The stab in the back first gained currency in Germany, as a means of explaining the nation’s stunning defeat in World War I. It was Field Marshal Paul von Hindenburg himself, the leading German hero of the war, who told the National Assembly, "As an English general has very truly said, the German army was ‘stabbed in the back.’"
Like everything else associated with the stab-in-the-back myth, this claim was disingenuous. The "English general" in question was one Maj. Gen. Neill Malcolm, head of the British Military Mission in Berlin after the war, who put forward this suggestion merely to politely summarize how Field Marshal Erich von Ludendorff–the force behind Hindenburg–was characterizing the German army’s alleged lack of support from its civilian government.
The full article traces the legend back to Old Norse folklore, discusses its adoption by the Nazis, and then tackles its use by the American right since the Second World War:
Since the end of World War II it has been the device by which the American right wing has both revitalized itself and repeatedly avoided responsibility for its own worst blunders. Indeed, the right has distilled its tale of betrayal into a formula: Advocate some momentarily popular but reckless policy. Deny culpability when that policy is exposed as disastrous. Blame the disaster on internal enemies who hate America. Repeat, always making sure to increase the number of internal enemies.
Read, and learn.