Here's the summary for the wikipedia article you mentioned in your comment:

A useful idiot is a person perceived as propagandizing for a cause—particularly a bad cause originating from a devious, ruthless source—without fully comprehending the cause's goals, and who is cynically being used by the cause's leaders. Erroneously attributed to Vladimir Lenin, the term was often used during the Cold War to describe non-communists regarded as susceptible to communist propaganda and psychological manipulation. Similar terms also exist in other languages.

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