It is said, that his father, apparently a scribe of a wealthy senator, might have been a former slave.
Diocles rose through the ranks of the military and achieved high position. Throughout the AD 270's he was military commander in Moesia. From AD 283 onwards, under Carus and his son and successor Numerian he acted as commander of the imperial bodyguard (protectores domestici) and appears a rather dubious figure in the deaths of both of those emperors.
In November AD 284, near Nicomedia he was chosen by the soldiers to avenge Numerian's death, which he did by charging Arrius Aper, the praetorian prefect, whom he sentenced to death. Thereafter he personally executed Aper in front of the troops.
Hailed emperor on 20 November AD 284, immediately, or shortly after this execution, Gaius Aurelius Valerius Diocletian - the name he assumed with the imperial title - crossed the Bosporus into Europe and met the forces of Numerian's brother and co-emperor Carinus at Margum on 1 April AD 285.
Diocletian was in fact losing the battle as the assassination of Carinus by one of his own officers, left the opposing army without a leader. With only one imperial candidate still left on the field, Carinus' army surrendered accepting Diocletian as emperor.
Carinus' murder would also suggest a possible involvement by Diocletian, connecting him (although solely by rumour) with the possible assassination of three emperors.
Seeing it necessary display goodwill to the supporters of Carinus, Diocletian kept Carinus' praetorian prefect, Aristobolus, as well as keeping many of the former emperor's government officials in place.
Then, much to everyone's surprise, Diocletian, in November AD 285 appointed his own comrade Maximian as Caesar and granted him control over the western provinces. Surprising as this development no doubt was, Diocletian urgently needed to give the problems on the Danubian borders his full attention. Meanwhile he needed someone in Rome to take care of government. Not having a son, it was a natural choice to pick one of his trusted military comrades to hold the fort for him.
With Maximian proving himself a worthy Caesar, Diocletian only several months later, on 1 April AD 286, promoted him to the rank of Augustus. Diocletian however remained the senior ruler, possessing a veto over any edicts made by Maximian.
The year Ad 286 however, should not only be remembered for the promotion of Maximian. It also should become known for the rebellion of Carausius, who was the commander of the North Sea fleet, who made himself emperor of Britain.
As dicolotean tried to abolish the religion of Christianity, more romans started converting, so diocletians reforms were unsuccessful.
People ignored his rules.
Diocletian was not a strong enough emperor to enforce them.