Cause and effect: please, explain ...
... how the following events are connected (1&2, 2&3, 3&4, 4&5):
- Mass migrations (from KF 2)
 - Fall of the Western Roman Empire (from KF 2)
 - Declining trade in Western Europe 
 - People flocked from the cities to the countryside 
 - A self-sufficient society arose on domains
 
In other words: how did event 1 cause event 2 to happen? How dit event 2 cause event 3 to happen? etc ...