During WWII, German films to be used in Germany for propaganda reasons were often intercepted by the Allies and sent to Canada. The National Film Board, under John Grierson, would then use the German footage in Allied propaganda.
Churchill's Island, perhaps the most famous of the Western propaganda, extolls the virtues of the Allied cause while using Nazi footage to show the evils of fascism.
While this film seems very dated (and with good reason, it's 60 years old), it's still a fascinating look at the use of a relatively new media as a tool of social control. 8/10.