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.

[11] The first time Diocletian's whereabouts are accurately established was in 282 when the Emperor Carus made him commander of the Protectores domestici, the elite cavalry force directly attached to the Imperial household.

[12] Carus's death, amid a successful war with Persia and in mysterious circumstances[13] – he was believed to have been struck by lightning or killed by Persian soldiers[14][15] – left his sons Numerian and Carinus as the new Augusti.

[16][18] In Emesa he was apparently still alive and in good health: he issued the only extant rescript in his name there,[19][Note 2] but after he left the city, his staff, including the prefect Aper (Numerian's father-in-law and the dominant influence in his entourage),[22] reported that he suffered from an inflammation of the eyes.

[25] Diocletian's elevation of Bassus symbolized his rejection of Carinus' government in Rome, his refusal to accept second-tier status to any other emperor,[31] and his willingness to continue the long-standing collaboration between the empire's senatorial and military aristocracies.

[51] In an act of clementia denoted by the epitomator of Aurelius Victor as unusual,[52] Diocletian did not kill or depose Carinus's traitorous praetorian prefect and consul Aristobulus, but confirmed him in both roles.

[60] Some historians state that Diocletian adopted Maximian as his filius Augusti, his "Augustan son", upon his appointment to the throne, following the precedent of some previous Emperors.

[96] It may be posited that Diocletian felt the need to bind Maximian closer to him, by making him his empowered associate, to avoid the possibility of him striking some sort of deal with Carausius.

[140] Diocletian's attempts to bring the Egyptian tax system in line with Imperial standards stirred discontent, and a revolt swept the region after Galerius's departure.

[149][150] He moved south into Roman Mesopotamia in 297, where he inflicted a severe defeat on Galerius in the region between Carrhae (Harran, Turkey) and Callinicum (Raqqa, Syria), suggested by the historian Fergus Millar to have been somewhere on the Balikh River.

[Note 10] Narseh retreated to Armenia to fight Galerius's force, putting himself at a disadvantage; the rugged Armenian terrain was favorable to Roman infantry, but not to Sassanid cavalry.

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.

[165] Many cities east of the Tigris came under Roman control, including Tigranokert, Saird, Martyropolis, Balalesa, Moxos, Daudia, and Arzan – though under what status is unclear.

[176] Diocletian found much to be offended by in Manichean religion: its novelty, its alien origins, its perceived corruption of Roman morals, and its inherent opposition to long-standing religious traditions.

[204][Note 11] Most in the crowd believed that Constantine and Maxentius, the only adult sons of reigning emperors, who had long been preparing to succeed their fathers, would be granted the title of Caesar.

[209] He moved into the expansive Diocletian's Palace, a heavily fortified compound located by the small town of Spalatum on the shores of the Adriatic Sea, and near the large provincial administrative center of Salona.

[210] The palace is preserved in great part to this day and forms the historic core of Split, modern-day Croatia, where it was designated a World Heritage Site by UNESCO in 1979.

[213] 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.

[215][216][Note 12] Diocletian saw his work as that of a restorer, a figure of authority whose duty it was to return the empire to peace, to recreate stability and justice where barbarian hordes had destroyed it.

[220] The cities where emperors lived frequently in this period – Milan, Trier, Arles, Sirmium, Serdica, Thessaloniki, Nicomedia and Antioch – were treated as alternate imperial seats, to the exclusion of Rome and its senatorial elite.

[235] To avoid the possibility of local usurpations,[236] to facilitate a more efficient collection of taxes and supplies, and to ease the enforcement of the law, Diocletian doubled the number of provinces from fifty to almost one hundred.

[244] In addition to their roles as judges and tax collectors, governors were expected to maintain the postal service (cursus publicus) and ensure that town councils fulfilled their duties.

[236] This led to a strained relationship between the central power and local elites: sometime during 303, attempted military sedition in Seleucia Pieria and Antioch prompted Diocletian to extract bloody retribution on both cities by putting to death a number of their council members for failing in their duties of keeping order in their jurisdiction.

[251] Admittedly, Diocletian's praetorian prefects – Afranius Hannibalianus, Julius Asclepiodotus, and Aurelius Hermogenianus – aided in regulating the flow and presentation of such paperwork, but the deep legalism of Roman culture kept the workload heavy.

[253] The sharp increase in the number of edicts and rescripts produced under Diocletian's rule has been read as evidence of an ongoing effort to realign the whole Empire on terms dictated by the imperial center.

[271] Lactantius criticized Diocletian for an excessive increase in troop sizes, declaring that "each of the four princes strove to maintain a much more considerable military force than any sole emperor had done in times past.

There began to be fewer men who paid taxes than there were who received wages; so that the means of the husbandmen being exhausted by enormous impositions, the farms were abandoned, cultivated grounds became woodland, and universal dismay prevailed".

He introduced an extensive new tax system based on heads (capita) and land (iugera) – with one iugerum equal to approximately 0.65 acres – and tied to a new, regular census of the empire's population and wealth.

[291] It appears that the edict was made in an attempt to preserve the current price of gold and to keep the Empire's coinage on silver, Rome's traditional metal currency.

[295] The fact that the edict began with a long rhetorical preamble betrays at the same time a moralizing stance as well as a weak grasp of economics – perhaps simply the wishful thinking that criminalizing a practice was enough to stop it.