The later Roman Empire
The Roman empire reached its greatest territorial extent during the 2nd century. The following two centuries witnessed the slow decline of Roman control over its outlying territories. The Emperor Diocletian split the empire into separately administered eastern and western halves in 286 AD. The division between east and west was encouraged by Constantine, who refounded the city of Byzantium as the new capital, Constantinople, in 330.
Military expenses increased steadily during the 4th century, even as Rome's neighbours became restless and increasingly powerful. Tribes who previously had contact with the Romans as trading partners, rivals, or mercenaries had sought entrance to the empire and access to its wealth throughout the 4th century.
Diocletian's reforms had created a strong governmental bureaucracy, reformed taxation, and strengthened the army. These reforms bought the Empire time, but they demanded money. Roman power had been maintained by its well-trained and equipped armies. These armies, however, were a constant drain on the Empire's finances. As warfare became more dependent on heavy cavalry, the infantry-based Roman military started to lose its advantage against its rivals. The defeat in 378 at the Battle of Adrianople, at the hands of mounted Gothic lancers, destroyed much of the Roman army and left the western empire undefended. Without a strong army, the empire was forced to accommodate the large numbers of Germanic tribes who sought refuge within its frontiers.
Known in traditional historiography collectively as the "barbarian invasions", the Migration Period, or the Volkerwanderung ("wandering of the peoples"), this migration was a complicated and gradual process. Some of these "barbarian" tribes rejected the classical culture of Rome, while others admired and aspired to emulate it. In return for land to farm and, in some regions, the right to collect tax revenues for the state, federated tribes provided military support to the empire. Other incursions were small-scale military invasions of tribal groups assembled to gather plunder. The Huns, Bulgars, Avars, and Magyars all raided the Empire's territories and terrorised its inhabitants. Later, Slavic and Germanic peoples would settle the lands previously taken by these tribes. The most famous invasion culminated in the sack of Rome by the Visigoths in 410, the first time in almost 800 years that Rome had fallen to an enemy.
By the end of the 5th century, Roman institutions were crumbling. Some early historians have given this period of societal collapse the epithet of "Dark Ages" because of the contrast to earlier times, (however, the term is avoided by current historians). The last emperor of the west, Romulus Augustulus, was deposed by the barbarian king Odoacer in 476. The Eastern Roman Empire (conventionally referred to as the "Byzantine Empire" after the fall of its western counterpart) had little ability to assert control over the lost western territories. Even though Byzantine emperors maintained a claim over the territory, and no "barbarian" king dared to elevate himself to the position of Emperor of the west, Byzantine control of most of the West could not be sustained; the renovatio imperii ("imperial restoration", entailing reconquest of the Italian peninsula and Mediterranean periphery) by Justinian was the sole, and temporary, exception.
As Roman authority disappeared in the west, cities, literacy, trading networks and urban infrastructure declined. Where civic functions and infrastructure were maintained, it was mainly by the Christian Church. Augustine of Hippo is an example of one bishop who became a capable civic administrator.