On the grounds that the territorial losses of Germany in the Treaty of Versailles were unfair,and current European leaders recognized this.
However,this was palpable nonsense - neither Austria nor the Sudetenland had EVER been a part of Germany.The real reason the leaders in France and Britain didn't do anything about German aggression was because they didn't want another war,so appeased Hitler instead,hoping that he would eventually be satisfied,and that there would be no need for another war to stop him.