Supposedly the most famous British atrocity story concerning the Germans during World War I was the "Corpse-Conversion Factory"-rumor; it was said the Germans produced soap out of corpses. A story, which got so well believed that it was repeated for years - without a clear evidence of reality at that time. (
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Black propaganda does not tell its source. The recipient cannot find out the correct source. Rather would it be possible to get a wrong idea about the sender. It is very helpful for separating two allies.