sep 14, 395 - The Roman Empire Split
In 395 CE, Rome was divided into two empires - The Western Roman Empire and the Eastern Roman Empire. The empire was divided to improve communications and military responses to external threats. This division let to the Western Roman Empire being poorer than the East. This led to the sacking of Rome and Odoacer becoming emperor. Arguably, the split of the Roman Empire was the start of the downfall of the empire as it had an immideate impact on the sacking of Rome and Odoacer becoming emperor.
Added to timeline:
Events that Led to the Growth and Fall of the Roman Empire