The third century was a tumultuous time for the Roman Empire. Between the death of Septimius Severus in 211 AD and the rise of Diocletian, almost all of the Roman Emperors died with their boots on – the exceptions Valerian as a prisoner in Persia, Claudius II Gothicus of the plague, Tacitus who was aged on accession may have died of old age and Carus allegedly hit by lightning though likely murdered. Diocletian himself has assumed power in a civil war after the death of Carus.
Then he did something no Emperor had done before. The strategic weakness of the Roman Empire was that it struggled with multiple front wars. Making things worse, armies in the field had developed a tendency to elevate their generals to the purple. With the Empire facing runaway inflation from the debasement of the coinage and threats along the Rhine, Danube, Persian borders and a breakaway empire in Britain the task was too big for one man to handle.
So Diocletian elevated an army buddy unrelated to him – Maximian – to co-emperor i.e. Augustus. The Diarchy was then turned into a Tetrarchy a few years later by each Augustus elevating a junior co-emperor as Caesar – Constantius I Chlorus for Maximian in the West and Galerius with Diocletian in the East. Each Augustus married off his daughter to his Caesar creating a familial tie – for Constantius it meant repudiating his first wife the mother of the future Emperor Constantine the Great.
Each Emperor was given a frontier to defend. Constantius subdued Britan, Maximian Germany and Africa, Diocletian the Balkans and Galerius defeated Persia. And then after 20 years on the throne Diocletian after a serious illness had enough and abdicated to grow cabbages. Both Caesars were elevated to Augustus and they chose two new Caesars.
And this ultimately doomed Diocletian’s experiment. Maximian was reluctant to abdicate and tried to seize the throne again. In elevating the new Caesars the adult sons of Constantius (Constantine) and Maximian (Maxentius) were overlooked. When Constantius died suddenly the next year his legions proclaimed Constantine emperor and the result was civil war. Diocletian lived in despair till 311 AD to see his life’s work fall apart. The civil war would rage intermittently for 20 years until Constantine killed all his rivals to become sole Emperor.