Peace on Earth is a 1939 Metro-Goldwyn-Mayer cartoon short subject directed by Hugh Harman, about a post-apocalyptic world populated by animals. Two young squirrels ask their grandfather on Christmas Eve what the "men" are in the lyric "Peace on Earth, good will to men." Then the grandfather tells about the human race that was always fighting and their last fight.

The animation was nominated for the 1939 Academy Award for Short Subjects and you can watch it here (9 minutes). It's naïf, as most part of cartoons for children, however it has a very positive message. Pay attention to how grandpa defines men, as There was always a fight. Sadly his definitions are more actual than ever. (via MilkandCookies)