he was Dux Moesiae, a commander of forces on the lower Danube. The often-unreliable Historia Augusta states that he served in Gaul, but this account is not corroborated by other sources, and is ignored by modern historians of the period. In 282, the legions of the upper Danube in Raetia and Noricum proclaimed the praetorian prefect M. Aurelius Carus as emperor, beginning a rebellion against emperor Probus. Probus' army, stationed in Sirmium (Sremska Mitrovica, Serbia), decided against fighting Carus, and assassinated Probus instead. Diocles soon gained Carus' trust. Carus soon appointed him to command the Protectores Domestici, the cavalry arm of the imperial bodyguard.
Carus, already sixty at his accession, wished to establish a dynasty that could outlive him. He immediately elevated his sons Carinus and Numerian to the rank of Caesar. In 283, Carus raised Carinus to the rank of Augustus, left him in charge of the care of the West, and moved with Numerian, Diocles, and the praetorian prefect Aper to the East, against the Sassanid Empire. The Sassanids had been embroiled in a succession dispute since the death of Shapur I in 272, and were in no position to oppose Carus' advance. According to Zonaras, Eutropius, and Festus, Carus won a major victory against the Persians, taking Seleucia and the Sassanid capital of Ctesiphon (near modern Al-Mada'in, Iraq), cities on opposite banks of the Tigris. In celebration, Carus and his sons took the title Persici maximi. Carus died in July or early August, reportedly struck by lightning.
Carus' death left his unpopular sons Numerian and Carinus as the new Augusti. Carinus quickly made his way to Rome from Gaul, and arrived by January 284; Numerian lingered in the East. The Roman retreat from Persia was orderly. Since the Persian King Bahram II was still struggling to establish his authority, he could not field any armies against them. The Romans left the country unopposed. By March 284 Numerian had only reached Emesa (Hims) in Syria; by November, only Asia Minor. In Emesa he was apparently still alive and in good health (he issued the only extant rescript in his name there),[notes 1] but after he left the city, his staff, including the prefect Aper, reported that he suffered from an inflammation of the eyes. He traveled in a closed coach from then on. When the army reached Bithynia, some of Numerian's soldiers smelled an odor reminiscent of a decaying corpse emanating from the coach. They opened its curtains. Inside, they found Numerian, dead.
Aper officially broke the news in Nicomedia (Izmit) in November. Numerianus' generals and tribunes called a council for the succession, and chose Diocles as emperor, in spite of Aper's attempts to garner support. On 20 November 284, the army of the east gathered on a hill 5 km (3.1 mi) outside Nicomedia. The army unanimously saluted their new Augustus, and Diocles accepted the purple imperial vestments. He raised his sword to the light of the sun, and swore an oath disclaiming responsibility for Numerian's death. He asserted that Aper had killed Numerian and concealed it. In full view of the army, Diocles drew his blade and killed Aper. According to the Historia Augusta, he quoted from Virgil while doing so. Soon after Aper's death, Diocles changed his name to the more Latinate "Diocletianus", in full Gaius Aurelius Valerius Diocletianus.
After his accession, Diocletian and Lucius Caesonius Bassus were named as consuls. They assumed the fasces in place of Carinus and Numerianus. Bassus was a member of a Campanian senatorial family, a former consul and a proconsul of Africa. He had been chosen by Probus for signal distinction. He was a man skilled in areas of government where Diocletian, presumably, had no experience. Diocletian's elevation of Bassus as consul symbolized his rejection of Carinus' government in Rome, his refusal to accept second-tier status to any other emperor, and his willingness to continue the long-standing collaboration between the empire's senatorial and military aristocracies. It also tied his success to that of the Senate, whose support he would need in an advance on Rome.
Diocletian was not the only challenger to Carinus' rule; the usurper M. Aurelius Julianus, Carinus' corrector Venetiae, took control of northern Italy and Pannonia after Diocletian's accession. He minted coins from the mint at Siscia (Sisak, Croatia) declaring himself as emperor and promising freedom. It was all good press for Diocletian, and aided in his portrayal of Carinus as a cruel and oppressive tyrant. Julianus' forces were weak, however, and were handily dispersed when Carinus' armies moved from Britain to northern Italy. As leader of the united East, Diocletian was clearly the greater threat. Over the winter of 284\endash 85, Diocletian advanced west across the Balkans. In the spring, some time before the end of May, his armies met Carinus' across the river Margus (Great Morava) in Moesia. In modern accounts, the site has been located between the Mons Aureus (Seone, west of Smederevo) and Viminacium, near modern Belgrade, Serbia.
Despite having the stronger army, Carinus held the weaker position. His rule was unpopular; it was subsequently alleged that Carinus had mistreated the Senate and seduced the wives of his officers. It is possible that Flavius Constantius, the governor of Dalmatia and Diocletian's associate in the household guard, had already defected to Diocletian in the early spring. When the Battle of the Margus began, Carinus' prefect Aristobulus also defected. In the course of the battle, Carinus was killed by his own men. Following Diocletian's victory, both the western and the eastern armies acclaimed him emperor. Diocletian exacted an oath of allegiance from the defeated army and departed for Italy.
Diocletian may have become involved in battles against the Quadi and Marcomanni immediately after the Battle of the Margus. He eventually made his way to northern Italy and made an imperial government, but it is not known whether Diocletian visited the city of Rome at this time. There is a contemporary issue of coins suggestive of an imperial adventus (arrival) for the city, but some modern historians state that Diocletian avoided the city, and that he did so on principle; the city and its Senate were no longer politically relevant to the affairs of the empire, and needed to be taught as much. Diocletian dated his reign from his elevation by the army, not the date of his ratification by the Senate, following the practice established by Carus, who had declared the Senate's ratification a useless formality. If Diocletian ever did enter Rome shortly after his accession, he did not stay long; he is attested back in the Balkans by November 2, 285, on campaign against the Sarmatians.
Diocletian replaced the prefect of Rome with his consular colleague Bassus. Most officials who had served under Carinus, however, retained their offices under Diocletian. In an act the epitomator Aurelius Victor denotes as unusual act of clementia, Diocletian did not kill or depose Carinus' traitorous praetorian prefect and consul Ti. Claudius Aurelius Aristobulus, but confirmed him in both roles, and later gave him the proconsulate of Africa and the rank of urban prefect. The other figures who retained their offices might have also betrayed Carinus.
Maximian's consistent loyalty to Diocletian proved an important component of the Tetrarchy's early successes.Recent history had demonstrated that sole rulership was dangerous to the stability of the empire. The assassinations of Aurelian (r. 270\endash 75) and Probus testified to that truth. Conflict boiled in every province of the empire, from Gaul to Syria, from Egypt to the lower Danube. It was too much for a single person to control, and Diocletian needed a lieutenant. At some time in 285 at Mediolanum (Milan, Italy), Diocletian raised his fellow-officer Maximian to the office of Caesar, making him co-emperor.
The concept of dual rulership was nothing new to the Roman Empire. Augustus, the first emperor (r. 27 BC\endash AD 14), had shared power with his colleagues, and more formal offices of co-emperor had existed from Marcus Aurelius (r. 161\endash 80) on. Most recently, the emperor Carus and his sons had ruled together, albeit unsuccessfully. Diocletian was in a less comfortable position than most of his predecessors, as he had a daughter, Valeria, but no sons. His co-ruler had to be from outside his family. He could not, therefore, be easily trusted. Some historians state that Diocletian, like some emperors before him, adopted Maximian as his filius Augusti, his "Augustan son", upon his appointment to the throne. This argument has not been universally accepted.
The relationship between Diocletian and Maximian was quickly couched in religious terms. Circa 287 Diocletian assumed the title Iovius, and Maximian assumed the title Herculius. The titles were probably meant to convey certain characteristics of their associated leaders; Diocletian, in Jovian style, would take on the dominating roles of planning and commanding; Maximian, in Herculian mode, would act as Jupiter's heroic subordinate. For all their religious connotations, the emperors were not "gods" in the tradition of the Imperial cult\emdash although they may have been hailed as such in Imperial panegyrics. Instead, they were seen as the gods' representatives, effecting their will on earth. The shift to divine sanctification from military acclamation took the power to appoint emperors away from the army. Religious legitimization elevated Diocletian and Maximian above potential rivals in a way military power and dynastic claims could not. After his acclamation, Maximian was dispatched to fight the rebel Bagaudae in Gaul. Diocletian returned to the East.
Diocletian progressed slowly. By November 2, he had only reached Citivas Iovia (Botivo, near Ptuj, Slovenia). In the Balkans during the autumn of 285, he encountered a tribe of Sarmatians who demanded assistance from the emperor. The Sarmatians requested that Diocletian either help them recover their lost lands or grant them pasturage rights within the empire. Diocletian refused and fought a battle with them, but was unable to secure a complete victory. The nomadic pressures of the European Plain remained, and could not be solved by a single war; soon the Sarmatians would have to be fought again. He wintered in Nicomedia.There may have been a revolt in the eastern provinces at this time, because Diocletian brought settlers from Asia to populate emptied farmlands in Thrace. He visited Judea the following spring.He probably returned to Nicomedia for the winter. Diocletian's stay in the East saw diplomatic success in the conflict with Persia: in 287, Bahram II granted him precious gifts, declared open friendship with the empire, and invited Diocletian to visit him. Roman sources insist that the act was entirely voluntary.
Around the same time, perhaps in 287, Persia relinquished claims on Armenia and recognized Roman authority over territory to the west and south of the Tigris. The western portion of Armenia was incorporated into the Roman empire and made a province. Tiridates III, Arsacid claimant to the Armenian throne and Roman client, had been disinherited and forced to take refuge in the Roman empire after the Persian conquest of 252/3. In 287, he returned to lay claim to the eastern half of his ancestral domain. He encountered no opposition. Bahram II's gifts were widely recognized as symbolic of a victory in the ongoing conflict with Persia; Diocletian was hailed as the "founder of eternal peace". The events might have represented a formal end to Carus' eastern campaign, which probably ended without an acknowledged peace. At the conclusion of discussions with the Persians, Diocletian re-organized the Mesopotamian frontier and fortified the city of Circesium (Buseire, Syria) on the Euphrates.
Maximian's campaigns were not proceeding as smoothly. The Bagaudae had been easily suppressed, but Carausius, the man he had put in charge of operations against Saxon and Frankish pirates on the Saxon Shore, had begun keeping the goods seized from the pirates for himself. Maximian issued a death-warrant for his larcenous subordinate. Carausius fled the Continent, proclaimed himself Augustus, and spurred Britain and northwestern Gaul into open revolt against Maximian and Diocletian. Spurred by the crisis, on April 1, 286, Maximian took up the title of Augustus. Maximian's appointment is unusual in that it was impossible for Diocletian to have been present to witness the event. It has even been suggested that Maximian usurped the title, and was only later recognized by Diocletian in hopes of avoiding civil war. Although this suggestion is unpopular, it is clear that Diocletian meant for Maximian to act with a certain amount of independence from Diocletian.
Maximian realized that he could not immediately suppress the rogue commander, and so, for the whole campaigning season of 287, campaigned against tribes beyond the Rhine instead. The following spring, as Maximian prepared a fleet for an expedition against Carausius, Diocletian returned from the East to meet Maximian. The two emperors agreed on a joint campaign against the Alamanni. Diocletian invaded Germania through Raetia while Maximian progressed from Mainz. Each emperor burned crops and food supplies as he went, destroying the Germans' means of sustenance. The two men added territory to the empire and allowed Maximian to continue preparations against Carausius without further disturbance. On his return to the East, Diocletian managed what was probably another rapid campaign against the resurgent Sarmatians. No details survive, but surviving inscriptions indicate that Diocletian took the title Sarmaticus Maximus after 289.
In the East, Diocletian engaged in diplomacy with desert tribes in the regions between Rome and Persia. He might have been attempting to persuade them to ally themselves with Rome, thus reviving the old, Rome-friendly, Palmyrene sphere of influence, or simply attempting to reduce the frequency of their incursions. No details survive for these events. Some of the princes of these states were Persian client kings; a disturbing fact in light of increasing tensions with that kingdom. In the West, Maximian lost the fleet built in 288 and 289, probably in the early spring of 290. The panegyrist who refers to the loss suggests that its cause was a storm, but this might simply be the panegyrist's attempt to conceal an embarrassing military defeat. Diocletian broke off his tour of the Eastern provinces soon thereafter. He returned with haste to the West, reaching Emesa by May 10, 290, and Sirmium on the Danube by July 1, 290.
Diocletian met Maximian in Milan in the winter of 290\endash 91, either in late December 290 or January 291. The meeting was undertaken with a sense of solemn pageantry. The emperors spent most of their time in public appearances. It has been surmised that the ceremonies were arranged to demonstrate Diocletian's continuing support for his faltering colleague. A deputation from the Roman Senate met with the emperors, renewing that body's infrequent contact with the imperial office. The choice of Milan over Rome further snubbed the capital's pride. But then it was already a long established practice that Rome itself was only a ceremonial capital, as the actual seat of the imperial administration was determined by the needs of defense. Long before Diocletian, Gallienus (r. 253\endash 68) had already chosen Milan as the seat of his headquarters. If the panegyric detailing the ceremony implied that the true center of the empire was not Rome, but where the emperor sat ("...the capital of the Empire appeared to be there, where the two emperors met"), it simply echoed what had already been stated by the historian Herodian in the early third century: "Rome is where the emperor is". During the meeting, decisions on matters of politics and war were probably made, but they were made in secret. The Augusti would not meet again until 303.
Some time after his return, and before 293, Diocletian transferred command of the war against Carausius from Maximian to Flavius Constantius. Constantius was a former governor of Dalmatia and a man of military experience stretching back to Aurelian's campaigns against Zenobia (272\endash 73). He was Maximian's praetorian prefect in Gaul, and the husband to Maximian's daughter, Theodora. On March 1, 293 at Milan, Maximian gave Constantius the office of Caesar. In the spring of 293, in either Philippopolis (Plovdiv, Bulgaria) or Sirmium, Diocletian would do the same for Galerius, husband to Diocletian's daughter Valeria, and perhaps Diocletian's praetorian prefect. Constantius was assigned Gaul and Britain. Galerius was assigned Syria, Palestine, Egypt, and responsibility for the eastern borderlands.
This arrangement is called the Tetrarchy, from a Greek term meaning "rulership by four". The Tetrarchic emperors were more or less sovereign in their own lands, and they travelled with their own imperial courts, administrators, secretaries, and armies. They were joined by blood and marriage; Diocletian and Maximian now styled themselves as brothers. The senior co-emperors formally adopted Galerius and Constantius as sons in 293. These relationships implied a line of succession. Galerius and Constantius would become Augusti after Diocletian and Maximian's departure. Maximian's son Maxentius, and Constantius' son Constantine would then become Caesars. In preparation for their future roles, Constantine and Maxentius were taken to Diocletian's court in Nicomedia.
Diocletian spent the spring of 293 traveling with Galerius from Sirmium to Byzantium (Istanbul, Turkey). Diocletian then returned to Sirmium, where he would remain for the following winter and spring. He campaigned against the Sarmatians again in 294, probably in the autumn, and won a victory against them. The Sarmatians' defeat kept them from the Danube provinces for a long time. Meanwhile, Diocletian built forts north of the Danube, at Aquincum (Budapest, Hungary), Bononia (Vidin, Bulgaria), Ulcisia Vetera, Castra Florentium, Intercisa (Dunaújváros, Hungary), and Onagrinum (Begec, Serbia). The new forts became part of a new defensive line called the Ripa Sarmatica. In 295 and 296 Diocletian campaigned in the region again, and won a victory over the Carpi in the summer of 296. Afterwards, during 299 and 302, as Diocletian was then residing in the East, it was Galerius' turn to campaign victoriously on the Danube. By the end of his reign, Diocletian had secured the entire length of the Danube, provided it with forts, bridgeheads, highways, and walled towns, and sent fifteen or more legions to patrol the region; an inscription at Sexaginta Prista on the Lower Danube extolled restored tranquilitas at the region. The defense came at a heavy cost, but was a significant achievement in an area difficult to defend.
Galerius, meanwhile, was engaged during 291-293 in disputes in Upper Egypt, where he suppressed a regional uprising . He would return to Syria in 295 to fight the revanchist Persian Empire. Diocletian's attempts to bring the Egyptian tax system in line with imperial standards stirred discontent, and a revolt swept the region after Galerius' departure. The usurper L. Domitius Domitianus declared himself Augustus in July or August 297. Much of Egypt, including Alexandria, recognized his rule. Diocletian moved into Egypt to suppress him, first putting down rebels in the Thebaid in the autumn of 297, then moving on to besiege Alexandria. Domitianus died in December 297, by which time Diocletian had secured control of the Egyptian countryside. Alexandria, whose defense was organized under Diocletian's former corrector Aurelius Achilleus, held out until a later date, probably March 298.
Bureaucratic affairs were completed during Diocletian's stay: a census took place, and Alexandria, in punishment for its rebellion, lost the ability to mint independently. Diocletian's reforms in the region, combined with those of Septimus Severus, brought Egyptian administrative practices much closer to Roman standards. Diocletian travelled south along the Nile the following summer, where he visited Oxyrhynchus and Elephantine. In Nubia, he made peace with the Nobatae and Blemmyes tribes. Under the terms of the peace treaty Rome's borders moved north to Philae and the two tribes received an annual gold stipend. Diocletian left Africa quickly after the treaty, moving from Upper Egypt in September 298 to Syria in February 299. He met up with Galerius in Mesopotamia.
In 294, Narseh, a son of Shapur who had been passed over for the Sassanid succession, came to power in Persia. Narseh eliminated Bahram III, a young man installed in the wake of Bahram II's death in 293. In early 294, Narseh sent Diocletian the customary package of gifts between the empires, and Diocletian responded with an exchange of ambassadors. Within Persia, however, Narseh was destroying every trace of his immediate predecessors from public monuments. He sought to identify himself with the warlike kings Ardashir (r. 226\endash 41) and Shapur (r. 241\endash 72), the same Shapur who had sacked Roman Antioch and skinned the Emperor Valerian (r. 253\endash 260) to decorate his war temple.
Narseh declared war on Rome in 295 or 296. He appears to have first invaded western Armenia, where he seized the lands delivered to Tiridates in the peace of 287. Narseh moved south into Roman Mesopotamia in 297, where he inflicted a severe defeat on Galerius in the region between Carrhae (Harran, Turkey) and Callinicum (Ar-Raqqah, Syria) (and thus, the historian Fergus Millar notes, probably somewhere on the Balikh river). Diocletian may or may not have been present at the battle, but he quickly divested himself of all responsibility. In a public ceremony at Antioch, the official version of events was clear: Galerius was responsible for the defeat; Diocletian was not. Diocletian publicly humiliated Galerius, forcing him to walk for a mile at the head of the imperial caravan, still clad in the purple robes of the emperor.
Galerius was reinforced, probably in the spring of 298, by a new contingent collected from the empire's Danubian holdings. Narseh did not advance from Armenia and Mesopotamia, leaving Galerius to lead the offensive in 298 with an attack on northern Mesopotamia via Armenia. It is unclear if Diocletian was present to assist the campaign; he might have returned to Egypt or Syria. Narseh retreated to Armenia to fight Galerius' force, to Narseh's disadvantage; the rugged Armenian terrain was favorable to Roman infantry, but unfavorable to Sassanid cavalry. In two battles, Galerius won major victories over Narseh. During the second encounter, Roman forces seized Narseh's camp, his treasury, his harem, and his wife. Galerius continued moving down the Tigris, and took the Persian capital at Ctesiphon before returning to Roman territory along the Euphrates.
Narseh sent an ambassador to Galerius to plead for the return of his wives and children in the course of the war, but Galerius had dismissed him. Serious peace negotiations began in the spring of 299. Diocletian and Galerius' magister memoriae (secretary) Sicorius Probus were sent to Narseh to present terms. The conditions of the peace were heavy; Armenia returned to Roman domination, with the fort of Ziatha as its border; Caucasian Iberia would pay allegiance to Rome under a Roman appointee; Nisibis, now under Roman rule, would become the sole conduit for trade between Persia and Rome; and Rome would exercise control over the five satrapies between the Tigris and Armenia: Ingilene, Sophanene (Sophene), Arzanene (Aghdznik), Corduene (Carduene), and Zabdicene (near modern Hakkâri, Turkey). These regions included the passage of the Tigris through the Anti-Taurus range; the Bitlis pass, the quickest southerly route into Persian Armenia; and access to the Tur Abdin plateau.
A stretch of land containing the later strategic strongholds of Amida (Diyarbakir, Turkey) and Bezabde came under firm Roman military occupation. With these territories, Rome would have an advance station north of Ctesiphon, and would be able to slow any future advance of Persian forces through the region. The Tigris was said to have become the boundary between the two empires, but what this means is unclear, as the satrapies listed all lie on the far side of the river. Millar suggests that the satrapies might have been held under a loose Roman hegemony, without military occupation. At the conclusion of the peace, Tiridates regained both his throne and the entirety of his ancestral claim. Rome secured a wide zone of cultural influence, which led to a wide diffusion of Syriac Christianity from a center at Nisibis in later decades, and the eventual Christianization of Armenia.
At the conclusion of the peace, Diocletian and Galerius returned to Syrian Antioch. At some time in 299, the emperors took part in a ceremony of sacrifice and divination in an attempt to predict the future. The haruspices were unable to read the entrails of the sacrificed animals, and blamed Christians in the imperial household. The emperors ordered all members of the court to perform a sacrifice to purify the palace. The emperors sent letters to the military command, demanding the entire army perform the required sacrifices or face discharge. Diocletian was conservative in matters of religion, a man faithful to the traditional Roman pantheon and understanding of demands for religious purification, but Eusebius, Lactantius and Constantine state that it was Galerius, not Diocletian, who was the prime supporter of the purge, and its greatest beneficiary. Galerius, even more devoted and passionate than Diocletian, saw political advantage in the politics of persecution. He was willing to break with a government policy of inaction on the issue.
Antioch was Diocletian's primary residence from 299 to 302, while Galerius swapped places with his Augustus on the Middle and Lower Danube. He visited Egypt once, over the winter of 301\endash 2, and issued a grain dole in Alexandria. Following some public disputes with Manicheans, Diocletian ordered that the leading followers of Mani be burnt alive along with their scriptures. In a March 31, 302 rescript from Alexandria, he declared that low-status Manicheans must be executed by the blade, and high-status Manicheans must be sent to work in the quarries of Proconnesus (Marmara Island, Turkey) or the mines of Phaeno in southern Palestine. All Manichean property was to be seized and deposited in the imperial treasury. Diocletian found much to be offended by in Manichean religion: its novelty, its alien origins, the way it corrupted the morals of the Roman race, and its inherent opposition to long-standing religious traditions. Manichaeanism was also supported by Persia at the time, compounding religious dissent with international politics. Excepting Persian support, the reasons why he disliked Manichaenism were equally applicable, if not more so, to Christianity, his next target.
Diocletian returned to Antioch in the autumn of 302. He ordered that the deacon Romanus of Caesarea have his tongue removed for defying the order of the courts and interrupting official sacrifices. Romanus was then sent to prison, where he was executed on November 17, 303. Diocletian believed that Romanus of Caesarea was arrogant, and he left the city for Nicomedia in the winter, accompanied by Galerius. According to Lactantius, Diocletian and Galerius entered into an argument over imperial policy towards Christians while wintering at Nicomedia in 302. Diocletian argued that forbidding Christians from the bureaucracy and military would be sufficient to appease the gods, but Galerius pushed for extermination. The two men sought the advice of the oracle of Apollo at Didyma. The oracle responded that "the just on earth" hindered Apollo's ability to provide advice. These "just", Diocletian was informed by members of the court, could only refer to the Christians of the empire. At the behest of his court, Diocletian acceded to demands for universal persecution.
On February 23, 303, Diocletian ordered that the newly built church at Nicomedia be razed. He demanded that its scriptures be burned, and seized its precious stores for the treasury. The next day, Diocletian's first "Edict against the Christians" was published. The edict ordered the destruction of Christian scriptures and places of worship across the Empire, and prohibited Christians from assembling for worship. Before the end of February, a fire destroyed part of the imperial palace. Galerius convinced Diocletian that the culprits were Christians, conspirators who had plotted with the eunuchs of the palace. An investigation was commissioned, but no responsible party was found. Executions followed anyway, and the palace eunuchs Dorotheus and Gorgonius were executed. One individual, Peter, was stripped, raised high, and scourged. Salt and vinegar were poured in his wounds, and he was slowly boiled over an open flame. The executions continued until at least April 24, 303, when six individuals, including the bishop Anthimus, were decapitated. A second fire occurred sixteen days after the first. Galerius left the city for Rome, declaring Nicomedia unsafe. Diocletian would soon follow.
Although further persecutionary edicts followed, compelling the arrest of the Christian clergy and universal acts of sacrifice, the persecutionary edicts were ultimately unsuccessful; most Christians escaped punishment, and even pagans were generally unsympathetic to the persecution. The martyrs' sufferings strengthened the resolve of their fellow Christians. Constantius and Maximian did not apply the later persecutionary edicts, and left the Christians of the West unharmed. Galerius rescinded the edict in 311, announcing that the persecution had failed to bring Christians back to traditional religion. The temporary apostasy of some Christians, and the surrendering of scriptures, during the persecution played a major role in the subsequent Donatist controversy. Within twenty-five years of the persecution's inauguration, the Christian emperor Constantine would rule the empire alone. He would reverse the consequences of the edicts, and return all confiscated property to Christians. Under Constantine's rule, Christianity would become the empire's preferred religion.Diocletian was demonized by his Christian successors: Lactantius intimated that Diocletian's ascendancy heralded the apocalypse, and in Serbian mythology, Diocletian is remembered as Dukljan, the adversary of God.
Diocletian entered the city of Rome in the early winter of 303. On November 20, he celebrated, with Maximian, the twentieth anniversary of his reign (vicennalia), the tenth anniversary of the Tetrarchy (decennalia), and a triumph for the war with Persia. Diocletian soon grew impatient with the city, as the Romans acted towards him with what Edward Gibbon, following Lactantius, calls "licentious familiarity". The Roman people did not give enough deference to his supreme authority; it expected him to act the part of an aristocratic ruler, not a monarchic one. On December 20, 303, Diocletian cut short his stay in Rome and left for the north. He did not even perform the ceremonies investing him with his ninth consulate; he did them in Ravenna on January 1, 304 instead. There are suggestions in the Panegyrici Latini and Lactantius' account that Diocletian arranged plans for his and Maximian's future retirement of power in Rome. Maximian, according to these accounts, swore to uphold Diocletian's plan in a ceremony in the temple of Jupiter.
From Ravenna, Diocletian left for the Danube. There, possibly in Galerius' company, he took part in a campaign against the Carpi. He contracted a minor illness while on campaign, but his condition quickly worsened and he chose to travel in a litter. In the late summer he left for Nicomedia. On November 20, he appeared in public to dedicate the opening of the circus beside his palace. He collapsed soon after the ceremonies. Over the winter of 304\endash 5 he kept within his palace at all times. Rumors alleging that Diocletian's death was merely being kept secret until Galerius could come to assume power spread through the city. On December 13, he seemed to have finally died. The city was sent into a mourning from which it was only retrieved by public declarations of his survival. When Diocletian reappeared in public on March 1, 305, he was emaciated and barely recognizable.
Galerius arrived in the city later in March. According to Lactantius, he came armed with plans to reconstitute the Tetrarchy, force Diocletian to step down, and fill the imperial office with men compliant to his will. Through coercion and threats, he eventually convinced Diocletian to comply with his plan. Lactantius also claims that he had done the same to Maximian at Sirmium. On May 1, 305, Diocletian called an assembly of his generals, traditional companion troops, and representatives from distant legions. They met at the same hill, 5 km (3.1 mi) out of Nicomedia, where Diocletian had been proclaimed emperor. In front of a statue of Jupiter, his patron deity, Diocletian addressed the crowd. With tears in his eyes, he told them of his weakness, his need for rest, and his will to resign. He declared that he needed to pass the duty of empire on to someone stronger. He thus became the first Roman emperor to voluntarily abdicate his title.
Most in the crowd believed they knew what would follow; Constantine and Maxentius, the only adult sons of a reigning emperor, men who had long been preparing to succeed their fathers, would be granted the title of Caesar. Constantine had traveled through Palestine at the right hand of Diocletian, and was present at the palace in Nicomedia in 303 and 305. It is likely that Maxentius received the same treatment. In Lactantius' account, when Diocletian announced that he was to resign, the entire crowd turned to face Constantine. It was not to be: Severus and Maximin were declared Caesars. Maximin appeared and took Diocletian's robes. On the same day, Severus received his robes from Maximian in Milan. Constantius succeeded Maximian as Augustus of the West, but Constantine and Maxentius were entirely ignored in the transition of power. This did not bode well for the future security of the Tetrarchic system.
Diocletian retired to his homeland, Dalmatia. He moved into the expansive palace he had built on the Adriatic near the administrative center of Salona. Maximian retired to villas in Campania or Lucania. Their homes were distant from political life, but Diocletian and Maximian were close enough to remain in regular contact with each other. Galerius assumed the consular fasces in 308 with Diocletian as his colleague. In the autumn of 308, Galerius again conferred with Diocletian at Carnuntum (Petronell-Carnuntum, Austria). Diocletian and Maximian were both present on November 11, 308, to see Galerius appoint Licinius to be Augustus in place of Severus, who had died at the hands of Maxentius. He ordered Maximian, who had attempted to return to power after his retirement, to step down permanently. At Carnuntum people begged Diocletian to return to the throne, to resolve the conflicts that had arisen through Constantine's rise to power and Maxentius' usurpation. Diocletian's reply: "If you could show the cabbage that I planted with my own hands to your emperor, he definitely wouldn't dare suggest that I replace the peace and happiness of this place with the storms of a never-satisfied greed."
He lived on for three more years, spending his days in his palace gardens. He saw his Tetrarchic system implode, torn by the selfish ambitions of his successors. He heard of Maximian's third claim to the throne, his forced suicide, his damnatio memoriae. In his own palace, statues and portraits of his former companion emperor were torn down and destroyed. Deep in despair and illness, Diocletian may have committed suicide.