I believe you're referring to the Dolchstoßlegende, (literally Dagger stab legend) which refers to a social theory popular in Germany in the period after World War I through World War II. It attributed Germany's defeat to a number of domestic factors. Most notably, the theory proclaimed that the public had failed to respond to its patriotic calling at the most crucial of times and some had even intentionally sabotaged the war effort.    The legend echoed the epic poem Nibelungenlied in which the dragon-slaying hero Siegfried is stabbed in the back by Hagen von Tronje.;)