Gaius Aurelius Valerius Diocletianus was probably born in Salona (Dalmatia) in 245 AD. and was originally called Diocles.
He was one of the last great Roman emperors: he tried to save what was left of the Roman empire in a period of bloody fights for power, with endless civil wars between different armed factions one against the other.

He undertook a military career, became Consul and then governor of Moesia and finally commander of the Praetorians at the service of the emperor Carus.
According to legend, he was predicted that he would become emperor if he killed a wild boar, Aper in Latin.
In 284 AD Emperor Carus was assassinated; his son Carinus became emperor of the Roman Western Empire. His other son Numerian had the Eastern Empire and died in mysterious circumstances, perhaps at the hands of his father-in-law who was actually called Aper. Dioclezan killed him with his sword, fulfilling the prophecy, and was acclaimed emperor.
In 285 the two armies of the East and the West clashed, Carinus won but was killed by one of his officers; Diocletian was acclaimed emperor of both empires.

The Roman empire was in a deep crisis, because the central authority had weakened and the army was in the hands of mercenaries. Several emperors/usurpers had minted their own coins to bribe soldiers and officials, causing the devaluation of the official coins.
The border provinces were threatened by the barbarians, in Britain but also along the Rhine and the Danube, and then in the East, in Persia, Egypt and Africa.
Insecurity had caused a serious economic and demographic crisis, the reduction of agricultural production, the devaluation of the currency and the increase in brigandage and piracy on the seas.

In that troubled era the commanders of a legion often transformed it into their own personal army and were acclaimed emperor. And since most of the legionaries were barbarians and mercenaries, the acclamations depended on gold donations and loot. If they were not enough, the new emperor was put to the sword and another one was chosen, which happened especially in the decade 275-285 AD.
That situation of total anarchy undermined the empire from within, much more than conflicts with historical and external enemies such as the Persians.

To deal with it, in 286 AD. Diocletian decided to share the command of the empire with Maximian, one of his best generals: he appointed him Augustus minor for the Western Empire while maintaining for himself the title of Augustus major for the Eastern Empire.

Both engaged in a series of military campaigns to quell the revolts, secure the borders again and revive the economy. They defeated the Persians, the Alamanni and the Franks, and successfully countered the Frankish and Saxon pirates. Then it was the turn of Rhaetia, of the Sarmatians; in the following years, Syria, Egypt and part of Africa were tamed.

In 292-293 the situation had improved and Diocletian faced the most important problem: the imperial succession. It could no longer be done on a family and dynastic basis, after the examples of Caracalla and Heliogabalus; it had to be guaranteed 'automatically' by choosing the successors in advance, to avoid the civil wars triggered by usurpers.

Thus the Tetrarchy was born. The two Augusti – Diocletian and Maximian – appointed two Caesars, Galerius and Constantius, who were adopted as sons and designated successors.
The empire was divided into four parts, but Diocletian retained supreme authority and issued laws for the entire empire, exercising absolute power.
Diocletian ruled the East (Egypt and Libya and Asia) with his capital in Nicomedia. Maximian reigned over Italy, Rhaetia, Sicily, Sardinia and part of Africa, with his capital in Milan. Galerius was given Pannonia, Moesia, Greece and Crete, with his capital at Sirmium. Finally Constantius had Britain, Gaul and Spain with its capital in Trier.

As Augustus and Hadrian had already done, Diocletian launched an ambitious program of public works, building fortifications and roads, but also palaces, circuses, baths and basilicas, especially in the new capitals: Trier, Milan, Sirmium, Thessalonica and Nicomedia.

Then he started major administrative and fiscal reforms, which did not always succeed as he wished. Italy lost its tax exemption; taxes were distributed more equally and were calculated every year. New coins were minted and the Price Edict was issued, which attempted to control the prices of goods. The army was also reformed, re-establishing discipline and loyalty as much as possible.

The imperial power, as always, needed a sacred legitimation, which since the time of Augustus consisted in proclaiming the divine genealogy of the imperial House and in the deification of the emperors, which occurred only after their death and with a specific decree of the Senate.
As the centuries passed, the power of the Senate was gradually but inexorably reduced, the capital was moved to Milan; Rome remained only a prestigious symbol, an empty shell devoid of real power.

Starting from Aurelian, the emperor was proclaimed divine during his lifetime, Dominus et Deus, being a descendant of the Sun god - Sol Invictus - that became the State cult. Aurelian adopted the oriental symbols of power such as the diadem and the cloak studded with precious stones, the pomp and complex court ceremonials.

Diocletian appropriated those same symbols, proclaiming himself «Dominus et Deus, rector orbis ac domino, fundator pacis aeternaem, providentissimo princeps» that is, «Master and god, governor of the world and lord, founder of eternal peace, providential prince».
He adopted and developed other court ceremonials that would be inherited by the Byzantine Empire, including the prostration before the ruler and extended the sacral dignity to the imperial palace and its counselors.

The deification in life of the Augusti had a very specific purpose: the sacred legitimation of the imperial power as divine, and also the legitimation of the succession by the Caesars already chosen by them as designated heirs.
In this way the other pretenders to the throne were eliminated because they did not have a divine status, and the armies no longer had the power to nominate and choose emperors.

The deification of the emperor did not prevent freedom of worship, but it placed Diocletian at odds with the Christians, who refused to worship him as a god and were harshly persecuted more for political than religious reasons because they undermined his supreme divine authority.

On 1 May 305 AD, Diocletian voluntarily abdicated, retiring to his Palace in Split (Croatia), whose construction had begun twelve years earlier.
Maximian also abdicated, but the succession after them was not smooth, with usurpations and betrayals that culminated in the final conflict between Maxentius and Constantine.