The many events which are being dealt with on these pages happened at different times, in different places and under quite different general historic conditions. One thing all types of forced migration do have in common, however, is that the members of an ethnically defined section of the population had to leave their homes because they belonged to that very group. This is why all these phenomena can be subsumed under the term forced migrations.

There is a number of terms used for forced migrations and related phenomena, terms which on sthe one hand describe different events but on the other hand can be used because they are linked to a specific interpretation of historic events. The terms "Aussiedlung" ("relocation") or "Umsiedlung" ("resettlement") suggest an orderly procedure. A similar connotation adheres to the Czech expression odsun ("expulsion") or the Polish term "repatriation", which was used during the communist era for the forced resettlement of the Polish population from those areas that had been annexed to the Soviet Union after World War II.

Forced migration always originates with governments (or de facto governments, as in former Yugoslavia), which use the national power structure in order to enforce these activities. Almost all forced migrations take place in connection with wars or civil wars.

- Forced migrations can be effected by governments who transfer members of their "own" ethnicity living in other countries to their own state (similar to the model of the "population exchange" conducted between Greece and Turkey). The basis for such procedures are international treaties between governments.

- During World War II, Germany, as an occupying power, had large areas of land cleared for German settlers, for instance in Poland and Czechoslovakia ("Generalplan Ost", or "General Plan for the East"). Deportations of people into forced labour camps, concentration camps and ghettos were a major part of German occupation policy.

- The forced resettlement of national minorities is the third important type of forced migrations. It happened, for instance, after World War II in the liberated countries of Eastern Europe or in former Yugoslavia in the 1990s.

Some connected phenomena, which often take place at the same time are- so-called "wild expulsions", which are not organised centrally, but come about locally and spontaneously (in many cases, however, they are tolerated or even secretly steered by the state).- Escape, e.g. from an approaching army. The term 'evacuation' implies that it is organized by governmental or military authorities.- Genocide, which does not aim at expulsion, but at the deliberate and systematic death of the members of an ethnic group and the extermination of the group as a whole.The interrelation between these types of events, the dividing line between which can become quite fluid is currently discussed very vividly by historians.In the Federal Republic of Germany, the term expulsion (Vertreibung) and the word expellee (Vertriebener) have become general usage after World War II. The Federal Expellee Act of 1953 was a major influence in this respect, while in the years prior to this act, other terms had been used, especially the expressions "escape" and "refugees" (c.f. the sources stemming from that time).

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