After the death of Roman Emperor Valens, Theodosius I (379-395 AD) was elected as an Emperor of the eastern part of the Roman Empire. Although he inherited a chaotic situation, he succeeded thanks to his talent and abilities, to bring an order to some extent in the very difficult condition of the Empire. He used his fortune and Goths priests in order to make an agreement with the Visigoths, and he had a very capable general, Flavius Stilicho, (who was the Vandal).
Stilicho would bring order in the places where diplomacy was stalled. At the same time, while he brought order in the relations with the barbarians, he had to fight with his competitors in the West. Almost at the end of his life, he managed to defeat his competitors. Theodosius was the last Emperor, who reined an entire Roman Empire.
Theodosius was often called the Great, mostly because of his good relationship towards the Church (similar as Constantine I). In fact, he finished a reform that was launched in the time of Constantine, and he proclaimed Christianity the only official religion of the Empire, a decision of the First Council of Nicaea was only one true faith. Arianism was placed out of the Law, and he ordered that all their churches had to be handed to the Catholic priests. He attacked all possible enemies of Christianity.
Therefore, since 391 AD, all pagan temples and events have been banned. This ended the tradition of Delphic oracle or the Olympic Games. Very shortly before his death (395 AD) Theodosius stripped many temples of old religion in Rome, and statues of ancient idols he had thrown out on the streets. He also ordered the senators to choose whether Jupiter or Jesus will manage an Empire – of course most of them converted to Christianity.
Theodosius the Great ruled the Empire very efficiently, but he was aware that neither of his two sons have his ability to govern an Empire. He decided therefore to divide the Empire 395 AD between the sons: ten-year-old Honorius received the Western half of the Empire, while eighteen-year-old Arcadius received the Eastern half of the Empire. There was no apparent reason for apprehension in the public, because the Empire has already been separated before (like earlier tetrarchy), and then it became united again.
However, this time it was different, this division became permanent, although that was not Theodosius intention. Honorius and Arcadius were quite useless as rulers. After them it did not appeared nor in the East or in the West powerful Emperor which could reunite two Empires. After that 395 AD, the West was practically left to itself, with little ability to withstand the full force of the barbarian peoples who insisted to cross the border.