WWII was a case for own, especially on Germans who were resettled by force from the Central and Eastern Europe into Germany. Some areas of Czech Republic and Poland still bear marks of sudden depopulation from 1945-1948 and had not recovered. Western Bohemia is one of the prime example, so is Silesia in Poland. Mass population movements even by violent means are nothing new in the world's history. During the movement of nations, several millions of people from Goths to Huns to Slavs to Saxons moved westward and in the process destroyed anything what was associated with Roman civilization. Often these people did NOT want to destroy Rome, they just wanted to live as good and prosperous as the Roman citizens, but they were unable to cope with the requirements of the Roman society. China experienced similar movement under the onslaught of Mongols in the 13th and 14th century where several millions of them migrated to south to escape them. Generally, the mass exodus is triggered by some widespread crisis. They spell nothing good and will destabilize everything in the path.
I believe the outcome from it will be collapse of the EU as political entity and each country will protect is borders from invasion. Hungary is already deploying militarized force, Czech Republic and Slovakia is very close to seal borders by its military. There is very close step the refugee crisis will be stopped by military means at any cost. I am old enough to remember Iron Curtain and how borders were sealed and very difficult to travel within Eastern Block or to the West. Germany, Sweden, France, and UK can very little do to stop them, and besides, these countries care less what the European powerhouses think. They see them as weak.