The Tetrarchy and Its Decline

In 293 CE, the Roman Emperor ("Augustus") Diocletian, recognizing that rule and administration of the sprawling principate was effectively beyond the powers of one executive, instituted the innovation later known as the Tetrarchy, or rule of four executives. He had raised Maximian first to caesar and then to augustus, or co-emperor, in 285 and 286. But seven years later, he expanded this ruling council to four men, by giving the title of caesar to the generals Galerius and Constatinus Chlorus. Each augustus ruled half the empire, and each caesar assisted an augustus by ruling part of his portion. This innovative system allowed each ruler to lead military forces in his section, while notionally avoiding the danger of a successful general challenging the power of a single emperor. The tetrarchy created political stability that permitted full-scale military action in several different portions of the empire at once, leading to new Roman successes against old enemies (like Persia) and new ones (the encroaching barbarians).

But the plan only lasted for one generation of leaders. In 305, Diocletian and Maximian stepped down and Galerius and Constatinus Chlorus became augusti. When the latter died a year later in York (after a successful campaign against the Picts), the tetrarchy was riven by conflict over his replacement. Politics and ambition took over, with popular generals declaring themselves augusti or caesars, retired tetrarchs returning to rulership, and bloody civil war all over. In 324, the augustus Constantine the Great defeated all his rivals and returned the empire to exclusive rule by a single augustus/emperor.