Diocletian
51st Emperor of the Roman Empire
Laureate bust of Diocletian
Reign	20 November 284 – 1 April 286 (alone) 1 April 286 – 1 May 305 (as Augustus of the east, with Maximian as Augustus of the west)[1]
Full name	Diocles (full name unknown) (from birth to accession); Caesar Gaius Aurelius Valerius Diocletianus Augustus (as emperor)[2]
Born	c. 22 December 244[3]
Birthplace	Salona (now Solin, Croatia)
Died	3 December 311 (aged 66)[4]
Place of death	Aspalathos (now Split, Croatia)
Buried	Diocletian's Palace in Aspalathos. His tomb was later turned into a Christian church, the Cathedral of St. Domnius, which is still standing within the palace at Split.
Predecessor	Numerian
Successor	Constantius Chlorus and Galerius
Consort to	Prisca
Offspring	Valeria

Diocletian (Latin: Gaius Aurelius Valerius Diocletianus Augustus;^{[5][notes 1]} c. 22 December 244^[3]  – 3 December 311^[4]), was a Roman Emperor from 284 to 305. Born to a family of low status in the Roman province of Dalmatia, Diocletian rose through the ranks of the military to become cavalry commander to the Emperor Carus. After the deaths of Carus and his son Numerian on campaign in Persia, Diocletian was proclaimed Emperor. The title was also claimed by Carus' other surviving son, Carinus, but Diocletian defeated him in the Battle of the Margus. With his accession to power, Diocletian ended the Crisis of the Third Century. He appointed fellow officer Maximian Augustus his senior co-emperor in 285.

Diocletian delegated further on 1 March 293, appointing Galerius and Constantius as Caesars, junior co-emperors. Under this "Tetrarchy", or "rule of four", each emperor would rule over a quarter-division of the Empire. Diocletian secured the Empire's borders and purged it of all threats to his power. He defeated the Sarmatians and Carpi during several campaigns between 285 and 299, the Alamanni in 288, and usurpers in Egypt between 297 and 298. Galerius, aided by Diocletian, campaigned successfully against Sassanid Persia, the Empire's traditional enemy. In 299 he sacked their capital, Ctesiphon. Diocletian led the subsequent negotiations and achieved a lasting and favorable peace. Diocletian separated and enlarged the Empire's civil and military services and reorganized the Empire's provincial divisions, establishing the largest and most bureaucratic government in the history of the Empire. He established new administrative centers in Nicomedia, Mediolanum, Antioch, and Trier, closer to the Empire's frontiers than the traditional capital at Rome had been. Building on third-century trends towards absolutism, he styled himself an autocrat, elevating himself above the Empire's masses with imposing forms of court ceremonies and architecture. Bureaucratic and military growth, constant campaigning, and construction projects increased the state's expenditures and necessitated a comprehensive tax reform. From at least 297 on, imperial taxation was standardized, made more equitable, and levied at generally higher rates.

Not all of Diocletian's plans were successful: the Edict on Maximum Prices (301), his attempt to curb inflation via price controls, was counterproductive and quickly ignored. Although effective while he ruled, Diocletian's Tetrarchic system collapsed after his abdication under the competing dynastic claims of Maxentius and Constantine, sons of Maximian and Constantius respectively. The Diocletianic Persecution (303–11), the Empire's last, largest, and bloodiest official persecution of Christianity, did not destroy the Empire's Christian community; indeed, after 324 Christianity became the empire's preferred religion under its first Christian emperor, Constantine.

In spite of his failures, Diocletian's reforms fundamentally changed the structure of Roman imperial government and helped stabilize the Empire economically and militarily, enabling the Empire to remain essentially intact for another hundred years despite having seemed near the brink of collapse in Diocletian's youth. Weakened by illness, Diocletian left the imperial office on 1 May 305, and became the only Roman emperor to voluntarily abdicate the position. He lived out his retirement in his palace on the Dalmatian coast, tending to his vegetable gardens. His palace eventually became the core of the modern-day city of Split.

Early life[link]

Panorama of amphitheatre in Salona

Diocletian was probably born near Salona in Dalmatia (Solin in modern Croatia), some time around 244.^[3] His parents named him Diocles, or possibly Diocles Valerius.^[6] The modern historian Timothy Barnes takes his official birthday, 22 December, as his actual birthdate. Other historians are not so certain.^[7] 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.^[8] The Byzantine chronicler Joannes Zonaras states that he was Dux Moesiae,^[9] a commander of forces on the lower Danube.^[10] 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.^[11]

Death of Numerian[link]

Aureus of Emperor Diocletian

Emperor 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.^[12] The Romans left the country unopposed.^[13] The Roman withdrawal from Persia was orderly. The Sassanid king Bahram II could not field any armies against them, as he was still struggling to establish his authority. By March 284, Numerian had only reached Emesa (Homs) in Syria; by November, only Asia Minor.^[14] In Emesa he was apparently still alive and in good health: he issued the only extant rescript in his name there,^{[15][notes 2]} 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.^[17] When the army reached Bithynia,^[12] some of the soldiers smelled an odor emanating from the coach.^[13] They opened its curtains and inside they found Numerian dead.^[18]

Aper officially broke the news in Nicomedia (İzmit) in November.^[19] Numerianus' generals and tribunes called a council for the succession, and chose Diocles as Emperor,^[20] in spite of Aper's attempts to garner support.^[19] On 20 November 284, the army of the east gathered on a hill 5 kilometres (3.1 mi) outside Nicomedia. The army unanimously saluted Diocles as their new Augustus, and he 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.^[21] In full view of the army, Diocles drew his sword and killed Aper.^[22] According to the Historia Augusta, he quoted from Virgil while doing so.^[23] Soon after Aper's death, Diocles changed his name to the more Latinate "Diocletianus",^[24] in full Gaius Aurelius Valerius Diocletianus.^[25]

Conflict with Carinus[link]

Head of Carinus at the Centrale Montemartini

After his accession, Diocletian and Lucius Caesonius Bassus^[26] were named as consuls.^[27] 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.^[28] He was a man skilled in areas of government where Diocletian, presumably, had no experience.^[19] 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,^[28] and his willingness to continue the long-standing collaboration between the Empire's senatorial and military aristocracies.^[19] It also tied his success to that of the Senate, whose support he would need in an advance on Rome.^[28]

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.^[29] Julianus minted coins from the mint at Siscia (Sisak, Croatia) declaring himself as Emperor and promising freedom. It was all good publicity for Diocletian, and it aided in his portrayal of Carinus as a cruel and oppressive tyrant.^[30] 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.^[31] Over the winter of 284–85, Diocletian advanced west across the Balkans. In the spring, some time before the end of May,^[32] 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,^[28] near modern Belgrade, Serbia.^[33]

Despite having the stronger army, Carinus held the weaker position. His rule was unpopular, and it was later alleged that he had mistreated the Senate and seduced his officers' wives.^[34] 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.^[35] When the Battle of the Margus began, Carinus' prefect Aristobulus also defected.^[19] 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 Augustus.^[36] Diocletian exacted an oath of allegiance from the defeated army and departed for Italy.^[37]

Early rule[link]

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 he visited the city of Rome at this time.^[38] There is a contemporary issue of coins suggestive of an imperial adventus (arrival) for the city,^[39] but some modern historians state that Diocletian avoided the city, and that he did so on principle, as 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,^[40] following the practice established by Carus, who had declared the Senate's ratification a useless formality.^[41] If Diocletian ever did enter Rome shortly after his accession, he did not stay long;^[42] he is attested back in the Balkans by 2 November 285, on campaign against the Sarmatians.^[43]

Diocletian replaced the prefect of Rome with his consular colleague Bassus. Most officials who had served under Carinus, however, retained their offices under Diocletian.^[44] In an act the epitomator Aurelius Victor denotes as unusual act of clementia,^[45] Diocletian did not kill or depose Carinus' traitorous praetorian prefect and consul Ti. Claudius Aurelius Aristobulus, but confirmed him in both roles,^[46] and later gave him the proconsulate of Africa and the rank of urban prefect.^[47] The other figures who retained their offices might have also betrayed Carinus.^[48]

Maximian made co-emperor[link]

Maximian's consistent loyalty to Diocletian proved an important component of the Tetrarchy's early successes.^[49]

Recent history had demonstrated that sole rulership was dangerous to the stability of the empire. The assassinations of Aurelian (r. 270–75) and Probus testified to that truth.^[19] 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.^[50] At some time in 285 at Mediolanum (Milan, Italy),^{[notes 3]} Diocletian raised his fellow-officer Maximian to the office of Caesar, making him co-emperor.^[53]

The concept of dual rulership was nothing new to the Roman Empire. Augustus, the first Emperor (r. 27 BC–AD 14), had shared power with his colleagues, and more formal offices of co-Emperor had existed from Marcus Aurelius (r. 161–80) on.^[54] 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.^[55] 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.^[56] This argument has not been universally accepted.^[57]

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.^[58] 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.^[59] For all their religious connotations, the emperors were not "gods" in the tradition of the Imperial cult—although they may have been hailed as such in Imperial panegyrics. Instead, they were seen as the gods' representatives, effecting their will on earth.^[60] 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.^[61] After his acclamation, Maximian was dispatched to fight the rebel Bagaudae in Gaul. Diocletian returned to the East.^[62]

Conflict with Sarmatia and Persia[link]

Diocletian progressed slowly. By 2 November, he had only reached Citivas Iovia (Botivo, near Ptuj, Slovenia).^[63] 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.^[64] He wintered in Nicomedia.^{[notes 4]} 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.^[66] He visited Judea the following spring.^{[notes 5]} 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.^[69] Roman sources insist that the act was entirely voluntary.^[70]

Around the same time, perhaps in 287,^[71] 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 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 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.^[72] 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.^[73] 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.^[74]

Maximian made Augustus[link]

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.^[75] Spurred by the crisis, on 1 April 286,^{[76][notes 6]} Maximian took up the title of Augustus.^[80] 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.^[81] This suggestion is unpopular, as it is clear that Diocletian meant for Maximian to act with a certain amount of independence.^[82]

Carausius, rebel emperor of Roman Britain. Most of the evidence for Carausius' reign comes from his coinage, which was of generally fine quality.^[83]

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.^[84] 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.^[85] The two men added territory to the Empire and allowed Maximian to continue preparations against Carausius without further disturbance.^[86] 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.^[87]

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,^[88] or simply attempting to reduce the frequency of their incursions.^[89] No details survive for these events.^[90] Some of the princes of these states were Persian client kings; a disturbing fact in light of increasing tensions with that kingdom.^[91] 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,^[92] but this might simply be the panegyrist's attempt to conceal an embarrassing military defeat.^[93] Diocletian broke off his tour of the Eastern provinces soon thereafter. He returned with haste to the West, reaching Emesa by 10 May 290,^[94] and Sirmium on the Danube by 1 July 290.^[95]

Diocletian met Maximian in Milan in the winter of 290–91, either in late December 290 or January 291.^[96] 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.^[88] A deputation from the Roman Senate met with the Emperors, renewing that body's infrequent contact with the Imperial office.^[97] 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. 253–68) had already chosen Milan as the seat of his headquarters.^[98] 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"),^[99] it simply echoed what had already been stated by the historian Herodian in the early third century: "Rome is where the emperor is".^[98] During the meeting, decisions on matters of politics and war were probably made, but they were made in secret.^[100] The Augusti would not meet again until 303.^[88]

Tetrarchy[link]

Foundation of the Tetrarchy[link]

Triumphal Arch of the Tetrarchy, Sbeitla, Tunisia

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 (272–73). He was Maximian's praetorian prefect in Gaul, and the husband to Maximian's daughter, Theodora. On 1 March 293 at Milan, Maximian gave Constantius the office of Caesar.^[101] 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.^{[notes 7]} Constantius was assigned Gaul and Britain. Galerius was assigned Syria, Palestine, Egypt, and responsibility for the eastern borderlands.^[103]

This arrangement is called the Tetrarchy, from a Greek term meaning "rulership by four".^[104] The Tetrarchic Emperors were more or less sovereign in their own lands, and they travelled with their own imperial courts, administrators, secretaries, and armies.^[105] 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.^[106]

Conflict in the Balkans and Egypt[link]

A Trajanic temple on the island of Philae, the newly established border between the Nobatae and Blemmyes and Roman Egypt^[107]

Diocletian spent the spring of 293 traveling with Galerius from Sirmium (Sremska Mitrovica, Serbia) 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,^[108] 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,^[109] at Aquincum (Budapest, Hungary), Bononia (Vidin, Bulgaria), Ulcisia Vetera, Castra Florentium, Intercisa (Dunaújváros, Hungary), and Onagrinum (Begeč, Serbia). The new forts became part of a new defensive line called the Ripa Sarmatica.^[110] In 295 and 296 Diocletian campaigned in the region again, and won a victory over the Carpi in the summer of 296.^[111] Afterwards, during 299 and 302, as Diocletian was then residing in the East, it was Galerius' turn to campaign victoriously on the Danube.^[112] 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.^[113] The defense came at a heavy cost, but was a significant achievement in an area difficult to defend.^[114]