Historical Background

Trajan and Hadrian were members of the long-lived Antonine dynasty, which came to an end with the death of Commodus in 192 C.E. The next dynasty was established by Septimius Severus, the first emperor of non-Italian descent (he was North African and his wife was Syrian). After the last member of the Severan dynasty died in 235, a prolonged period of civil war broke out. Over the next fifty years there was a rapid succession of claimants to the Roman throne, only one of whom died a natural death (the others were murdered or killed in battle). The instability on the throne affected all aspects of Roman life, resulting in inflation and devaluation of the currency, as well as hostile invasions as barbarians overran the borders of the empire. For the first time in centuries, a new fortification wall was built around the city of Rome.

The crisis of the third century ended when a general named Diocletian became emperor in 284. Diocletian is known as one of the last great persecutors of Christians. He instituted wide-ranging reforms that affected nearly every aspect of Roman life, including changing the monetary system and establishing a line of border forts to protect the empire. Diocletian even reformed the system of government. He realized that the empire had grown too large for one man to manage alone, and that the principle of dynastic succession was a source of instability. Therefore, Diocletian split the empire into two halves, east and west, and appointed an emperor (called an Augustus) to rule over each half. He also appointed two co-rulers with the title Caesar to assist the emperors, one each for the east and west. The Caesars were intended to ensure a peaceful and orderly transition to the throne by replacing the Augusti when they retired. Diocletian's system of rule by four men (two Augusti and two Caesars) is called the Tetrarchy, and each of the four rulers is called a tetrarch.