Diocletian

On July 21, 285 Diocletian appoints Marcus Aurelius Valerius Maximianus as Caesar and co-ruler. An excerpt from the article:

"Dividing the Empire

Diocletian found a solution to the age-old problem of succession: the tetrarchy.
Diocletian understood that a major problem in ruling a territory of the extent of the Roman Empire was its immense size. It was far too large to be ruled by just one person, so one of the first actions taken by the new emperor was to split the empire into two parts. Lacking an heir, in November of 285 CE, shortly after securing the imperial throne for himself, he named an Illyrian officer (who happened to be his son-in-law) named Maximian as Caesar in the west. The new Caesar, who would be promoted to Augustus one year later, immediately assumed the name Marcus Aurelius Valerius. Diocletian, who was never very fond of the city of Rome, would remain emperor in the east. The appointment of Maximian afforded Diocletian the time to deal with the continuing problems in the east, however, despite Maximian's position as co-emperor, Diocletian considered himself to be the senior emperor (something to which Maximian agreed), retaining the ability to veto any of Maximian's decisions. Gone was Augustus's principate; in its place was the dominate.

Unfortunately for both Diocletian and Maximian, peace in the empire could not be kept for long. The difficulties that had plagued the empire for the past several decades remained. As with his predecessors, problems soon erupted along the Danube River in Moesia and Pannonia. For the next five years, Diocletian would spend most of that time campaigning throughout the eastern half of the empire. An eventual victory in 286 CE would bring him not only a long awaited peace but the title of Germanicus Maximus. Diocletian demonstrated similar skills in Persia by defeating the Sarmatians in 289 CE and Saracens in 292 CE.

Maximian was plagued by similar problems in the west. A rogue officer named Carausius, the commander of the Roman North Sea fleet, seized control of Britain and part of northern Gaul, proclaiming himself as emperor. He had been awarded his command after helping Maximian defeat the renegade Bagaudae in Gaul. Later, when it was learned that he was keeping much of the “spoils of war” for himself, he was declared an outlaw and a death warrant was issued by Maximian. But, like many of the men who proclaimed themselves emperor, he met his death at the hands of someone under his own command, in this case his finance minister Allectus.

The concept of a divided empire was apparently working. However, a situation that had faced every emperor since Augustus had to be addressed and that was succession. Diocletian's solution to this age-old problem was the tetrarchy - an idea that preserved the empire in its present state, with two emperors, but allowing for a smooth transition should an emperor die or abdicate. The new proposal called for two Augusti - Diocletian in the east and Maximian in the west - and a Caesar to serve under each emperor. This “Caesar” would then succeed the “Augustus” should he die or resign. Each of the four would administer his own territory and have his own capital. Although the empire remained split, each Caesar was answerable to both Augusti. To fill these new positions, Maximian adopted and then named his praetorian commander Constantius as his Caesar. Constantius had gained a reputation for himself after he led a number of successful campaigns against Carausius. Diocletian chose as his Caesar Galerius who had served with distinction under Emperors Aurelian and Probus."