The Tetrarchy, or "rule of four", was a reform of the Roman imperial government initiated by the emperor Diocletian in 286 c.e. This was a total change of the princeps system begun by the emperor Augustus Caesar in 30 b.c.e. Augustus, despite his name of "revered one", was addressed while emperor as "princeps", or "first citizen", and his imperial rule was built informally around government offices of the Roman Republic.

His true power had come from a combination of the office of tribune and his role in society as "first citizen". Diocletian, on the other hand, changed the position of Roman emperor from one cloaked in republican virtue and office into a full Eastern-style monarchy.

Access to the emperor was limited under Diocletian and he was addressed as "Dominus Noster", or "our lord". This change in the imperial system had two main goals. The first was a more efficient military command and control, and the second was an attempt to change the way the emperor was chosen.

Diocletian had good reason for instituting new changes for the Roman world. Prior to Diocletian’s imperial rule, beginning in 286, the Roman world had seen what historians now call the Crisis of the Third Century.

Since only 235 c.e., which had seen the death of the emperor Alexander Severus, Rome had been ruled by generals, by some accounts as many as 50. During the exceedingly short rules of these generals, Rome had been raided by Vandals and Goths and attacked outright on her eastern border by the Sassanid Empire.

While the Roman Empire was both politically and militarily unstable, Rome was also in terrible financial straits. The Roman Empire had seen years of hyperinflation due to the debasing of the coinage. When Diocletian came to power, he had ample reason to introduce reforms, reforms that some scholars say saved the empire for two more centuries.

This new imperial system was designed first to provide for an increased military command capacity within the empire. No longer would the Roman army sit only on the frontiers, but there would now be a "defense in depth" policy. The army would patrol the farthest reaches of the empire and also provide a defense for all Roman territory.

Diocletian secured these changes with two fundamental alterations to military policy. The first was a de facto division of the empire into an eastern and western half, with each half having its own emperor. Beneath these two emperors, who held the title of Augustus, were two junior emperors who held the title of Caesar. In all, the new imperial college would contain four members: two Augusti and two Caesari.

For the military this allowed for a total of four military commanders who could campaign on the very edge of empire, without the others having to worry about a victorious general being elevated to the rank of emperor by his troops. In addition to the edges of empire, there would be military resources and military commanders for the new defense in depth policy.

The imperial college was also intended to create a sense of stability in the empire. The political division between East and West was not intended to be a true division. In fact, all imperial decrees continued to be made in the name of all four men of the imperial college.

This allowed for a sense of stability during the sometimes-unstable transfer of power between Roman rulers. The previous princeps system of government had for hundreds of years left the empire with no formal way to choose a new emperor after the death of the previous reigning emperor.

Diocletian’s reforms sought to rectify this. In theory, when the senior Augustus died, his Caesar would be elevated and would in turn choose a new Caesar. In this manner the Caesar would gain both experience and legitimacy with the Roman populace.

In practice, however, Diocletian’s reforms did not even last for one full transfer. In order that he see his system of succession put into effect, Diocletian decided to retire after 20 years as emperor and forced his co-Augustus, Maximian, to retire as well.

When this occurred, each man’s Caesar was elevated to the imperial throne, and two new Caesari were chosen. Maximian did not agree to this forced abdication, and eventually he attempted to regain his position as head of the Western Roman Empire.

This failure by Maximian marked the beginning of the end of the Tetrarchy. By the end of Constantine the Great’s reign in 337, most of Diocletian’s reforms had failed. The rule of Constantine and his progeny was marked by civil war and competing imperial claims, just as had been the case before Diocletian’s reforms of 286.