How were britain and france drawn into war with germany during world war ii?

In 1938, the prime ministers of Britain and France signed the Munich Pact with Adolph Hitler of Germany, allowing his annexation of Sudetenland territory. Others termed this "appeasement" of Hitler, and it turned out to be the case that Hitler didn't stop with the Sudetenland. He then took over all of Czechoslovakia. Finally, when the Germans invaded Poland in September, 1939, the British and French felt compelled to go to war against the German aggression.