Because they were going to join Germany.
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The German army moved into the Sudetenland 16 March 1939, the Munich Agreement, which gave the land to Germany happened in September 1939. The Czechoslovakian government circulated 21 September and a few days later the Nazi's installed a puppet government.
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in 1938, Britain and France signed the Munich pact with Germany and Italy. The pact permitted German annexation of border areas of Czechoslovakia (sudetenland).
The Sudetenland
In 1938, 28.8% of the Sudetenland population were Germans.
Hitler believed that there was a vast majority of Sudeten Germans in the Sudeten Land, and that by taking over it he was only complying with Wilson's idea of self-determination. He also thought that if in a later war he were to be attacked by the Russians that it would act as a first line of defence. Also by taking over Czechoslovakia he would be that one step closer to Poland and his policy of expanding East.
The Sudetenland crisis began at the beginning of 1938 and chamberlain flew out to meet Hitler on September the 15th.