4th century A.D Roman Britain
In the 4th century A.D Roman Britain was a sophisticated Latin-speaking culture with impressive technologically advanced cities, engaged in extensive and lucrative commerce throughout the known western world, and supported by the wealth of the empire. But around the beginning of the 5th century, the steepest and most rapid and precipitous decline in British history began. As trouble back home and elsewhere in the empire forced the Romans to withdraw, the British cities fell into ruin and were abandoned. The people lost not only the ability to read and write but also the ability to build with stone. With the collapse of commerce and with no skilled labor available, there was no way to get pottery or manufactured goods and with the loss of all higher knowledge, there was no ability to make those things themselves.
As the cities were abandoned the people returned to subsistence farming. Attacked from all sides by Picts, Scots, Irish, Angles, Saxons, and others, the Pax Romana was replaced by brigandry and looting, and anarchy. Christianity disappeared, replaced by the Germanic and Scandinavian religions of new conquerors. Amid widespread misery and economic, cultural, and intellectual decline, the population of the place the Romans had named Britannia plummeted. The population of Britannia when the Romans began to withdraw was about 4 million. Six hundred years later, the population was only about half of that. Not until the 13th century, nine hundred years later, did the population return to the level it had been during the time of Roman rule.
World history has generally been marked by a trajectory of progress and improvement. But as the history of Britain shows, that progress will not necessarily occur in a straight line and can be interrupted by periods of sudden and dramatic decline.
The image is from the Canterbury Roman Museum.