Emperor Wu, having become more concerned about whether his brother Prince You would seize the throne if he died, had in 282 sent Prince You to his principality, even though there was no evidence that Prince You had such ambitions.

Having grown ill, Prince You dies in anger in 283.

The old contest for northern Mesopotamia, with its fortified cities of Carrhae, Nisibis, and Edessa, continues.

The Sassanians are even more eager to regain and retain control of Armenia because there the Arsacid dynasty still survives and turns for protection to Rome, with which, in consequence, new wars continually break out.

Carus, having announced in the winter of 282-83 that he seeks to achieve the reconquest of Mesopotamia planned by Probus, sets out for Persia, again accompanied by Numerian, leaving Carinus in charge of the western portion of the empire.

Bahram II is still preoccupied with defending his position against his brother Hormizd, viceroy of the eastern provinces.

Probus’s successor Carus, proceeding through Thrace and Asia Minor, invades Mesopotamia unopposed in 283, annexing the province and capturing first Seleucia, then …

…the Persian capital, Ctesiphon.

The reconquest of Mesopotamia avenges all the previous defeats suffered by the Romans against the Sassanids; for this, Carus receives the title of Persicus Maximus.

The emperor makes an augustus of his elder son Carinus, left in charge of governing the west of the empire in Carus' absence.

Wwhile preparing to follow up his success against the Persians and carry arms yet further into Sassanian territory in the summer of 283, Carus dies suddenly and mysteriously during a violent storm, allegedly struck by lightning, though rumor names Praetorian prefect Arius Aper as his assassin.

He had reigned for less than a year.

His sudden death, variously attributed to disease, the effects of lightning, or a wound received in the campaign against the Persians, causes the Roman army to conclude a peace and withdraw.

Numerian’s acclamation in December, without opposition, as emperor in the East, suggests that his father’s death may well have been due to natural causes.

…Naqsh-e Rustam to commemorate his victory.

Carinus, ruling the West after the death of his father Carus, who had succeeded the murdered Probus, has conducted a campaign on the Rhine, fighting with success against the Quadi tribes, but soon leaves the defense of the Upper Rhine to his legates and returns in January 284 to Rome, where he abandons himself to all kinds of debauchery and excess.

Carinus celebrates the annual Roman Games (ludi Romani) in September 284 on a scale of unexampled magnificence in honor of his late father's victories in Persia the year before.

Numerian lingers in the East.

The Roman retreat from Persia is orderly and unopposed, for the Persian King is still struggling to establish his authority.

Numerian has by March 284 only reached Emesa (Homs) in Syria, where he is apparently still alive and in good health, as he issues the only extant rescript in his name there. (Coins are issued in his name in Cyzicus at some time before the end of 284, but it is impossible to know whether he is still in the public eye by this point.)

After Emesa, Numerian's staff, including the prefect Aper, report that Numerian suffers from a disabling inflammation of the eyes, and has to travel in a closed coach.

Some of Numerian's soldiers smell an odor reminiscent of a decaying corpse emanating from his coach after the army has traveled through Bithynia to reached the Bosporus late in 284.

They open its curtains and inside find Numerian, dead.

His father-in-law and adoptive father, the praetorian prefect Flavius Aper, assumes command but is accused having killed the emperor in order to seize power.

Instead, Diocles, who may have been a member of Carinus' bodyguard, is acclaimed as emperor by his fellow soldiers.

The thirty-nine-year old army chief, whose father had been a scribe or the emancipated slave of a senator called Anullinus, has risen from modest Dalmatian origins through the military ranks to become Dux Moesiae, with responsibility for defending the lower Danube.

Appearing for the first time in public dressed in the imperial purple, he declares himself innocent of Numerian's murder, designates Aper as the criminal and kills him personally.

Contemporaries accept Aper's guilt, but it is also the case that a prediction had been made to Diocletian previously, telling him that he would become emperor on the day he killed a boar (Latin: aper).

By eliminating Aper, Diocletian has rid himself of an eventual competitor and, retroactively, provided his act with sacred meaning.

Acclaimed emperor on November 17, 284, Diocletian possesses real power only in those countries that are dominated by his army (i.e., in Asia Minor and possibly Syria).

The rest of the empire is obedient to Numerian's brother Carinus.

Diocletian is not the only challenger to Carinus' rule; the usurper M. Aurelius Julianus, Carinus' corrector Venetiae, had taken control of northern Italy and Pannonia after Diocletian's accession.

He mints coins from the mint at Siscia (Sisak, Croatia) declaring himself as emperor and promising freedom.

It is all good press for Diocletian, and aids in his portrayal of Carinus as a cruel and oppressive tyrant.

Julianus' forces are weak, however, and are handily dispersed when Carinus' armies move from Britain to northern Italy, defeating and killing Julianus near Verona in the spring of 285.