Countries that seen their borders changed as a result of the WW I and WW II are numerous, as almost all countries involved, or that found in the middle of the wars got some changes, be it big ones or minimal ones.
Some of them are:
Greece; got 51% of the ethnic Macedonian territory, performing genocide in meantime, and retaining the territory, also got Southern Thrace and lot of islands from Turkey.
Finland; lost part of its territory on the southeast by the Soviet Union.
Poland; Lost a lot of territory, mostly by the Soviet Union.
Soviet Union; gained territories in the Baltic and Eastern Europe, often violating the human rights in the new territories.
Germany; initially gained a lot of territory, than lost most of it, and expreinced some slight adjustments in its borders.
Yugoslavia; increased with the territory of Macedonia (the Vardar part of it), thus extending further south.
