Peace on Earth is a one-reel 1939 Metro-Goldwyn-Mayer cartoon short directed by Hugh Harman, about a post-apocalyptic world populated only by animals, as it is claimed in the short that human beings have gone extinct due to war.

Two young squirrels ask their grandfather (voiced by Mel Blanc) on Christmas Eve who the “men” are in the lyric “Peace on Earth, good will to men.” The grandfather squirrel then tells them a history of the human race, focusing on the never-ending wars men waged. Ultimately the wars do end, with the deaths of the last men on Earth, two soldiers shooting each other, one shoots the other soldier and the injured soldier kills the last, but slowly dies as he sinks into a watery foxhole while his hand grasps into the water. Afterwards, the surviving animals discover a copy of an implied Bible in the ruins of a church. Inspired by the book’s teachings, they decide to rebuild a society dedicated to peace and non-violence (using the helmets of the soldiers to construct houses). The short features a version of “Hark! The Herald Angels Sing” with rewritten lyrics, and a trio of carolers sing this song outside of the squirrels’ house.

(From Wikipedia)