For total control, Diocletian divided the Roman Empire into two

Diocletian was well aware that the Empire's enormous size posed a challenge to his rule. He saw that the Empire could not be ruled by a single ruler. Diocletian divided the Roman Empire into two halves, East and West, and he started looking for a co-emperor to lead the other half.

The general Maximian Diocletian's son-in-law was named co-emperor in the first stage of Diocletian's political reorganization, which is also known as the "diarchy" or "rule of two"—first as Caesar (the junior emperor) in 285 and subsequently Augustus in 286. This restructuring reduced the amount of administrative labor needed to run an empire the size of Rome's by allowing Diocletian to handle affairs in the eastern parts of the empire while Maximian handled comparable things in the western parts. Marcus Aurelius Valerius was given to him in his honor. Maximian was able to handle all the challenges and problems of the Western Roman Empire, therefore Diocletian was pleased with his choice to appoint him. With Maximian's approval, Diocletian increased the imperial college in 293 by choosing two Caesars (one in charge of each Augustus), Galerius and Constantius Chlorus, arguing that both civic and military issues need more attention.