Following the civil wars of the first decades of the fourth century, the Emperor Constantine I recognised that the empire as a whole could no longer effectively be ruled from Rome. He moved his capital eastwards, to the site of the ancient Megaran colony of Byzantium, and renamed it Konstantinoupolis, the city of Constantine. Its strategic position was attractive, for the emperor could remain in contact with both eastern and western affairs from its site on the Bosphorus. The city was expanded, new walls were constructed and the emperor undertook an expensive building programme. Begun in 326, the city was formally consecrated in 330.
Constantine inaugurated a series of important reforms within both the military and civil establishment of the empire. The fiscal system was overhauled and a new gold coin, the solidus introduced in a successful effort to stabilise the monetary economy of the state. Military and civil offices were separated, the central administration was restructured and placed under a series of imperially-chosen senior officers responsible to the emperor directly. The armies were reorganised into two major sections, those based in frontier provinces and along the borders, and several field armies of more mobile troops attached directly to the emperor’s court as a field reserve, ready to meet any invader who broke through the outer defences. The provincial administration was reformed, more and smaller provincial and intermediate units being established, the better to permit central control and supervision of fiscal matters. Finally, with the toleration of Christianity and its positive promotion under Constantine at the expense of many of the established non-Christian cults, the church began to evolve into a powerful social and political force which was, in the course of time, to dominateeast Roman society and to vie with the state for authority in many aspects of civil law and justice.
In spite of Constantine’s efforts at reform, the size of the empire and the different concerns of west and east resulted in a continuation of a split government, with one ruler in each part, although the tetrarchic system was never revived. Upon Constantine’s death in May 337, his three sons succeeded to his authority with the support of the armies. Constantine II, the eldest, was recognised as senior and ruled the west. Constantius ruled in the east and Constans, the youngest, was allotted the central provinces (Africa, Italy, Illyricum). Tension between Constans and Constantine resulted in war in 340 and the defeat and death of the latter, with the result that Constans became ruler of the western regions as well. As a result of popular discontent among both the civilian population and the army in the west, however, Constans was deposed in 350 and his place taken by a certain Magnentius, a high-ranking officer of barbarian origin. Magnentius was not recognised by Constantius, and he invaded Illyricum. But he was defeated in 351, escaping to Italy where, after further defeats, he took his own life. Constantius ruled the empire alone until his death in 360.
In 355 Constantius had appointed his cousin Julian to represent him in Gaul; in 357, he was given the command against the invading Franks and Alamanni and, following a series ofvictories, he was acclaimed by his soldiers as Augustus. Constantius was campaigning against the Persian king Shapur who had invaded the eastern provinces in 359, and the acclamation may have been stimulated by the emperor’s demand that Julian send him his best troops for the Persian war. Julian marched east, but on the way to meet him Constantius died in 361, naming Julian as his successor. Although a competent general and efficient administrator, Julian may have been unpopular with some of his soldiers because ofhis attempts to revive paganism, often at the financial expense of the church. During the Persian campaign of 363 he was mortally wounded, although it is not clear in what circumstances. The troops acclaimed the commander of Julian’s guards, a certain Jovian, as emperor. Having made peace with Shapur, Jovian marched back to Constantinople, dying in Bithynia a mere eight months later.
Jovian’s successors were Valentinian and Valens, brothers from Pannonia (roughly modern Austria and Croatia), the former having been elected by the military at Constantinople then appointing his brother as co-emperor. Valentinian ruled in the west and established his capital at Milan, while Valens had to face a rebellion almost immediately, led by the usurper Procopius and caused by the soldiers loyal to Julian, whose favourite Procopius had been. But the rebellion petered out in 366.
The two new emperors each had substantial military challenges to overcome. But Valentinian died in 375 while dealing with the Quadi in Pannonia, and was succeeded by his chosen successor, Gratian. In the east, Valens had to deal with repeated Gothic invasions of Thrace, where in 378 he was disastrously defeated and killed near Adrianople (mod. Edirne) in Thrace.
Gratian appointed as Valens’ successor the general Theodosius, son of a successful general of the same name and himself an experienced commander, initially as commander-inchief and then Augustus; and by a combination of diplomacy and strategy Theodosius was able to make peace with the Goths, permitting them to settle within the empire under their own laws, providing troops for the imperial armies in return for annual food subsidies. Following the death of Gratian in 383 as the result of a coup, and the eventual overthrow of the usurper, Magnus Maximus, by Theodosius in 388, Theodosius became sole ruler. He was, however, the last emperor to hold this position. At his death in 395 his two sons Arcadius (in the east) and Honorius (in the west) ruled jointly.
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