What movie won an Oscar for the signal motion picture during WWII?

"Prelude to War" is significant because it was produced as part of the Why We Fight series, created under the direction of Frank Capra for the United States government during World War II. This film served a very specific purpose: to educate American soldiers about the conflict and to rally public support for the war effort. Its combination of documentary footage, propaganda, and narrative storytelling was impactful in shaping public perception and morale. Winning an Oscar in this context highlights its importance not only as entertainment but also as an educational tool during a critical time in history.

The other films listed, while notable in their own right, either do not fall under the Oscar-winning category or did not serve the same primary purpose of educating and informing the public and military during WWII.