POD - Late fourth century - see below
Barbarian Invasions - Fifth Century
Age of Justinian - Sixth Century
When Western Emperor Valentinian I (364-375) died, the empire was divided between his sons - Gratian got Gaul (OTL France), Britain and Hispania (OTL Spain), while Valentinian II got Italy and the African provinces. Meanwhile, Valens, Valentinian's brother, was ruling in the west. In the year Valentinian died, a group of Huns invaded Eastern Europe. The Goths, fleeing from them, asked Valens if they could settle in Illyria (OTL Serbia and Croatia) in 376. Valens allowed a small group led by a cheiftain called Fritigern to settle there. However, more Goths, Alans and even Huns migrated into the borders of the East Roman Empire. When the Roman generals treated the tribes badly, they started a rebellion early the next year.
At the same time, Gratian and Valentinian II decided that they wanted the whole empire for themselves, not just the Western part. They gathered an army, and leaving General Mallobaudes behind to look after any barbarian threats, marched off east. When they got there, they found that Valens, having withdrawn all but a very small force to defend against the Persians, was in Constantinople (OTL Istanbul) about to launch an attack on the rebellious Goths. Gratian formed an alliance with the Goths and they jointly attacked Constantinople. After a very short siege, Gratian entered the city triumphant.
Then, he turned on the Goths. A surprise attack was launched consisting of the army Gratian brought to deal with Valens and the army Valens brought to deal with the Goths. The Goths were defeated, but the battle was long and bloody, and Gratian was killed. He managed to nominate his successors - Valentinian II would be Western Emperor, and a general named Theodosius Eastern Emperor.
In 383, a general named Magnus Maximus revolted in Britain and invaded Gaul. He then took over Hispania. After three years, he then invaded Italy. Valentinian tried to flee to Theodosius' Eastern Empire with his mother, but was killed. Theodosius, succeeding Valentinian, invaded and defeated Magnus. He now ruled over the whole empire from Constantinople.
Theodosius was a Christian. He made Christianity the state religion and tore down pagan temples and altars. He persecuted anyone who was seen practicing a pagan religion. He ended the Greek Olympic games, which were seen as being pagan.
Theodosius died in 395. He thought about dividing the empire, but after all the civil wars that had resulted from the empire's division, he decided that Arcadius, his eldest son, shold rule the empire, maybe with Honorius, his younger son, as his junior. Arcadius, when he became emperor, was dominated by Rufinus, one of his ministers, because he was only 18. Stilicho, the magister militum (master of the soldiers) wanted control of the emperor, so Rufinus was quickly assassinated, but another man called Eugenius became Arcadius' advisor.
Next: Barbarian Invasions