Division of the Roman Empire
When Emperor Theodosius died in 395 he left the empire to his two sons, splitting it into the Western Roman Empire and the Eastern Roman (Byzantine) Empire.
William Smith / George Grove · Public domain · Source
Theodosius I settled the Goths inside the empire by treaty, made Christianity the state religion and banned other cults. He was the last emperor to rule both halves of the empire, and when he died in Milan in January 395 he left the east to his elder son Arcadius and the west to his younger son Honorius.
In law the two remained one Roman Empire, but they were never reunited. The richer, more populous east, with its capital at Constantinople, lasted until 1453, while the west, which bore the full brunt of the Germanic peoples, fell in 476. The division foreshadowed the split of Europe into Greek Orthodox east and Roman Catholic west. At this time King Gwanggaeto was expanding Goguryeo’s territory.