Defence of the Realm
Diocletian came to power in 284 AD and held his position for over 20 years, despite the pattern of his predecessors, most of whom were killed while holding the position of emperor.
Diocletian was faced with two security concerns while emperor: the security of the empire itself and the frontiers surrounding it and the imperial office. He addressed the problem of security of the empire by increasing the size of the army. Diocletian created new legions though the size of these legions were substantially smaller (as earlier legions were made up of 5,000 officers, these legions were only composed of 1,000 men). However, the overall size of the army was considerably larger as Diocletian increased the amount of men by approximately one-third to close to 400,000 men. This increase in the size of the army led to much strengthening of the Empire's frontiers.
The strengthening of the army forced Diocletian to implemented tax reforms to counteract the decline in resources. These taxes were partly paid in coin, but also in kind, a reflection of the declining emphasis that was placed on the monetary economy.
The Expression of Power
The frequent assassinations on the emperor had destabilized the empire during the 3rd Century AD. As a result, when Diocletian came into power, he altered the concept of the emperor to make the emperor appear remote and aloof. When the emperor appeared in public, he wore a jeweled diadem, jeweled shoes and robes of purple and gold. Those who wished to approach the emperor were forced to kiss the hem of the emperor's robe. Diocletian changed the position of emperor from being merely a princeps (a "first citizen") into a dominus et deus ("lord and god"). The emperor became the absolute monarch; he was was only advised by a council that was appointed by himself.
Changing the organization in the empire was another improvement in imperial security. Previously, rebellions occurred in important frontiers because the provincial governor had civil and military forces at his disposal. This made it possible for that province to defy the central government in such a way that minimized any local opposition. Diocletian changed this by separating command of both civil and military forces. Now, each province had a both a civil governor and a military commander (or "dux"). The Severan provinces were further subdivided so those provincial governors there had less authority than before. For instance, Britain, which had previously been divided from one province into two, had been further divided into four by Diocletian. Within the empire, the provinces were grouped into 12 larger units, known as dioceses that were controlled by vicars who were directly responsible to the imperial administration.
Diocletian's co-option of colleagues was the most radical change imposed on the position of the emperor. This was a result of a problem facing the empire: no one person could control the actions of the entire empire. As a result, Diocletian appointed his first colleague, Maximian to the position of "Caesar" (or junior emperor) and later to "Augustus" (senior emperor). Soon after, two others were appointed as junior emperors to administer both the east and west portions of the empire. This division of power, or "tetrarchy", had important consequences on the future.
Constantine and Christianity
Diocletian's imperial reforms set the pattern for ruling for decades to come. However, the tetrarchy fell victim to ambition. When Diocletian abdicated in 305 AD, he forced Maximian to do so as well, leaving the junior colleagues, Constantius and Galerius to become the senior emperors. The tetrarchy was maintained when Constantius and Galerius appointed new junior emperors. However, when Constantius died in 306 AD, the tetrarchy fell apart. The western provinces recognized Constantine as the successor, grudgingly accepted by the other tetrarchs. Meanwhile, Maxentius, the son of Diocletian's colleague, Maximian, declared himself as the emperor of Rome, by seizing Italy and North Africa. The result of this conflict was a series of civil wars. It was finally decided that Constantine would be the sole ruler of the Roman Empire in 324 AD.
Administrative and military reform continued under Constantine. He ordered the division of the Roman army into frontier troops and mobile field units. Some thought that this weakened the frontiers of the empire. Constantine's decisions were based on previous experience, as he had much experience with war, especially with both the Germans and the Goths. However, his most famous innovation was the adoption of Christianity as the religion of the state.
In February of 303 AD, the eastern emperors Diocletian and Galerius had issued an edict ordering the destruction of churches and scriptures. This culminated in the command that everyone must offer sacrifice to the pagan gods. The Christians refused to do so, and subsequently were killed by the thousands for their disobedience. This persecution continued until 312 AD, but was spasmodic in nature after the first onslaught and didn't have much affect upon the western provinces.
Constantine adopted Christianity as his religion in October of 312 AD, shortly after his victory over Maxentius. He claimed to have seen a vision of the cross in the sky with the divine command "Conquer by this". Whether by policy or personal conviction, Constantine adopted the religion of Christianity shortly thereafter.
Constantine became involved in the matters of the Christian church and even went so far as to admit Christian bishops into his inner circle of counsellors. Temple treasures were confiscated and used to fund programs of church building, including the first St. Peter's in Rome and churches built in the Holy Places in Jerusalem and Bethlehem. Paganism didn't disappear, though edicts were issued discouraging and prohibiting pagan practices. Yet, non-Christians still held many high positions throughout the 4th Century.
The final action of note in Constantine's reign was the shift of the capital of the Roman Empire from Rome (a city known for the pagan religion) to Constantinople on the Bosphorus in 330 AD. Because Constantinople was a Christian capital, it marked the shift from paganism to Christianity. As well, it also marked a shift in the increasing importance of the eastern provinces, and less emphasis being put on the western ones.
The Successors of Constantine
Constantine died in 337 AD. By this time, the empire was divided into three distinct parts, each of which was given to his three surviving sons, Constantine II, Constantius II and Constans. His brother's stepson, Flavius Iulius Dalmatius, who was to be a fourth Caesar, was killed by Constantine's sons. Constantine II was killed in battle against his brother, Constans in 340 AD. Constans was later killed in 350 AD by Magnetius. Constantius was the sole survivor, who died a natural death in November 360 AD, but was about to do battle with his cousin Julian before he died.
All of Constantine's sons claimed to be Christian; however, Julian was a staunch advocate of traditional religion and did his best to revert the empire back to paganism. He removed the tax exemption of Christian clergy, and renewed the practice of pagan sacrifice. He provoked Christians by closing the Great Church at Antioch and threatened to rebuild the Temple at Jerusalem as a counterpoise to Constantine's church of the Holy Sepulchre. These actions made even pagan believers uneasy; however Julian was unable to carry out all of his wishes, as he died in battle against the Persians in 363 AD.
Successors to Julian were Christian, and they tried desperately to eradicate the empire of paganism by the end of the 4th Century AD. In 384 AD, the regional governor Cynegius ordered the closure of temples in Egypt. Emperor Theodosius prohibited pagan sacrifice and withdrew subsidies from pagan priests seven years later. The abhorrence of the pagans drove groups of Christians to attack pagan temples and synagogues. Christianity was so powerful that a prominent bishop such as Ambrose Milan could impose humiliating public penance on the Emperor Theodosius himself.
The 4th century AD was a time of increasing repression for those such as farmers. A law was passed in 332 AD, preventing farmers from avoiding payment of the poll tax. This was one example of the increasing authoritarianism of the empire. As well, the rich seemed to get wealthier, while the poor were being increasingly taxed and often suffered from much hardship. Some regions of the empire experienced economic hardships, while others, like Syria and North Africa, experienced prosperity at the end of the century.
The Gothic Invasions
Militarily, the final decades of the 4th century AD were dominated by the menace of the Goths. The Germans had settled north of the Black Sea and were already invading Asia Minor and the Balkans during the 3rd century. By the later 4th century AD, the Goths found themselves under pressure from a new enemy, the Huns, who were a nomadic tribe. The Goths subsequently sought refuge within the territories of the empire. Valens, the eastern emperor, allowed one group to enter, but later they rebelled after being maltreated. In 378 AD in a battle fought at Adrianople, the eastern empire's army suffered a defeat and Valens was killed.
At this time, the empire was divided into two distinct parts: the east and the west. The division took its final form when Valentinian I (364-375 AD) gave control of the east to his brother Valens (364-378 AD). However, in 378 AD, when the empire was in the state of crisis, the authority reverted to Valentinian's son and successor in the west, Emperor Gratian (367-383 AD). Theodosius I became the new eastern emperor. Theodosius's first task was to clear the Goths from the Balkans or at least bring them under control. This was only achieved by allowing them to settle within the empire under their own king, normally as an ally of Rome but effectively as an armed and autonomous people.
The Sack of Rome
Theodosius died in 395 AD. His young sons Arcadius and Honorius became the east and west emperors respectively. At this point, the Goths chose to rebel. With Alaric as their leader, the Goths proceeded to Constantinople; en route, they looted and killed many in Greece. In 397 AD, the Goths settled in Epirus (northwest Greece) for four years until 401 AD when they invaded Italy. The attack was turned back by Stilicho, a commander appointed by Theodosius. A second attack occurred in 407 AD and returned a year later for a third attack. While attempting to negotiate with the vacillating government of Honorius, the Goths lost patience and sacked Rome.
The event was considered a catastrophe even though the government was not located in Rome (it had been moved to Ravenna). The western empire was in crisis, not only from the Goths, but from rival emperors and by armies of Vandals, Alans, and Suebi who had crossed the Rhine and were ravaging Gaul. The Goths left Italy and ceded a kingdom centered on Toulouse in 418. Honorius survived five more years, dying of disease in 423 AD. By that time, Britain, with large areas of Gaul and Spain, were effectively beyond his control.
Honorius' successors in the west did not fare any better. The long reign of Valentinian III (423-455 AD) saw the defeat of the Hunnish leader Attila at the battle of the Catalaunian Fields in 453 AD, but failed to turn back the trend to fragmentation. The Vandals took over North Africa in 439 AD. The following western emperors Valentinian gradually yielded more power to the Germanic commanders who controlled their armies, eventually becoming little more than figureheads. The least of all, Romulus Augustus, abdicated in 476 AD, leaving his position with a pension to Campania.
The abdication of Romulus Augustus marks the end of the Roman Empire in the west, which henceforth was a mosaic of Germanic kingdoms ruled by Ostrogoths, Visigoths, Vandals, Franks, Saxons and others. With these territories, the Roman aristocracy survived, reading and writing in Latin and putting their administrative skills to the service of their new masters. By contrast, the Roman Empire remained strong in the east. The emperors frequently intervened in western affairs; the most powerful of whom, Justinian I (527-565 AD) re-conquered a substantial part of the lost western provinces. This territory was lost later, but the Byzantine realm survived, its Greek-speaking rulers continuing to style themselves "Emperors of the Romans" until the last of them died by the city walls when the Ottoman Turks conquered Constantinople in 1453 AD.