He means the fall of Rome and organized state, and the regression due to barbaric attacks during the middle ages.

It took several centuries for the germanic, etc. tribes that defeated Rome to become civilized enough to Roman standards, and build equal structures and quality of life.

(As a point of comparison, Egyptians, Greeks, Romans, etc. had centuries of written history, whereas e.g. the British written history starts with the Roman invasion of the island, and starts by Romans).