Germany's invasion of Poland (September 1939) led Britain and France to abandon the policy of appeasement.
The policy of appeasement was signed by the prime ministers of Britain and France with Hitler in Munich in September, 1938. They had given in to Germany's annexation of the Sudentland as a German territory, including the evacuation of any Czech population from the region. After signing the Munich Pact, Hitler took control of all of Czechoslovakia (in March, 1939). Britain and France still did not pursue war with Germany when that happened. But when Germany invaded Poland in September, 1939, it was beyond clear that appeasing Hitler hadn't worked, and war was pursued.