A further massive transfer of Bastarnae was carried out by emperor Diocletian ( ruled 284-305 ) after he and his colleague Galerius defeated a coalition of Bastarnae and Carpi in 299.

Diocletian delegated further on 1 March 293, appointing Galerius and Constantius as Caesars, junior co-emperors.

Galerius, aided by Diocletian, campaigned successfully against Sassanid Persia, the Empire's traditional enemy.

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.

Galerius and Constantius would become Augusti after Diocletian and Maximian's departure.

Diocletian spent the spring of 293 traveling with Galerius from Sirmium ( Sremska Mitrovica, Serbia ) to Byzantium ( Istanbul, Turkey ).

Afterwards, during 299 and 302, as Diocletian was then residing in the East, it was Galerius ' turn to campaign victoriously on the Danube.

In a public ceremony at Antioch, the official version of events was clear: Galerius was responsible for the defeat ; Diocletian was not.

Diocletian and Galerius ' magister memoriae ( secretary ) Sicorius Probus were sent to Narseh to present terms.

At the conclusion of the peace, Diocletian and Galerius returned to Syrian Antioch.

Diocletian was conservative in matters of religion, a man faithful to the traditional Roman pantheon and understanding of demands for religious purification, but Eusebius, Lactantius and Constantine state that it was Galerius, not Diocletian, who was the prime supporter of the purge, and its greatest beneficiary.

Galerius, even more devoted and passionate than Diocletian, saw political advantage in the politics of persecution.

Diocletian believed that Romanus of Caesarea was arrogant, and he left the city for Nicomedia in the winter, accompanied by Galerius.

According to Lactantius, Diocletian and Galerius entered into an argument over imperial policy towards Christians while wintering at Nicomedia in 302.

Diocletian argued that forbidding Christians from the bureaucracy and military would be sufficient to appease the gods, but Galerius pushed for extermination.

Galerius convinced Diocletian that the culprits were Christians, conspirators who had plotted with the eunuchs of the palace.

Galerius assumed the consular fasces in 308 with Diocletian as his colleague.

In the autumn of 308, Galerius again conferred with Diocletian at Carnuntum ( Petronell-Carnuntum, Austria ).

Diocletian and Maximian were both present on 11 November 308, to see Galerius appoint Licinius to be Augustus in place of Severus, who had died at the hands of Maxentius.

Compendium extract: Diocletian to the Death of Galerius: 284 – 311

Afterward, the persecutions under Diocletian and Galerius directed his attention to the martyrs of his own time and the past, and this led him to the history of the whole Church and finally to the history of the world, which, to him, was only a preparation for ecclesiastical history.

The point of the work is to describe the deaths of the persecutors of Christians: Nero, Domitian, Decius, Valerian, Aurelian, and the contemporaries of Lactantius himself, Diocletian, Maximian, Galerius, and Maximinus.

* 293 – Roman Emperors Diocletian and Maximian appoint Galerius as Caesar to Diocletian, beginning the period of four rulers known as the Tetrarchy.

* 286 – Emperor Diocletian elevates his general Maximian to co-emperor with the rank of Augustus and gives him control over the Western regions of the Roman Empire.

The title was also claimed by Carus ' other surviving son, Carinus, but Diocletian defeated him in the Battle of the Margus.

At some time in 285 at Mediolanum ( Milan, Italy ), Diocletian raised his fellow-officer Maximian to the office of Caesar, making him co-emperor.

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.

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.

Diocletian moved into Egypt to suppress him, first putting down rebels in the Thebaid in the autumn of 297, then moving on to besiege Alexandria.

Diocletian soon grew impatient with the city, as the Romans acted towards him with what Edward Gibbon, following Lactantius, calls " licentious familiarity ".

Nothing is known of him or of the date at which he lived ; the times of Trajan, Hadrian, Antoninus Pius, the beginning of the 3rd century, and the age of Diocletian and Constantine have all been suggested.

* April 1 – Emperor Diocletian elevates his friend Maximian to co-emperor, giving him the title Augustus.

Christian tradition makes him a native of the Dalmatian city of Salona, today Solin near Split, the son of a man also named Caius, and a member of a noble family related to the Emperor Diocletian.

In another account, the battle is represented as having resulted in a complete victory for Diocletian, for Carinus ' army deserted him.

On his arrival at Antioch, Galerius was rebuked by Diocletian who disgraced him for his shameful defeat at the hands of Narses.

He issued the first proper silver coins that had appeared in the Roman Empire for generations, knowing that good quality bullion coinage would enhance his legitimacy and make him look more successful than Diocletian and Maximian.

Whereas before emperors had worn only a purple toga ( toga purpura ) and been greeted with deference, Diocletian wore jewelled robes and shoes, and required those who greeted him to kneel ( proskynesis ) and kiss the hem of his robe ( adoratio ).

Diocletian established the Tetrarchy, a system by which the Roman Empire was divided into East and West, with each having an Augustus to rule over it and a Caesar to assist him.

The first is that the Greek and Latin legends of Saint Christopher identify him as belonging to the Third Valerian Cohort of the Marmantae ( Latin: Cohors tertia Valeria, at Marmantarum ), a military unit of Northern Africa of Marmarica ( between modern day Libya and Egypt ), recruited by none other than the Emperor Diocletian.

Diocletian reproached Sebastian for his supposed betrayal, and he commanded him to be led to a field and there to be bound to a stake to be shot at.

Sebastian then stood on a step and harangued Diocletian as he passed by ; the emperor had him beaten to death and his body thrown into a privy.

Diocletian characterizes the emperor as an interchangeable authority figure by depicting him with a generalized image.

Jacobus de Voragine in the Golden Legend credited him as a bishop at Formia over all the Italian Campania, as a hermit on Mount Lebanon, and a martyr in the persecutions under Eastern Roman Emperor Diocletian.

Diocletian had him thrown in another pit, but an angel came and slew all the vipers and worms.