Germany's annexation of the Sudentland, a portion of Czechoslovakia.
In the spring of 1938, Germany annexed Austria to become part of Germany. (Austria and Germany had always been closely related.) Then Adolph Hitler made public his desires to bring the Sudentland into Germany as well. The region was in Czechoslovakia, but had a large German population and was seeking to align itself with Germany. The British and French prime ministers met in Munich with Hitler, and signed the Munich Pact with Hitler on September 30, 1938. The agreement granted Germany permission to annex the Sudentland -- which is where the British and French hoped Hitler would stop. But soon after, he took over all of Czechoslovakia. Their policy of "appeasement" hadn't worked. When Germany later invaded Poland, in September, 1939, the Second World War began.