Czechoslovakia was a country in central Europe, lasting roughly from the end of World War I until the Fall of the Soviet Union. Before then it was part of Austria-Hungary and since then it has split into Czechia and Slovakia.
Czechoslovakia is notable for having produced some very interesting postage stamps over the years and for having been on the losing end of the Munich Agreement just prior to World War II. The Skoda Works a large armaments factory, was located there.
The Sudetenland was a mountainous region in northwestern Czechoslovakia that contained a majority of German-speakers.