an antisemitic and anticommunist conspiracy theory that was widely believed and promulgated in Germany after 1918. It maintained that the Imperial German Army did not lose World War I on the battlefield, but was instead betrayed by certain citizens on the home front – especially Jews, revolutionary socialists who fomented strikes and labor unrest,[1] and republican politicians who had overthrown the House of Hohenzollern in the German Revolution

https://en.wikipedia.org/wiki/Stab-in-the-back_myth

Stab in the back myth