The Sudetenland was the area of mountains and hills forming a "rim" around the western part of Czechoslovakia (corrsponding to the modern Czech Republic). Although the area was inhabited mainly by ethnic Germans, it became part of the new state of Czechoslovakia when the Austro-Hungarian Empire disintegrated in 1918. Elements within the Sudeten German population, led by Henlein, agitated for union with Germany claiming discrimination by the majority Czech population of the new state; and they were supported in this by Adolf Hitler following his rise to power in 1933.
In 1938, by a mixture of threats and bluff, Hitler persuaded Britain and France to agree to the transfer of the Sudetenland to Germany (already enlarged by the annexation of Austria, or Anschluss, earlier the same year). This was immortalized by British Prime Minister Neville Chamberlain's quote that he had "bought peace in our time". The issue was never discussed with the Czechs themselves.
This not only gained Hitler kudos within Germany and with the Sudeten Germans, but it robbed Czechoslovakia of a defensible border and made the nation easy prey for the German subversion and aggression which led to its dismemberment in 1939. Following Germany's defeat in the Second World War, the Czech population and armed forces in 1945 turned on the now defenseless Sudeten German population, killing thousands and expelling the remainder.