Diocletian’s Tetrarchy
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By the middle of the Third Century CE, the Roman Empire had reached its greatest extent. It spread from the Atlantic Ocean to Mesopotamia and from the Danube to the edge of the Sahara. It was a massive territory with many different cultures, religions, and societies all brought together by their political overlord, Rome. However, many people did not necessarily perceive themselves as Roman. Outside of Italy, more of the people under Rome’s authority spoke Greek than Latin, not to mention other local languages. For many on the edges of the Empire, the only visual identifications they had with Rome were the images on coins and the centurions who maintained the borders.
Into this comes Diocletian. He was a soldier who had risen through the ranks to become head of the Praetorian Guard for the Emperor Carus. In 283, he was accompanying the Emperor on his way home from a campaign against the Sassanid Empire (roughly modern Iran) when Carus died, reportedly of a lightning strike. As you might imagine, this is a highly unlikely cause of death. Carus’ two sons were with them and immediately declared themselves his successors, together in a joint emperorship. Shortly after, the younger of the two men was discovered to have died while travelling by one of his litter-bearers. He had reportedly been killed by a praetorian who happened to be an enemy of Diocletian. The elder son, Carinus, recognized that Diocletian was attempting to usurp his position and so gathered an army against him. They met in July 285 and Carinus was defeated.
Diocletian came to the throne recognizing a fundamental problem with the Roman Empire: its size. Past military coups had seen responses from across the empire, with generals from the borders marching inward to either make their own claim or to support another claimant. While Diocletian did deal with some small rebellions, it can be doubted those on the far Western edges of the Empire were even aware there had been a change of government. While that benefitted him, it would not be beneficial in the future, especially if he sought to create a long-lasting dynasty. There was also the issue of the loyalty of the legions on the outskirts: locally recruited, locally trained, and locally commanded, there was no reason to believe they wouldn’t choose their own communities over the concept of Rome because, for them, Rome was merely a concept.
Diocletian decided the best way to make sure that loyalty to Rome could be held at least in equality to loyalty to their home was to split the empire in half. One emperor in the East and another in the West. In a way, this reflected the governmental design of the Republic, which had been led by two consuls who served as the executives. He chose as his colleague a fellow praetorian named Maximian. Diocletian took the eastern part of the empire while Maximian held the west. They were, in theory, of equal stature and power, but Diocletian was the more powerful of the two.
The system worked well and in 293, another element was added: two more sub-emperors. These men were given the title “Caesar” while the Diocletian and Maximian now each became “Augustus.” Diocletian chose a man named Galerius and Maximian chose Constantius. The relationship between the men was made familial with Constantius marrying Maximian’s daughter and Galerius marrying Diocletian’s. This instituted the Tetrarchy, rule by four. The idea behind it was to ensure a smooth and peaceful transfer of power to men who were already well-versed in the running of the empire. This also would somewhat dissociate heredity from the ruling of the Empire; the son of an emperor could not be assumed to be his heir. The next in line had to be officially named as such and would be expected to learn the job alongside the elder emperor.
After a little over a decade of successful rule, Diocletian took the last logical step in this new way of running the empire: abdication. If the imperial seat was not based on heredity, then one didn’t need to wait for the death of the ruler. By stepping down Diocletian established that the office was more important than the man inhabiting it. This also harkened back to the Republic, with its one-year terms for magistracies with regular changes in who held offices (though I’m sure the idea of giving the people a vote never crossed Diocletian’s mind). So, in May of 305, both Diocletian and Maximian retired from public life. Galerius and Constantius were elevated to the positions of Augusti and their choices for their successors, Maximinius Daia and Severus, respectively.
The transition was a smooth one and it appeared that Diocletian’s design for the massive Empire would be a successful and long-lasting one. And it may well have been had it not been for the death of Constantius the next year. Instead of a smooth transition to his successor, a civil war broke out between Severus and those who wanted to see Constantius’ son Constantine on the throne. This war went on for six years, leaving Constantine victorious and a new element introduced as a means to unify Rome: Christianity.