The Severan dynasty, which had lasted for 43 years, was brought to an abrupt end at the hands of Maximinus, a Thracian barbarian of enormous physical strength who had risen to a high position in the emperor's private guard. He had served the Severan family for over thirty years and was completely trusted when he murdered Alexander Severus, seized the throne, and plunged the imperial government into chaos. He killed his enemies, which included virtually anyone from the upper classes, without mercy. He was murdered by his own troops after less than three wretched years in power, but the empire never recovered from this upheaval. The military anarchy which followed lasted until the reign of Diocletian, saw over twenty emperors in the space of 50 years, only one of whom died a natural death. None were distinguished, and the only notable event of the period was the rebellion of Queen Zenobia of Palmyra in Syria. She came close to conquering the eastern half of the Roman empire, but was put down by the emperor Aurelian in 272.
Diocletian, who came to the throne in 284, did a masterful job of bringing order to an empire in chaos. He divided the empire into four districts, two in the east and two in the west, and appointed a junior and senior governor of each division (called caesar and emperor respectively). Upon the death or retirement of the emperor, the caesar would be elevated to emperor and appoint another caesar. This system worked for exactly one generation, but it allowed Diocletian to retire, and live out his natural life unmolested, an accomplishment nearly unprecedented in imperial history.
One of the caesars appointed by Diocletian was Constantius, the father of Constantine. When Constantius died, Constantine was elected to replace him. He spent the early part of his reign consolidating power by fighting off rivals from both the east and west. The second half of his reign was dedicated to civil reforms and building his new capital at Constantinople. Most notably, Constantine was the first Christian emperor, and his edict of Milan in 313 made Christian worship legal throughout the empire. From this point on, with the exception only of Julian the Apostate, the imperial court was at least nominally Christian.
The peace and prosperity which took root during the thirty year reign of Constantine was short lived. On his death the empire was divided among his three sons but they quarreled among themselves while the empire sunk slowly back into disorder. All of Constantine's sons died without heirs, and after the death of his nephew, Julian the Apostate, the empire was permanently divided into an eastern and a western half. The only remaining emperor of note was Theodosius, who ably governed the eastern empire from 379 to 395, and put down some of the early Visigoth invaders. He is remembered for his willingness to do public penance for the slaughter of the Thessalonians, which was imposed on him by Ambrose, Bishop of Milan. The idea that even emperors were subject to the laws of God was a radically new idea that made a permanent mark upon Western civilization.
Meanwhile, the empire of the west was already suffering from waves of barbarian invaders that the government was powerless to put down. By the time that the city of Rome was overrun by the Visigoths in 410, most of Gaul had already been abandoned to the invading Franks and the legions had been pulled from Britain in hopes of defending Italy.
The waves of barbarians that descended upon Italy during the fifth century only finished off a process that was already under way. The Western empire had ceded much of its territory without a fight, most wealthy families had moved away from Rome and the emperor himself had moved to Ravenna. By the time the city of Rome was invaded there was not even an army to send in its defense, since the cowardly Honorius, who sought only to appease the Visigoths, had murdered Stilicho, his most capable general. Still, the Visigoth invasion of 410 was mild compared to that of the Vandals, who plundered the city to ruin in 455. The Visigoths were at least Christian, semi-civilized, and desired a treaty with the Western Empire that would allow them self-governing territories. This they eventually obtained, and a Visigoth empire was established in Spain shortly after the death of Alaric the Visigoth. The Visigoths were allies of the Western Empire as long as it lasted and helped to ward off Attila the Hun, who overran Western Europe in 450 A.D.
By this time the area actually controlled by the Western emperor was reduced to Italy, and when it passed from the last Roman emperor, Romulus Augustus, to the barbarian chief Odoacer in 476, it caused hardly a ripple. Odoacer was soon overthrown by Theodoric the Ostrogoth, who ruled Italy for many years but made no pretense of being an emperor. Ten years later, the last Gallo-Roman province of France, was conquered by Clovis. At that point, all of the old Roman provinces in Gaul, Hispania, and Italy were controled by German chieftains, who preserved some of the old Roman customs, but governed as independent commanders.
As the western empire collapsed, the power and influence of Christianity increased. Because of the fluid organization of the church, it was able to adapt and grow in an environment of political unrest. Kingdoms, chieftains, and empires might come and go, but the church provided a degree of continuity and civilization that was increasingly attractive to citizens of the collapsed empire. Many important leaders of the church who arose during this time of chaos while political powers rose and fell. Some of the influential christian leaders who lived during the decline of the Roman Empire were Saint Jerome, Saint Augustine , Saint Ambrose, Eusebius, Saint Athanasius, and Saint Benedict.
Several generations after the fall of the Western Empire, the Eastern Empire enjoyed a brief resurgence. Between 530 and 560 A.D., under the rule of Justinian the Great, Constantinople won back much of the territory that had been lost to barbarians in the west. These included the reconquest of North Africa from the Vandals, the reconquest of Italy from the Ostrogoths, and several important victories against the Sassanid Empire in Persia. These victories were almost all due to the efforts of Belisarius, one of the greatest generals in Roman history, and for a brief time it looked as if the Roman Empire would reemerge as a dominant power. But a long period of decline followed the brilliant career of Belisarius, and Northern Italy was overrun by the Lombards only a few years after his death.