The external wars that sharply undermined prestige of the central authorities and scattered the myth about invincibility of roman weapon, had brought hardest defeats to the Romans. In 251 y. A. D. Roman Army suffered a terrible defeat in a buttle with the Goths at Abritte, where emperor Decius was also killed. In 260 y. A. D. persian king Shapur had broken the Romans and captured emperor Valerian. For the first time Rome felt such disgrace. The captive emperor was forced to be a servant of the king-winner.
Such cruel failures promoted reinforcement of social struggle of oppressed classes and that frightened big land-owners in provinces very much. From the second half of III centure A. D. they being to search for an alliance with Roman government and to rally around of it. The rallying of main forces of the slave-holding class allowed to restore the unity of Roman State.
Emperor Klaudius The Second put a shattering defeat to the Goths in the battle at Naiss in 269 y., Emperor Aurelian (270–275 y. A. D.) destroyed the Palmirian and Gaullian empires, Emperor Prob (276–282 y. A. D.) expelled The Franks from Gaullia and successfully waged on the Moors in Africa.
At the first years of reign of Diokletian (284–305 y. A. D.) the integrity of the Empire was finally established. Maximian, the fellow-governer of Diokletian, suppressed the social movements in Gaullia and Nothern Africa, Diokletian himself quelled the revolt in Egypt, the Domination of Rome was restored in Britain. The successful wars with the Persian, the Franks, the Alemmans lifted the foreing-policy authority of Rome impossible to fix these success without complete transformstion of all state organism.
The events of II centure A. D. testified about deep changes in the economic and social life of the Empire. The change of political organization was inevitable. Only amplification of the state authority, based on a powerful army and a remified bureaucratic machinery of the State, could prevent recurrence of the events of middle of III centure A. D.
The task of complite transformation of political organization of the roman society also was carried out by Emperor Diocletian.
First of all Diocletian decided to create a new system of the imperial authority. He established the constant institute of co-governors. At first — diarchy, when the Empire is operated by two emperors one in the east, other on the west, and then — tetrarchy, when together with two senior emperors — Augustus — appear the younger ones — Caesars. With Augustus Diocletian the Caesar was Galerius, with Augustus Maximian — Constantius Chlorus. In the first, and, by the way, in the last time in the history of Rome Diocletian attempted to set up an absolutely precise system of succession to the throne — two Augustus would be replaced by Caesars, which becoming augustus select new caesars for themselves. Also there was established a temporary terminator of government of Augustus-20 years. The status of the emperor finally changed too. Henceforth the ruler of Rome was not princeps — the first in Senate, but he was dominus now — the Master. Dominus et Deus — Master and Lord — it is an official title of Roman emperors in the epoch of Dominate. For sthrenghtening prestige of the imperial authority Diocletian sets up imperial ceremonial at his court, borrowed it from assanian Persia.
Under Diocletian, in connection with establishment of Tetrarchy, the administrative reform of the Empire was conducted too. The extensive territories are fixed for each of the tetrarchies: for Diocletian — Thrakia, Asia Minor, Siria, Mesopotamia, Egypt; for Maximian — Italy, and Nothern Africa; for Galerius — Greece, Macedonia, Illiria; for Constantius Chlorus — Gaullia, Britain and Spain. The number of provinces is doubled — now there are 100 of them. Italy loses its special status, turning on an ordinary area of the Empire. Rome, keeping the status of the capital, loses its role as a residence of the Emperor, The Roman Senate forteits the status of supreme body of the Empire, turning to a council on administration of Rome-city. The centre of the Empire moves to the east.
Diocletian establishes the new tax system, that survived the Rome and was preserved in Byzantine. An attachment farmers to land and citizens to residence becomes its consequence. The finance is reformed the monetary system renovates. However the grandious attempt of Diocletian to set up the rigid state control over prices and incomes of the population had failed. The last great prosecution of the Christians finished with the failure too. The Diocletian's favourite child — Tetrarchy — also couldn't survive his reign.
At the same time the political system as a whole, the principal of taxation, the attachement of estates — all of this outlived Diocletian for many centuries.