Answer: Sudetenland; Czechoslovakia
The Sudetenland is a historical area of former Czechoslovakia which was inhabited by Sudeten Germans. After WWI, Austria-Hungary was dismembered and the Sudeten Germans found themselves in the new country of Czechoslovakia.
With the rise of Pan-Germanism before and during WWII, Germany claimed the Sudetenland, and annexated it after the Munich Agreement. After the war, Czechoslovakia expelled the Sudeten Germans from their territory, and the area is now almost exclusively inhabited by Czech speakers.