What event led to the development of the two different European civilizations of the Western Roman Empire and the Eastern Roman Empire?

Answer: Invasions by Barbarian tribes.

During the 4th century, the Empire remained unified, with its headquarters in Constantinople. At the end of the century, Emperor Theodosius established, in 395, the definitive division: Roman Empire of the West, with capital in Rome, and Roman Empire of the East, also called Byzantine Empire, with capital in Constantinople.