Panorama of amphitheatre in Salona
Diocletian was born near Salona in Dalmatia (Solin in modern Croatia), some time around 244. His parents gave him the Greek name 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. His parents were of low status; Eutropius records "that he is said by most writers to have been the son of a scribe, but by some to have been a freedman of a senator called Anulinus." 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. The first time Diocletian's whereabouts are accurately established, in 282, the Emperor Carus made him commander of the Protectores domestici, the elite cavalry force directly attached to the Imperial household – a post that earned him the honour of a consulship in 283. As such, he took part in Carus' subsequent Persian campaign.
Death of Numerian
Carus's death, amid a successful war with Persia and in mysterious circumstances – he was believed (perhaps as a result of later Diocletianic propaganda) to have been struck by lightning – left his sons Numerian and Carinus as the new Augusti. Carinus quickly made his way to Rome from his post in Gaul as imperial commissioner and arrived there by January 284, becoming legitimate Emperor in the West. Numerian lingered in the East. The Roman withdrawal from Persia was orderly and unopposed. The Sassanid king Bahram II could not field an army 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. In Emesa he was apparently still alive and in good health: he issued the only extant rescript in his name there,[Note 1] but after he left the city, his staff, including the prefect (Numerian's father-in-law, and as such the dominant influence in the Emperor's entourage) Aper, reported that he suffered from an inflammation of the eyes. He travelled in a closed coach from then on. When the army reached Bithynia, some of the soldiers smelled an odor emanating from the coach. They opened its curtains and inside they found Numerian dead. Both Eutropius and Aurelius Victor describe Numerian's death as an assassination.
Aper officially broke the news in Nicomedia (İzmit) 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 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. In full view of the army, Diocles drew his sword 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.
Conflict with Carinus
After his accession, Diocletian and Lucius Caesonius Bassus were named as consuls and assumed the fasces in place of Carinus and Numerianus. Bassus was a member of a senatorial family from Campania, a former consul and proconsul of Africa, chosen by Probus for signal distinction. He was 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 his 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. Julianus minted coins from the mint at Siscia (Sisak, Croatia) declaring himself 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. 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 284–85, 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, more powerful 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. 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 as Emperor. Diocletian exacted an oath of allegiance from the defeated army and departed for Italy.