Complex sentence: Atrocity propaganda does not fulfill these requirements and is thus known to be the least effective form of propaganda in the long-term if the lies are discovered, and has even been proven to have a negative effect on individuals.
Split into simpler sentences: Atrocity propaganda does not fulfill these requirements and is thus known to be the least effective form of propaganda in the long-term. If the lies are discovered atrocity propaganda has even been proven to have a negative effect on individuals.