Thesis published after World War I, which says, that parts of the German population and not the "undefeated" Wehrmacht is the reason for the defeat.
Dolchstoß (dagger thrust), is a symbol for this: The Wehrmacht treacherously murdered by its own people. Often used by right-winged politicans in the Weimarer Republik against the republic, socialists and social-democrats.
This thesis has been falsified by research.