The Fall of the (Western) Roman Empire
Spread of Christianity
Barbarians attacked Rome
By this time Christianity is the main religion of the Roman Empire. Thier loyalty was no longer towards the emperor or Rome but to Jesus.
Barbarians (anyone outside of the Roman Empire) attacked Rome. This was because Vikings, Turks, and the Hans were attacking them.
In 476, the last Roman Empire is dethroned.
Some believe that this is the reason the Middle Ages began.