Sudeten -Germans after 1920
The Sudeten Mountains are in Bohemia, which was once arguably attached to the Frankish kingdoms ruled by the Carolingians via a tributary relationship. The people who lived there in the ninth and tenth centuries were mostly Slavs. It was an independent Kingdom during the early modern period, where there was the normal interbreeding between local Slavic rulers and the rest of Europe's ruling families (that same interbreeding that later put a bunch of Germanic types on the English throne, and made a Battenburg a Greek prince.) Later, the Austro-Hungarian Empire absorbed it.
Because of the constant movement of peoples in response to the changing boundaries resulting from regular warfare and re-drawing of boundaries that plagued Central Europe from the Middle Ages to the present time, an enclave of German people ended up living in these Sudeten mountains and surrounding areas. Their occupancy has continued through the establishment of the state of Czechoslovakia and later, through the division of that state into its successor republics, the Czech Republic and Slovakia. Hitler used the excuse of a native German population in the Sudetenland as one of his excuses for his invasion of that area. These people are sometimes known as Bohemia-Germans ("Boehmen-Deutsche").