<P> The Roman Empire reached its greatest territorial extent during the second century AD; the following two centuries witnessed the slow decline of Roman control over its outlying territories . Economic issues, including inflation, and external pressure on the frontiers combined to create the Crisis of the Third Century, with emperors coming to the throne only to be rapidly replaced by new usurpers . Military expenses increased steadily during the third century, mainly in response to the war with the Sasanian Empire, which revived in the middle of the third century . The army doubled in size, and cavalry and smaller units replaced the Roman legion as the main tactical unit . The need for revenue led to increased taxes and a decline in numbers of the curial, or landowning, class, and decreasing numbers of them willing to shoulder the burdens of holding office in their native towns . More bureaucrats were needed in the central administration to deal with the needs of the army, which led to complaints from civilians that there were more tax - collectors in the empire than tax - payers . </P> <P> The Emperor Diocletian (r . 284--305) split the empire into separately administered eastern and western halves in 286; the empire was not considered divided by its inhabitants or rulers, as legal and administrative promulgations in one division were considered valid in the other . In 330, after a period of civil war, Constantine the Great (r . 306--337) refounded the city of Byzantium as the newly renamed eastern capital, Constantinople . Diocletian's reforms strengthened the governmental bureaucracy, reformed taxation, and strengthened the army, which bought the empire time but did not resolve the problems it was facing: excessive taxation, a declining birthrate, and pressures on its frontiers, among others . Civil war between rival emperors became common in the middle of the 4th century, diverting soldiers from the empire's frontier forces and allowing invaders to encroach . For much of the 4th century, Roman society stabilised in a new form that differed from the earlier classical period, with a widening gulf between the rich and poor, and a decline in the vitality of the smaller towns . Another change was the Christianisation, or conversion of the empire to Christianity, a gradual process that lasted from the 2nd to the 5th centuries . </P> <P> In 376, the Goths, fleeing from the Huns, received permission from Emperor Valens (r . 364--378) to settle in the Roman province of Thracia in the Balkans . The settlement did not go smoothly, and when Roman officials mishandled the situation, the Goths began to raid and plunder . Valens, attempting to put down the disorder, was killed fighting the Goths at the Battle of Adrianople on 9 August 378 . As well as the threat from such tribal confederacies from the north, internal divisions within the empire, especially within the Christian Church, caused problems . In 400, the Visigoths invaded the Western Roman Empire and, although briefly forced back from Italy, in 410 sacked the city of Rome . In 406 the Alans, Vandals, and Suevi crossed into Gaul; over the next three years they spread across Gaul and in 409 crossed the Pyrenees Mountains into modern - day Spain . The Migration Period began, when various peoples, initially largely Germanic peoples, moved across Europe . The Franks, Alemanni, and the Burgundians all ended up in northern Gaul while the Angles, Saxons, and Jutes settled in Britain, and the Vandals went on to cross the strait of Gibraltar after which they conquered the province of Africa . In the 430s the Huns began invading the empire; their king Attila (r . 434--453) led invasions into the Balkans in 442 and 447, Gaul in 451, and Italy in 452 . The Hunnic threat remained until Attila's death in 453, when the Hunnic confederation he led fell apart . These invasions by the tribes completely changed the political and demographic nature of what had been the Western Roman Empire . </P> <P> By the end of the 5th century the western section of the empire was divided into smaller political units, ruled by the tribes that had invaded in the early part of the century . The deposition of the last emperor of the west, Romulus Augustulus, in 476 has traditionally marked the end of the Western Roman Empire . By 493 the Italian peninsula was conquered by the Ostrogoths . The Eastern Roman Empire, often referred to as the Byzantine Empire after the fall of its western counterpart, had little ability to assert control over the lost western territories . The Byzantine emperors maintained a claim over the territory, but while none of the new kings in the west dared to elevate himself to the position of emperor of the west, Byzantine control of most of the Western Empire could not be sustained; the reconquest of the Mediterranean periphery and the Italian Peninsula (Gothic War) in the reign of Justinian (r . 527--565) was the sole, and temporary, exception . </P>

What were the defining characteristics of the european middle ages