On January 17, 395 Theodosius The Great passed away, leading to the final partition of the Roman Empire. Theodosius I was the last Emperor who ruled over the whole Roman Empire.
One of the few Roman Emperors to be honoured with the epithet “Great”, Theodosius was not born in purple, nor did he have strong ties to the Imperial Family; he was the son of a senior military officer, Theodosius the Elder. He was an able commander from very young age; among other campaigns, he accompanied his father to suppress the Great Conspiracy of 368 in Britannia, aged just 20 at the time. Just a couple of years later, he became a military commander in his own right.
At the time of his rise to power, the Roman Empire was ruled by co-Emperors. First, brothers Valentinian I and Valens ruled the Empire. After Valentinian died in 375, his sons – Valentinian II and Gratian – succeeded him as rulers of the Western Roman Empire, while Valens remained ruler of the Eastern part. However, Valens himself passed away just three years later, in 378; he had no successors. To fill the vacuum and knowing he would not be able to control the Easter Empire alone, Gratian invited Theodosius to take command of the Illyrian army, thus effectively becoming his co-ruler.
Emperor Gratian died five years later, in 383. As he had no sons and had not appointed a successor, Theodosius became the de-facto sole ruler of the East. Before long, however, he appointed his elder son, Arcadius (then a 5-year-old child) his co-ruler, thus cementing the succession of his own line. During the reigns of Theodosius in the East and Valentinian II in the West, the Empire lived through relatively harmonious times; Gratian was always a political supporter of Valentinian and the latter paid back with similar loyalty.
Valentinian II died in 392; he was widely believed to have been assassinated by or on orders of one Arbogast. Arbogast knew he had no chances to become an Emperor himself among other reasons because he was a Frank and not Roman. And thus, he arranged the elevation of his protégé, Eugenius to the purple. Theodosius immediately rejected the new Emperor and rather easily (and without much opposition) conquered the Western Empire. Nominally, he had co-rulers in both parts; his elder son Arcadius in the East and his younger son Honorius in the East. However, from 392 and until his death in 395, Theodosius was the de-facto sole ruler of the entire Empire.
When Theodosius died in 395, his sons (and co-Emperors) were proclaimed Emperors in respectively Eastern and Western Empires; at the time, Arcadius was not yet 18 and Honorius was only 10. The elder boy became the sole ruler of the Easter Empire with his capital in Constantinople, while Honorius became ruler of the Western part with his capital in Milan. Although the two Empires were politically different states and were ruled by different Emperors, throughout the 5th century two halves were nominally, culturally and historically the same state.