The fall of the Western Roman Empire (476)
Visigoths, Vandals, Huns. In 410, Rome is sacked for the first time in 800 years. In 476, the last Western emperor is deposed.
The philosophical cause
Augustine — Pagans said: the Christians betrayed the gods, and the gods turned away from Rome. Augustine writes the City of God as his answer: earthly empires always fall.
The Church survives where the state has collapsed. It is the Church that will preserve culture through the Middle Ages.