Tetrarchy

Tetrarchy, from Greek tetra (four) and archē (rule), is the term for the system of imperial government introduced by Roman Emperor Diocletian around 293 CE to administer the sprawling empire. The arrangement divided authority among four rulers: two senior emperors, called Augusti, who governed the eastern and western halves; and two junior emperors, called Caesares, designated heirs and deputies to the Augusti. Each ruler controlled civil administration, taxation, and military forces within his assigned region.

Diocletian and Maximian served as Augusti. Galerius and Constantius Chlorus were appointed Caesares in the East and West, respectively. The tetrarchy was designed to stabilize succession, strengthen frontier defense, and streamline governance across a difficult and overextended empire, by matching administrative boundaries to military command and by providing a clear line of succession.

After Diocletian and Maximian abdicated in 305, Galerius and Constantius became Augusti, and Severus and Maximinus Daia were appointed Caesares. The system proved unstable; rival ambitions and civil wars eroded its authority, and by the early 4th century the arrangement collapsed as Constantine the Great and Licinius fought for supremacy. Constantine eventually became sole ruler in 324 CE, ending the tetrarchy as a governing structure, though the concept influenced later administrative ideas and remains a reference point in discussions of plural leadership.