Don't Be a Sucker - 1947
An American who has been listening to a racist and bigoted rabble-rouser is warned off by a naturalized Hungarian immigrant, who explains to him how racist and bigoted demagogy allowed the Nazis to rise to power in Germany, and warns Americans not to fall for similar demagogy propagated by American racists and bigots.
https://en.wikipedia.org/wiki/Don't_Be_a_Sucker!
Financed and produced by the United States War Department, and shot at the Warners studio, although it was distributed through all of the major studios' film exchanges and also by National Screen Services free to the theatre exhibitors: A young, healthy American Free Mason is taken in by the message of a soap-box orator who asserts that all good jobs in the United States are being taken by the so-called minorities, domestic and foreign. He falls into a conversation with a refugee professor who tells him of the pattern of events that brought Hitler to power in Germany and how Germany's anti-democratic groups split the country into helpless minorities, each hating the other. The professor concludes by pointing out that America is composed of many minorities, but all are united as Americans. (Reissued in 1946 following the end of World War II. Written by Les Adams <
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http://www.imdb.com/title/tt0275310/