 When Diocletian became Roman Emperor in 284 , the military situation had recently stabilized , which allowed him to enact badly needed constitutional reforms . Diocletian resurrected the " collegial " system that Marcus Aurelius had first used , and divided the empire into east and west . Each half was to be ruled by one of two co @-@ emperors , called the Augusti . He then resurrected the precedent set by Hadrian , and ensured that each emperor named his successor early in his reign . Diocletian called that successor a Caesar . Diocletian then created a bureaucratic apparatus that was similar to the system that Hadrian had created , wherein each office had a defined set of responsibilities , a set rank , and a set path of promotion . In this administrative system , Diocletian followed the example that had been set by Domitian , and divided the Empire into small administrative units . He also assigned to the four tetrarchs ( the two Augusti and the two Caesares ) honorary titles and insignia that had been used by Domitian . One important consequence of these reforms was the fact that the image of a free republic had finally given way , and the centuries @-@ old reality of monarchy now became obvious . 
