1930s American and European cinema

Because hindsight is 20/20 it is very difficult to see how anyone could allow a political party to take over their country, abolish any piece of democracy, establish a dictatorship, get the country into a full-blown world war, and carry out a mass annihilation of over 11 million people, six million of which belong to one cultural group the way Adolf Hitler and the Nazis did in the 1930s-1940s.  Because this is a cinema history class we, naturally, focussed on how films played a role in this.  Please comment on how this happened (cinematically) including who the main players were.  Further comment on how we, now, can see something like this coming and avoid buying into this type of propaganda in the future either here or abroad.