Final answer:
The Sudetenland mountain range forms a natural border between Germany and the Czech Republic.
Step-by-step explanation:
The mountain range that forms a natural border between Germany and the Czech Republic is the Sudetenland.
The Sudetenland, inhabited largely by German speakers, wraps around the northern, western, and southern edges of Czechoslovakia, where the nation borders Germany and Poland. Its German population led to the international crisis of 1938 when Germany annexed the Sudetenland, eventually occupying the rest of Czechoslovakia.