stab in the back

stick a knife in someone's back; betray someone

The stab-in-the-back legend (German: Dolchstoßlegende, literally "Dagger stab legend") refers to a social myth and persecution-propaganda 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 instead of failed militarist geostrategy. 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."
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say or do something unfair that harms a friend or someone who trusts you My friend stabbed me in the back even after I made an effort to help him get a job.

PUGNALARE ALLE SPALLE