Useful idiot is a derogatory expression stated to mean "(Originally) a citizen of a non-communist country sympathetic to communism who is regarded (by communists) as naive and susceptible to manipulation for propaganda or other purposes; (more widely) any person similarly manipulable for political purposes."
The earliest use of the expression has been stated to be in the New York Times in the 1940s. 
- Fellow traveler
- Great Purge - Included widely publicized show trials meant to be propaganda aimed at foreign Communist sympathizers.
- McCarthyism - On the widespread Communist infiltration just in the United States.
- Willi Münzenberg - Created numerous Communist front organizations that he termed "Innocents' Clubs".
- xforddictionaries.com: Useful idiot https://www.oxforddictionaries.com/us/definition/american_english/useful-idiot