What event caused the policy of appeasement to be viewed as a failure

Okay, so appeasement was applied to Hitler. Hitler wanted the Sudetenland and British Prime Minister believed that if he gave him the Sudetenland that would appease him and he wouldn't want any more land. So, he and Hitler signed the Munich Pact in which Hitler he promised he wouldn't invade the rest of Czechoslovakia, but he broke that promise and did indeed invade Czechoslovakia. This is why appeasement would be considered a failed policy.