Why was appeasement a foreign policy failure for Great Britain and France?

The appeasement policy, in the international sphere, consists on making political or material concessions to an aggressive country or organization in order to avoid starting a conflict (war).

European Allied powers (UK and France), during the 1930s, made territorial concessions to Hitler as, they were still in the aftermath of WWI and they wanted to avoid a new war by all means. After signing the Munich Agreement, also known as Munich Betrayal, in 1938, Hitler managed to obtain the Sudetenland in Czechoslovakia with no opposition from the allies. It is significant to mention that France even had a prior alliance with Czechoslovakia and dishonoured it when embracing this agreement.

Of course Hitler was not willing to stop with these annexations and, in the end, the Allied powers had to declare war to Germany as he continued his territorial expansions unilaterally but, before, they had allowed Hitler to became stronger with no opposition.