Tetrarchy
During the 3rd century, three crises threatened the Roman Empire: external invasions, internal civil wars and an economy riddled with weaknesses and problems. The city of Rome gradually became less important as an administrative centre. The crisis of the 3rd century displayed the defects of the heterogeneous system of government that Augustus had established to administer his immense dominion. His successors had introduced some modifications, but events made it clearer that a new, more centralized and more uniform system was required.
Diocletian was responsible for creating a new administrative system (the tetrarchy). He associated himself with a co-emperor, or Augustus. Each Augustus was then to adopt a young colleague, or Caesar, to share in the rule and eventually to succeed the senior partner. After the abdication of Diocletian and Maximian, however, the tetrachy collapsed, and Constantine I replaced it with the dynastic principle of hereditary succession.
Read more about this topic: History Of The Byzantine Empire