In the later part of the third century AD, the Roman empire was in the midst of civil wars and a severe economic crisis. Generals were being declared as emperors by their troops, dangerously destabilizing the established system of centralized power. During this period of upheaval, Diocles, later Diocletian, was born in the province of Dalmatia, in modern day Croatia. Born of simple stock, Diolcles distinguished himself in the ranks of the roman army, even becoming a cavalry commander under emperor Carus. This was not an uncommon path for an emperor at the time, with many previous emperors using the army to rise socially, with high amounts of social mobility a defining characteristic of this era. His rise culminated in 284 AD, when he became princeps (emperor), inheriting an empire in chaos. He immediately set about to solve these problems through a series of political, military, social and economic reforms.
For the army, he changed the traditional structure of 5000 men legions into more fluid and efficient units of 1000 men. Also, legions became more specialized and their equipment was to be provided by the state through the fabricae, which were groups of state-owned warehouses and workshops. The equipment of the soldiers morphed as they started adopting more oval shields and spears with longer spathas as secondary weapons. These changes may have been caused by many of Rome’s enemies being cavalry soldiers such as the Sassanids, the Sarmatians and the Scythians. The organization of the army into Comitatenses and Limitanei may have also began with the reign of Diocletian. The Cohortes Praetoriae, were relegated by the emperor to a secondary role, forming the garrison for the city of Rome, as they were considered too corrupt and ambitious to be trusted in the field. The new imperial guard was formed by the Ioviani and Herculiani regiments, chosen from the legio Iovia and legio Herculiana, famous for their loyalty to emperor Diocletian.
Diocletian, being a man of simple origins, born outside the confines of old aristocracy, disliked the senate and the old oligarchy it represented. He deprived the Senate of most of its powers, relegating it to the role of governing the capital. Power became increasingly centralized in the hands of the new military elites, which were forming the empire. This began the transition towards the separation of the military and civilian roles within the Roman world, which will happen during the reign of Valentianian I. The court system around the emperor became intricate and extremely bureaucratic, as nets of administrators and officials effectively shielded the emperor from the outside world, eventually making it difficult for him to connect to his empire.
The emperor, considered the first of the roman citizens, was now to be considered a dominus, an authoritarian father figure with links to the gods. Diocletian noticed that the empire was too large to be controlled by a single emperor. It simply was not possible due to its vastness. As a result, in 286 AD Diocletian elevated his old war companion Maximian to a Caesar in the West, based in Mediolanum. Maximian was a perfect candidate for the job as he was a loyal man as well as an able commander. Maximian was later promoted to the rank of Augustus, a full-fledged colleague of Diocletian. The Roman empire thereafter turned into a diarchy, with Maximian ruling the West and Diocletian ruling the East from Nicomedia. Diocletian quickly realized that two emperors alone were not able to maintain stability in the empire. Because of this, he formed the famous tetrarchy, rule of four, an effective division of the empire into four spheres of influence. Maximian had Constantius Chlorus as his Caesar (junior colleague) in the West, based in Trier, modern day Germany, while Diocletian chose Galerius, a peasant who had risen from the ranks of the army as his Caesar, based in Thessalonika.
Diocletian wanted to stabilize the economy but did not have competent experts at his disposal, so he only managed to debase the Roman coinage by creating a new series of imperial mints. Diocletian later died in 313 AD at his fortified palace at Split, Dalmatia, knowing that his tetrarchic project was doomed to fail. The rise of Diocletian and the tetrarchic period marks a new point in Roman history and it brings the crises of the third century to an end by projecting the empire into an era of political and military stability. The tetrarchic model revealed itself to be a political failure in that it did not account for the ambitions of men, and the impeccable order installed by Diocletian could not be maintained. Diocletian can be considered a pillar of the Roman state as he was an excellent legislator and reformer. The period from the rise of Diocletian onwards would be called the Dominate instead of the Principate as the authoritarian emperor created by Diocletian would become the norm.